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What is going on with 'gets(stdin)' on the site coderbyte?
The Definitive C++ Book Guide and ListWhy does gets(stdin) return an integer? And other errorsWhat is the difference between #include <filename> and #include “filename”?What are the differences between a pointer variable and a reference variable in C++?What does the explicit keyword mean?What is the “-->” operator in C++?What are move semantics?What is the copy-and-swap idiom?What is The Rule of Three?What are the basic rules and idioms for operator overloading?What is a lambda expression in C++11?Why is reading lines from stdin much slower in C++ than Python?
Coderbyte is an online coding challenge site (I found it just 2 minutes ago).
The first C++ challenge you are greeted with has a C++ skeleton you need to modify:
#include <iostream>
#include <string>
using namespace std;
int FirstFactorial(int num)
// Code goes here
return num;
int main()
// Keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
If you are little familiar with C++ the first thing* that pops in your eyes is:
int FirstFactorial(int num);
cout << FirstFactorial(gets(stdin));
So, ok, the code calls gets
which is deprecated since C++11 and removed since C++14 which is bad in itself.
But then I realize: gets
is of type char*(char*)
. So it shouldn't accept a FILE*
parameter and the result shouldn't be usable in the place of an int
parameter, but ... not only it compiles without any warnings or errors, but it runs and actually passes the correct input value to FirstFactorial
.
Outside of this particular site, the code doesn't compile (as expected), so what is going on here?
*Actually the first one is using namespace std
but that is irrelevant to my issue here.
c++ input gets standards-compliance
|
show 1 more comment
Coderbyte is an online coding challenge site (I found it just 2 minutes ago).
The first C++ challenge you are greeted with has a C++ skeleton you need to modify:
#include <iostream>
#include <string>
using namespace std;
int FirstFactorial(int num)
// Code goes here
return num;
int main()
// Keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
If you are little familiar with C++ the first thing* that pops in your eyes is:
int FirstFactorial(int num);
cout << FirstFactorial(gets(stdin));
So, ok, the code calls gets
which is deprecated since C++11 and removed since C++14 which is bad in itself.
But then I realize: gets
is of type char*(char*)
. So it shouldn't accept a FILE*
parameter and the result shouldn't be usable in the place of an int
parameter, but ... not only it compiles without any warnings or errors, but it runs and actually passes the correct input value to FirstFactorial
.
Outside of this particular site, the code doesn't compile (as expected), so what is going on here?
*Actually the first one is using namespace std
but that is irrelevant to my issue here.
c++ input gets standards-compliance
Note thatstdin
in the standard library is aFILE*
, and a pointer to any type converts tochar*
, which is the type of the argument ofgets()
. However, you should never, ever, ever write that kind of code outside an obfuscated C contest. If your compiler even accepts it, add more warning flags, and if you’re trying to fix a codebase that has that construct in it, turn warnings into errors.
– Davislor
2 days ago
@Davislor no it doesn't "candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument"
– bolov
yesterday
2
@Davislor huh, that might be true for ancient C, but definitely not for C++.
– Quentin
yesterday
@Quentin Yeah. That shouldn’t compile. The intended challenge might’ve been, “Take this broken code, read my mind about what it’s supposed to do, and fix it,” but in that case there should be a real specification. With test cases.
– Davislor
yesterday
4
I’m surprised no-one tried this, butgets(stdin )
(with an extra space) produces the expected C++ error.
– Roman Odaisky
yesterday
|
show 1 more comment
Coderbyte is an online coding challenge site (I found it just 2 minutes ago).
The first C++ challenge you are greeted with has a C++ skeleton you need to modify:
#include <iostream>
#include <string>
using namespace std;
int FirstFactorial(int num)
// Code goes here
return num;
int main()
// Keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
If you are little familiar with C++ the first thing* that pops in your eyes is:
int FirstFactorial(int num);
cout << FirstFactorial(gets(stdin));
So, ok, the code calls gets
which is deprecated since C++11 and removed since C++14 which is bad in itself.
But then I realize: gets
is of type char*(char*)
. So it shouldn't accept a FILE*
parameter and the result shouldn't be usable in the place of an int
parameter, but ... not only it compiles without any warnings or errors, but it runs and actually passes the correct input value to FirstFactorial
.
Outside of this particular site, the code doesn't compile (as expected), so what is going on here?
*Actually the first one is using namespace std
but that is irrelevant to my issue here.
c++ input gets standards-compliance
Coderbyte is an online coding challenge site (I found it just 2 minutes ago).
The first C++ challenge you are greeted with has a C++ skeleton you need to modify:
#include <iostream>
#include <string>
using namespace std;
int FirstFactorial(int num)
// Code goes here
return num;
int main()
// Keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
If you are little familiar with C++ the first thing* that pops in your eyes is:
int FirstFactorial(int num);
cout << FirstFactorial(gets(stdin));
So, ok, the code calls gets
which is deprecated since C++11 and removed since C++14 which is bad in itself.
But then I realize: gets
is of type char*(char*)
. So it shouldn't accept a FILE*
parameter and the result shouldn't be usable in the place of an int
parameter, but ... not only it compiles without any warnings or errors, but it runs and actually passes the correct input value to FirstFactorial
.
Outside of this particular site, the code doesn't compile (as expected), so what is going on here?
*Actually the first one is using namespace std
but that is irrelevant to my issue here.
c++ input gets standards-compliance
c++ input gets standards-compliance
edited 15 hours ago
Peter Mortensen
13.8k1987113
13.8k1987113
asked 2 days ago
bolovbolov
32.6k676140
32.6k676140
Note thatstdin
in the standard library is aFILE*
, and a pointer to any type converts tochar*
, which is the type of the argument ofgets()
. However, you should never, ever, ever write that kind of code outside an obfuscated C contest. If your compiler even accepts it, add more warning flags, and if you’re trying to fix a codebase that has that construct in it, turn warnings into errors.
– Davislor
2 days ago
@Davislor no it doesn't "candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument"
– bolov
yesterday
2
@Davislor huh, that might be true for ancient C, but definitely not for C++.
– Quentin
yesterday
@Quentin Yeah. That shouldn’t compile. The intended challenge might’ve been, “Take this broken code, read my mind about what it’s supposed to do, and fix it,” but in that case there should be a real specification. With test cases.
– Davislor
yesterday
4
I’m surprised no-one tried this, butgets(stdin )
(with an extra space) produces the expected C++ error.
– Roman Odaisky
yesterday
|
show 1 more comment
Note thatstdin
in the standard library is aFILE*
, and a pointer to any type converts tochar*
, which is the type of the argument ofgets()
. However, you should never, ever, ever write that kind of code outside an obfuscated C contest. If your compiler even accepts it, add more warning flags, and if you’re trying to fix a codebase that has that construct in it, turn warnings into errors.
– Davislor
2 days ago
@Davislor no it doesn't "candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument"
– bolov
yesterday
2
@Davislor huh, that might be true for ancient C, but definitely not for C++.
– Quentin
yesterday
@Quentin Yeah. That shouldn’t compile. The intended challenge might’ve been, “Take this broken code, read my mind about what it’s supposed to do, and fix it,” but in that case there should be a real specification. With test cases.
– Davislor
yesterday
4
I’m surprised no-one tried this, butgets(stdin )
(with an extra space) produces the expected C++ error.
– Roman Odaisky
yesterday
Note that
stdin
in the standard library is a FILE*
, and a pointer to any type converts to char*
, which is the type of the argument of gets()
. However, you should never, ever, ever write that kind of code outside an obfuscated C contest. If your compiler even accepts it, add more warning flags, and if you’re trying to fix a codebase that has that construct in it, turn warnings into errors.– Davislor
2 days ago
Note that
stdin
in the standard library is a FILE*
, and a pointer to any type converts to char*
, which is the type of the argument of gets()
. However, you should never, ever, ever write that kind of code outside an obfuscated C contest. If your compiler even accepts it, add more warning flags, and if you’re trying to fix a codebase that has that construct in it, turn warnings into errors.– Davislor
2 days ago
@Davislor no it doesn't "candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument"
– bolov
yesterday
@Davislor no it doesn't "candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument"
– bolov
yesterday
2
2
@Davislor huh, that might be true for ancient C, but definitely not for C++.
– Quentin
yesterday
@Davislor huh, that might be true for ancient C, but definitely not for C++.
– Quentin
yesterday
@Quentin Yeah. That shouldn’t compile. The intended challenge might’ve been, “Take this broken code, read my mind about what it’s supposed to do, and fix it,” but in that case there should be a real specification. With test cases.
– Davislor
yesterday
@Quentin Yeah. That shouldn’t compile. The intended challenge might’ve been, “Take this broken code, read my mind about what it’s supposed to do, and fix it,” but in that case there should be a real specification. With test cases.
– Davislor
yesterday
4
4
I’m surprised no-one tried this, but
gets(stdin )
(with an extra space) produces the expected C++ error.– Roman Odaisky
yesterday
I’m surprised no-one tried this, but
gets(stdin )
(with an extra space) produces the expected C++ error.– Roman Odaisky
yesterday
|
show 1 more comment
3 Answers
3
active
oldest
votes
I'm the founder of Coderbyte and also the guy who created this "gets(stdin)" hack.
The comments on this post are correct that is is a form of find-and-replace, so let me explain why I did this really quickly.
Back in the day when I first created the site (around 2012), it only supported JavaScript. There was no way to "read in input" in JavaScript running in the browser, and so there would be a function foo(input) and I used the readline() function from Node.js to call it like foo(readline()). Except I was a kid and didn't know better, so I literally just replaced readline() with the input at run-time. So foo(readline()) became foo(2) or foo("hello") which worked fine for JavaScript.
Around 2013/2014 I added more languages and used third-party service to evaluate code online, but it was very difficult to do stdin/stdout with the services I was using, so I stuck with the same silly find-and-replace for languages like Python, Ruby, and eventually C++, C#, etc.
Fast forward to today, I run the code in my own containers, but never updated the way stdin/stdout works because people have gotten used to the weird hack (some people have even posted in forums explaining how to get around it).
I know it is not best practice and it isn't helpful for someone learning a new language to see hacks like this, but the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem. One common complaint about coding challenge sites years ago was that new programmers would spend a lot of time just figuring out how to read from stdin or read lines from a file, so I wanted new coders to avoid this problem on Coderbyte.
I'll be updating the entire editor page soon along with the default code and stdin reading for languages. Hopefully then C++ programmers will enjoy using Coderbyte more :)
New contributor
13
"[B]ut the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem" - and it didn't occur to you to, instead of writing something that resembles "real" code, just put a made up function name or an obvious placeholder in that spot? Genuinely curious.
– Ruther Rendommeleigh
yesterday
Suppose that the input is a string instead of a number, let's say "hello". You replacegets(stdin)
with "hello" and use that as input to the function to see if the test passes. If my code is wrong and it only passes with the string "world" as input, I can just put a#define gets(string) "world"
and every test will pass. Right?
– ChatterOne
yesterday
21
I genuinely didn't expect I was going to choose an answer other than my own when I posted this. Thank you for proving me wrong in such a great way. It's really a pleasure to see your answer.
– bolov
yesterday
3
Very interesting! I would recommend, if you want to keep this hack, that you replace the function call with something likeTAKE_INPUT
, then use your find-replace to insert#define TAKE_INPUT whatever_here
at the top.
– Draconis
yesterday
4
We need more answers starting with "I'm the founder of x and also the guy who created this".
– pipe
20 hours ago
|
show 2 more comments
I am intrigued. So, time to put the investigation goggles on and since I don't have access to the compiler or compilation flags I need to get inventive. Also because nothing about this code makes sense it's not a bad idea question every assumption.
First let's check the actual type of gets
. I have a little trick for that:
template <class> struct Name;
int main()
Name<decltype(gets)> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
And that looks ... normal:
/tmp/613814454/Main.cpp:16:19: warning: 'gets' is deprecated [-Wdeprecated-declarations]
Name<decltype(gets)> n;
^
/usr/include/stdio.h:638:37: note: 'gets' has been explicitly marked deprecated here
extern char *gets (char *__s) __wur __attribute_deprecated__;
^
/usr/include/x86_64-linux-gnu/sys/cdefs.h:254:51: note: expanded from macro '__attribute_deprecated__'
# define __attribute_deprecated__ __attribute__ ((__deprecated__))
^
/tmp/613814454/Main.cpp:16:26: error: implicit instantiation of undefined template 'Name<char *(char *)>'
Name<decltype(gets)> n;
^
/tmp/613814454/Main.cpp:12:25: note: template is declared here
template <class> struct Name;
^
1 warning and 1 error generated.
gets
is marked as deprecated and has the signature char *(char *)
. But then how is FirstFactorial(gets(stdin));
compiling?
Let's try something else:
int main()
Name<decltype(gets(stdin))> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
Which gives us:
/tmp/286775780/Main.cpp:15:21: error: implicit instantiation of undefined template 'Name<int>'
Name<decltype(8)> n;
^
Finally we are getting something: decltype(8)
. So the entire gets(stdin)
was textually replaced with the input (8
).
And the things get weirder. The compiler error continues:
/tmp/596773533/Main.cpp:18:26: error: no matching function for call to 'gets'
cout << FirstFactorial(gets(stdin));
^~~~
/usr/include/stdio.h:638:14: note: candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument
extern char *gets (char *__s) __wur __attribute_deprecated__;
So now we get the expected error for cout << FirstFactorial(gets(stdin));
I checked for a macro and since #undef gets
seems to do nothing it looks like it isn't a macro.
But
std::integral_constant<int, gets(stdin)> n;
It compiles.
But
std::integral_constant<int, gets(stdin)> n; // OK
std::integral_constant<int, gets(stdin)> n2; // ERROR wtf??
Doesn't with the expected error at the n2
line.
And again, almost any modification to main
makes the line cout << FirstFactorial(gets(stdin));
spit out the expected error.
Moreover the stdin
actually seems to be empty.
So I can only conclude and speculate they have a little program that parses the source and tries (poorly) to replace gets(stdin)
with the test case input value before actually feeding it into the compiler. If anybody has a better theory or actually knows what they are doing please share!
This is obviously a very bad practice. While researching this I found there is at least a question here (example) about this and because people have no idea that there is a site out there who does this their answer is "don't use gets
use ... instead" which is indeed a good advice but only confuses the OP more since any attempt at a valid read from stdin will fail on this site.
TLDR
gets(stdin)
is invalid C++. It's a gimmick this particular site uses (for what reasons I cannot figure out). If you want to continue to submit on the site (I am neither endorsing it neither not endorsing it) you have to use this construct that otherwise would not make sense, but be aware that it is brittle. Almost any modifications to main
will spit out an error. Outside of this site use normal input reading methods.
24
I'm genuinely amazed. Maybe this Q/A can be a canonical post on why not to learn from coding challenge sites.
– alter igel
2 days ago
1
The "8" comes from the left box at the bottom of the screen. Try typing a text string in there (I tried "maplemaple") and see the result...
– Stobor
2 days ago
24
Something really evil is happening, and I think it's at the level of text replacement in the source code outside of the compiler. Try this:std::cout << "gets(stdin)";
and the output is8
(or whatever you type into the 'input' field. This is a disgraceful abuse of the language.
– alter igel
2 days ago
11
@Stobor note the quotes around"gets(stdin)"
. That's a string literal that even the preprocessor wouldn't touch
– alter igel
2 days ago
1
To quote James Kirk: "This is damn peculiar."
– ApproachingDarknessFish
yesterday
|
show 6 more comments
I tried the following addition to main
in the Coderbyte editor:
std::cout << "gets(stdin)";
Where the mysterious and enigmatic snippet gets(stdin)
appears inside a string literal. This shouldn't possibly be transformed by anything, not even the preprocessor, and any C++ programmer should expect this code to print the exact string gets(stdin)
to the standard output. And yet we see the following output, when compiled and run on coderbyte:
8
Where the value 8
is taken straight from the convenient 'input' field under the editor.
From this, it's clear that this online editor is performing blind find-and-replace operations on the source code, substitution appearances of gets(stdin)
with the user's 'input'. I would personally call this a misuse of the language that's worse than careless preprocessor macros.
In the context of an online coding challenge website, I'm worried by this because it teaches unconventional, non-standard, meaningless, and at least unsafe practices like gets(stdin)
, and in a manner that can't be repeated on other platforms.
I'm sure it can't be this hard to just use std::cin
and just stream input to a program.
and it's not even a blind "find and replace" because sometimes it replaces it sometimes it does not.
– bolov
2 days ago
3
@bolov could it be just the first occurrence ofgets(stdin)
that is replaced? I meant 'blind' in the sense that it appears to be unaware of the language's syntax or grammar.
– alter igel
2 days ago
yes, you are right. It replaces the first occurence. I tried putting one before main and that's what I got indeed.
– bolov
2 days ago
1
Further research suggests that that site does it for all languages, not just C++ - python/ruby it uses the function call ("raw_input()" or "STDIN.gets") which would typically return a string from stdin, but ends up doing a string substitution of that string instead. I guess finding a regex match for the getline function was too hard, so they went with gets(stdin) for C/C++.
– Stobor
2 days ago
3
@Stobor dang, you're right. I can confirm this happens for Java too, the lineSystem.out.print(FirstFactorial(s.nextLine()9));
prints89
even whens
is undefined.
– alter igel
2 days ago
add a comment |
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3 Answers
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3 Answers
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active
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active
oldest
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active
oldest
votes
I'm the founder of Coderbyte and also the guy who created this "gets(stdin)" hack.
The comments on this post are correct that is is a form of find-and-replace, so let me explain why I did this really quickly.
Back in the day when I first created the site (around 2012), it only supported JavaScript. There was no way to "read in input" in JavaScript running in the browser, and so there would be a function foo(input) and I used the readline() function from Node.js to call it like foo(readline()). Except I was a kid and didn't know better, so I literally just replaced readline() with the input at run-time. So foo(readline()) became foo(2) or foo("hello") which worked fine for JavaScript.
Around 2013/2014 I added more languages and used third-party service to evaluate code online, but it was very difficult to do stdin/stdout with the services I was using, so I stuck with the same silly find-and-replace for languages like Python, Ruby, and eventually C++, C#, etc.
Fast forward to today, I run the code in my own containers, but never updated the way stdin/stdout works because people have gotten used to the weird hack (some people have even posted in forums explaining how to get around it).
I know it is not best practice and it isn't helpful for someone learning a new language to see hacks like this, but the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem. One common complaint about coding challenge sites years ago was that new programmers would spend a lot of time just figuring out how to read from stdin or read lines from a file, so I wanted new coders to avoid this problem on Coderbyte.
I'll be updating the entire editor page soon along with the default code and stdin reading for languages. Hopefully then C++ programmers will enjoy using Coderbyte more :)
New contributor
13
"[B]ut the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem" - and it didn't occur to you to, instead of writing something that resembles "real" code, just put a made up function name or an obvious placeholder in that spot? Genuinely curious.
– Ruther Rendommeleigh
yesterday
Suppose that the input is a string instead of a number, let's say "hello". You replacegets(stdin)
with "hello" and use that as input to the function to see if the test passes. If my code is wrong and it only passes with the string "world" as input, I can just put a#define gets(string) "world"
and every test will pass. Right?
– ChatterOne
yesterday
21
I genuinely didn't expect I was going to choose an answer other than my own when I posted this. Thank you for proving me wrong in such a great way. It's really a pleasure to see your answer.
– bolov
yesterday
3
Very interesting! I would recommend, if you want to keep this hack, that you replace the function call with something likeTAKE_INPUT
, then use your find-replace to insert#define TAKE_INPUT whatever_here
at the top.
– Draconis
yesterday
4
We need more answers starting with "I'm the founder of x and also the guy who created this".
– pipe
20 hours ago
|
show 2 more comments
I'm the founder of Coderbyte and also the guy who created this "gets(stdin)" hack.
The comments on this post are correct that is is a form of find-and-replace, so let me explain why I did this really quickly.
Back in the day when I first created the site (around 2012), it only supported JavaScript. There was no way to "read in input" in JavaScript running in the browser, and so there would be a function foo(input) and I used the readline() function from Node.js to call it like foo(readline()). Except I was a kid and didn't know better, so I literally just replaced readline() with the input at run-time. So foo(readline()) became foo(2) or foo("hello") which worked fine for JavaScript.
Around 2013/2014 I added more languages and used third-party service to evaluate code online, but it was very difficult to do stdin/stdout with the services I was using, so I stuck with the same silly find-and-replace for languages like Python, Ruby, and eventually C++, C#, etc.
Fast forward to today, I run the code in my own containers, but never updated the way stdin/stdout works because people have gotten used to the weird hack (some people have even posted in forums explaining how to get around it).
I know it is not best practice and it isn't helpful for someone learning a new language to see hacks like this, but the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem. One common complaint about coding challenge sites years ago was that new programmers would spend a lot of time just figuring out how to read from stdin or read lines from a file, so I wanted new coders to avoid this problem on Coderbyte.
I'll be updating the entire editor page soon along with the default code and stdin reading for languages. Hopefully then C++ programmers will enjoy using Coderbyte more :)
New contributor
13
"[B]ut the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem" - and it didn't occur to you to, instead of writing something that resembles "real" code, just put a made up function name or an obvious placeholder in that spot? Genuinely curious.
– Ruther Rendommeleigh
yesterday
Suppose that the input is a string instead of a number, let's say "hello". You replacegets(stdin)
with "hello" and use that as input to the function to see if the test passes. If my code is wrong and it only passes with the string "world" as input, I can just put a#define gets(string) "world"
and every test will pass. Right?
– ChatterOne
yesterday
21
I genuinely didn't expect I was going to choose an answer other than my own when I posted this. Thank you for proving me wrong in such a great way. It's really a pleasure to see your answer.
– bolov
yesterday
3
Very interesting! I would recommend, if you want to keep this hack, that you replace the function call with something likeTAKE_INPUT
, then use your find-replace to insert#define TAKE_INPUT whatever_here
at the top.
– Draconis
yesterday
4
We need more answers starting with "I'm the founder of x and also the guy who created this".
– pipe
20 hours ago
|
show 2 more comments
I'm the founder of Coderbyte and also the guy who created this "gets(stdin)" hack.
The comments on this post are correct that is is a form of find-and-replace, so let me explain why I did this really quickly.
Back in the day when I first created the site (around 2012), it only supported JavaScript. There was no way to "read in input" in JavaScript running in the browser, and so there would be a function foo(input) and I used the readline() function from Node.js to call it like foo(readline()). Except I was a kid and didn't know better, so I literally just replaced readline() with the input at run-time. So foo(readline()) became foo(2) or foo("hello") which worked fine for JavaScript.
Around 2013/2014 I added more languages and used third-party service to evaluate code online, but it was very difficult to do stdin/stdout with the services I was using, so I stuck with the same silly find-and-replace for languages like Python, Ruby, and eventually C++, C#, etc.
Fast forward to today, I run the code in my own containers, but never updated the way stdin/stdout works because people have gotten used to the weird hack (some people have even posted in forums explaining how to get around it).
I know it is not best practice and it isn't helpful for someone learning a new language to see hacks like this, but the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem. One common complaint about coding challenge sites years ago was that new programmers would spend a lot of time just figuring out how to read from stdin or read lines from a file, so I wanted new coders to avoid this problem on Coderbyte.
I'll be updating the entire editor page soon along with the default code and stdin reading for languages. Hopefully then C++ programmers will enjoy using Coderbyte more :)
New contributor
I'm the founder of Coderbyte and also the guy who created this "gets(stdin)" hack.
The comments on this post are correct that is is a form of find-and-replace, so let me explain why I did this really quickly.
Back in the day when I first created the site (around 2012), it only supported JavaScript. There was no way to "read in input" in JavaScript running in the browser, and so there would be a function foo(input) and I used the readline() function from Node.js to call it like foo(readline()). Except I was a kid and didn't know better, so I literally just replaced readline() with the input at run-time. So foo(readline()) became foo(2) or foo("hello") which worked fine for JavaScript.
Around 2013/2014 I added more languages and used third-party service to evaluate code online, but it was very difficult to do stdin/stdout with the services I was using, so I stuck with the same silly find-and-replace for languages like Python, Ruby, and eventually C++, C#, etc.
Fast forward to today, I run the code in my own containers, but never updated the way stdin/stdout works because people have gotten used to the weird hack (some people have even posted in forums explaining how to get around it).
I know it is not best practice and it isn't helpful for someone learning a new language to see hacks like this, but the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem. One common complaint about coding challenge sites years ago was that new programmers would spend a lot of time just figuring out how to read from stdin or read lines from a file, so I wanted new coders to avoid this problem on Coderbyte.
I'll be updating the entire editor page soon along with the default code and stdin reading for languages. Hopefully then C++ programmers will enjoy using Coderbyte more :)
New contributor
edited 15 hours ago
Peter Mortensen
13.8k1987113
13.8k1987113
New contributor
answered 2 days ago
Daniel BorowskiDaniel Borowski
436123
436123
New contributor
New contributor
13
"[B]ut the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem" - and it didn't occur to you to, instead of writing something that resembles "real" code, just put a made up function name or an obvious placeholder in that spot? Genuinely curious.
– Ruther Rendommeleigh
yesterday
Suppose that the input is a string instead of a number, let's say "hello". You replacegets(stdin)
with "hello" and use that as input to the function to see if the test passes. If my code is wrong and it only passes with the string "world" as input, I can just put a#define gets(string) "world"
and every test will pass. Right?
– ChatterOne
yesterday
21
I genuinely didn't expect I was going to choose an answer other than my own when I posted this. Thank you for proving me wrong in such a great way. It's really a pleasure to see your answer.
– bolov
yesterday
3
Very interesting! I would recommend, if you want to keep this hack, that you replace the function call with something likeTAKE_INPUT
, then use your find-replace to insert#define TAKE_INPUT whatever_here
at the top.
– Draconis
yesterday
4
We need more answers starting with "I'm the founder of x and also the guy who created this".
– pipe
20 hours ago
|
show 2 more comments
13
"[B]ut the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem" - and it didn't occur to you to, instead of writing something that resembles "real" code, just put a made up function name or an obvious placeholder in that spot? Genuinely curious.
– Ruther Rendommeleigh
yesterday
Suppose that the input is a string instead of a number, let's say "hello". You replacegets(stdin)
with "hello" and use that as input to the function to see if the test passes. If my code is wrong and it only passes with the string "world" as input, I can just put a#define gets(string) "world"
and every test will pass. Right?
– ChatterOne
yesterday
21
I genuinely didn't expect I was going to choose an answer other than my own when I posted this. Thank you for proving me wrong in such a great way. It's really a pleasure to see your answer.
– bolov
yesterday
3
Very interesting! I would recommend, if you want to keep this hack, that you replace the function call with something likeTAKE_INPUT
, then use your find-replace to insert#define TAKE_INPUT whatever_here
at the top.
– Draconis
yesterday
4
We need more answers starting with "I'm the founder of x and also the guy who created this".
– pipe
20 hours ago
13
13
"[B]ut the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem" - and it didn't occur to you to, instead of writing something that resembles "real" code, just put a made up function name or an obvious placeholder in that spot? Genuinely curious.
– Ruther Rendommeleigh
yesterday
"[B]ut the idea was for new programmers to not worry about reading input at all and just focus on writing the algorithm to solve the problem" - and it didn't occur to you to, instead of writing something that resembles "real" code, just put a made up function name or an obvious placeholder in that spot? Genuinely curious.
– Ruther Rendommeleigh
yesterday
Suppose that the input is a string instead of a number, let's say "hello". You replace
gets(stdin)
with "hello" and use that as input to the function to see if the test passes. If my code is wrong and it only passes with the string "world" as input, I can just put a #define gets(string) "world"
and every test will pass. Right?– ChatterOne
yesterday
Suppose that the input is a string instead of a number, let's say "hello". You replace
gets(stdin)
with "hello" and use that as input to the function to see if the test passes. If my code is wrong and it only passes with the string "world" as input, I can just put a #define gets(string) "world"
and every test will pass. Right?– ChatterOne
yesterday
21
21
I genuinely didn't expect I was going to choose an answer other than my own when I posted this. Thank you for proving me wrong in such a great way. It's really a pleasure to see your answer.
– bolov
yesterday
I genuinely didn't expect I was going to choose an answer other than my own when I posted this. Thank you for proving me wrong in such a great way. It's really a pleasure to see your answer.
– bolov
yesterday
3
3
Very interesting! I would recommend, if you want to keep this hack, that you replace the function call with something like
TAKE_INPUT
, then use your find-replace to insert #define TAKE_INPUT whatever_here
at the top.– Draconis
yesterday
Very interesting! I would recommend, if you want to keep this hack, that you replace the function call with something like
TAKE_INPUT
, then use your find-replace to insert #define TAKE_INPUT whatever_here
at the top.– Draconis
yesterday
4
4
We need more answers starting with "I'm the founder of x and also the guy who created this".
– pipe
20 hours ago
We need more answers starting with "I'm the founder of x and also the guy who created this".
– pipe
20 hours ago
|
show 2 more comments
I am intrigued. So, time to put the investigation goggles on and since I don't have access to the compiler or compilation flags I need to get inventive. Also because nothing about this code makes sense it's not a bad idea question every assumption.
First let's check the actual type of gets
. I have a little trick for that:
template <class> struct Name;
int main()
Name<decltype(gets)> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
And that looks ... normal:
/tmp/613814454/Main.cpp:16:19: warning: 'gets' is deprecated [-Wdeprecated-declarations]
Name<decltype(gets)> n;
^
/usr/include/stdio.h:638:37: note: 'gets' has been explicitly marked deprecated here
extern char *gets (char *__s) __wur __attribute_deprecated__;
^
/usr/include/x86_64-linux-gnu/sys/cdefs.h:254:51: note: expanded from macro '__attribute_deprecated__'
# define __attribute_deprecated__ __attribute__ ((__deprecated__))
^
/tmp/613814454/Main.cpp:16:26: error: implicit instantiation of undefined template 'Name<char *(char *)>'
Name<decltype(gets)> n;
^
/tmp/613814454/Main.cpp:12:25: note: template is declared here
template <class> struct Name;
^
1 warning and 1 error generated.
gets
is marked as deprecated and has the signature char *(char *)
. But then how is FirstFactorial(gets(stdin));
compiling?
Let's try something else:
int main()
Name<decltype(gets(stdin))> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
Which gives us:
/tmp/286775780/Main.cpp:15:21: error: implicit instantiation of undefined template 'Name<int>'
Name<decltype(8)> n;
^
Finally we are getting something: decltype(8)
. So the entire gets(stdin)
was textually replaced with the input (8
).
And the things get weirder. The compiler error continues:
/tmp/596773533/Main.cpp:18:26: error: no matching function for call to 'gets'
cout << FirstFactorial(gets(stdin));
^~~~
/usr/include/stdio.h:638:14: note: candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument
extern char *gets (char *__s) __wur __attribute_deprecated__;
So now we get the expected error for cout << FirstFactorial(gets(stdin));
I checked for a macro and since #undef gets
seems to do nothing it looks like it isn't a macro.
But
std::integral_constant<int, gets(stdin)> n;
It compiles.
But
std::integral_constant<int, gets(stdin)> n; // OK
std::integral_constant<int, gets(stdin)> n2; // ERROR wtf??
Doesn't with the expected error at the n2
line.
And again, almost any modification to main
makes the line cout << FirstFactorial(gets(stdin));
spit out the expected error.
Moreover the stdin
actually seems to be empty.
So I can only conclude and speculate they have a little program that parses the source and tries (poorly) to replace gets(stdin)
with the test case input value before actually feeding it into the compiler. If anybody has a better theory or actually knows what they are doing please share!
This is obviously a very bad practice. While researching this I found there is at least a question here (example) about this and because people have no idea that there is a site out there who does this their answer is "don't use gets
use ... instead" which is indeed a good advice but only confuses the OP more since any attempt at a valid read from stdin will fail on this site.
TLDR
gets(stdin)
is invalid C++. It's a gimmick this particular site uses (for what reasons I cannot figure out). If you want to continue to submit on the site (I am neither endorsing it neither not endorsing it) you have to use this construct that otherwise would not make sense, but be aware that it is brittle. Almost any modifications to main
will spit out an error. Outside of this site use normal input reading methods.
24
I'm genuinely amazed. Maybe this Q/A can be a canonical post on why not to learn from coding challenge sites.
– alter igel
2 days ago
1
The "8" comes from the left box at the bottom of the screen. Try typing a text string in there (I tried "maplemaple") and see the result...
– Stobor
2 days ago
24
Something really evil is happening, and I think it's at the level of text replacement in the source code outside of the compiler. Try this:std::cout << "gets(stdin)";
and the output is8
(or whatever you type into the 'input' field. This is a disgraceful abuse of the language.
– alter igel
2 days ago
11
@Stobor note the quotes around"gets(stdin)"
. That's a string literal that even the preprocessor wouldn't touch
– alter igel
2 days ago
1
To quote James Kirk: "This is damn peculiar."
– ApproachingDarknessFish
yesterday
|
show 6 more comments
I am intrigued. So, time to put the investigation goggles on and since I don't have access to the compiler or compilation flags I need to get inventive. Also because nothing about this code makes sense it's not a bad idea question every assumption.
First let's check the actual type of gets
. I have a little trick for that:
template <class> struct Name;
int main()
Name<decltype(gets)> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
And that looks ... normal:
/tmp/613814454/Main.cpp:16:19: warning: 'gets' is deprecated [-Wdeprecated-declarations]
Name<decltype(gets)> n;
^
/usr/include/stdio.h:638:37: note: 'gets' has been explicitly marked deprecated here
extern char *gets (char *__s) __wur __attribute_deprecated__;
^
/usr/include/x86_64-linux-gnu/sys/cdefs.h:254:51: note: expanded from macro '__attribute_deprecated__'
# define __attribute_deprecated__ __attribute__ ((__deprecated__))
^
/tmp/613814454/Main.cpp:16:26: error: implicit instantiation of undefined template 'Name<char *(char *)>'
Name<decltype(gets)> n;
^
/tmp/613814454/Main.cpp:12:25: note: template is declared here
template <class> struct Name;
^
1 warning and 1 error generated.
gets
is marked as deprecated and has the signature char *(char *)
. But then how is FirstFactorial(gets(stdin));
compiling?
Let's try something else:
int main()
Name<decltype(gets(stdin))> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
Which gives us:
/tmp/286775780/Main.cpp:15:21: error: implicit instantiation of undefined template 'Name<int>'
Name<decltype(8)> n;
^
Finally we are getting something: decltype(8)
. So the entire gets(stdin)
was textually replaced with the input (8
).
And the things get weirder. The compiler error continues:
/tmp/596773533/Main.cpp:18:26: error: no matching function for call to 'gets'
cout << FirstFactorial(gets(stdin));
^~~~
/usr/include/stdio.h:638:14: note: candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument
extern char *gets (char *__s) __wur __attribute_deprecated__;
So now we get the expected error for cout << FirstFactorial(gets(stdin));
I checked for a macro and since #undef gets
seems to do nothing it looks like it isn't a macro.
But
std::integral_constant<int, gets(stdin)> n;
It compiles.
But
std::integral_constant<int, gets(stdin)> n; // OK
std::integral_constant<int, gets(stdin)> n2; // ERROR wtf??
Doesn't with the expected error at the n2
line.
And again, almost any modification to main
makes the line cout << FirstFactorial(gets(stdin));
spit out the expected error.
Moreover the stdin
actually seems to be empty.
So I can only conclude and speculate they have a little program that parses the source and tries (poorly) to replace gets(stdin)
with the test case input value before actually feeding it into the compiler. If anybody has a better theory or actually knows what they are doing please share!
This is obviously a very bad practice. While researching this I found there is at least a question here (example) about this and because people have no idea that there is a site out there who does this their answer is "don't use gets
use ... instead" which is indeed a good advice but only confuses the OP more since any attempt at a valid read from stdin will fail on this site.
TLDR
gets(stdin)
is invalid C++. It's a gimmick this particular site uses (for what reasons I cannot figure out). If you want to continue to submit on the site (I am neither endorsing it neither not endorsing it) you have to use this construct that otherwise would not make sense, but be aware that it is brittle. Almost any modifications to main
will spit out an error. Outside of this site use normal input reading methods.
24
I'm genuinely amazed. Maybe this Q/A can be a canonical post on why not to learn from coding challenge sites.
– alter igel
2 days ago
1
The "8" comes from the left box at the bottom of the screen. Try typing a text string in there (I tried "maplemaple") and see the result...
– Stobor
2 days ago
24
Something really evil is happening, and I think it's at the level of text replacement in the source code outside of the compiler. Try this:std::cout << "gets(stdin)";
and the output is8
(or whatever you type into the 'input' field. This is a disgraceful abuse of the language.
– alter igel
2 days ago
11
@Stobor note the quotes around"gets(stdin)"
. That's a string literal that even the preprocessor wouldn't touch
– alter igel
2 days ago
1
To quote James Kirk: "This is damn peculiar."
– ApproachingDarknessFish
yesterday
|
show 6 more comments
I am intrigued. So, time to put the investigation goggles on and since I don't have access to the compiler or compilation flags I need to get inventive. Also because nothing about this code makes sense it's not a bad idea question every assumption.
First let's check the actual type of gets
. I have a little trick for that:
template <class> struct Name;
int main()
Name<decltype(gets)> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
And that looks ... normal:
/tmp/613814454/Main.cpp:16:19: warning: 'gets' is deprecated [-Wdeprecated-declarations]
Name<decltype(gets)> n;
^
/usr/include/stdio.h:638:37: note: 'gets' has been explicitly marked deprecated here
extern char *gets (char *__s) __wur __attribute_deprecated__;
^
/usr/include/x86_64-linux-gnu/sys/cdefs.h:254:51: note: expanded from macro '__attribute_deprecated__'
# define __attribute_deprecated__ __attribute__ ((__deprecated__))
^
/tmp/613814454/Main.cpp:16:26: error: implicit instantiation of undefined template 'Name<char *(char *)>'
Name<decltype(gets)> n;
^
/tmp/613814454/Main.cpp:12:25: note: template is declared here
template <class> struct Name;
^
1 warning and 1 error generated.
gets
is marked as deprecated and has the signature char *(char *)
. But then how is FirstFactorial(gets(stdin));
compiling?
Let's try something else:
int main()
Name<decltype(gets(stdin))> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
Which gives us:
/tmp/286775780/Main.cpp:15:21: error: implicit instantiation of undefined template 'Name<int>'
Name<decltype(8)> n;
^
Finally we are getting something: decltype(8)
. So the entire gets(stdin)
was textually replaced with the input (8
).
And the things get weirder. The compiler error continues:
/tmp/596773533/Main.cpp:18:26: error: no matching function for call to 'gets'
cout << FirstFactorial(gets(stdin));
^~~~
/usr/include/stdio.h:638:14: note: candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument
extern char *gets (char *__s) __wur __attribute_deprecated__;
So now we get the expected error for cout << FirstFactorial(gets(stdin));
I checked for a macro and since #undef gets
seems to do nothing it looks like it isn't a macro.
But
std::integral_constant<int, gets(stdin)> n;
It compiles.
But
std::integral_constant<int, gets(stdin)> n; // OK
std::integral_constant<int, gets(stdin)> n2; // ERROR wtf??
Doesn't with the expected error at the n2
line.
And again, almost any modification to main
makes the line cout << FirstFactorial(gets(stdin));
spit out the expected error.
Moreover the stdin
actually seems to be empty.
So I can only conclude and speculate they have a little program that parses the source and tries (poorly) to replace gets(stdin)
with the test case input value before actually feeding it into the compiler. If anybody has a better theory or actually knows what they are doing please share!
This is obviously a very bad practice. While researching this I found there is at least a question here (example) about this and because people have no idea that there is a site out there who does this their answer is "don't use gets
use ... instead" which is indeed a good advice but only confuses the OP more since any attempt at a valid read from stdin will fail on this site.
TLDR
gets(stdin)
is invalid C++. It's a gimmick this particular site uses (for what reasons I cannot figure out). If you want to continue to submit on the site (I am neither endorsing it neither not endorsing it) you have to use this construct that otherwise would not make sense, but be aware that it is brittle. Almost any modifications to main
will spit out an error. Outside of this site use normal input reading methods.
I am intrigued. So, time to put the investigation goggles on and since I don't have access to the compiler or compilation flags I need to get inventive. Also because nothing about this code makes sense it's not a bad idea question every assumption.
First let's check the actual type of gets
. I have a little trick for that:
template <class> struct Name;
int main()
Name<decltype(gets)> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
And that looks ... normal:
/tmp/613814454/Main.cpp:16:19: warning: 'gets' is deprecated [-Wdeprecated-declarations]
Name<decltype(gets)> n;
^
/usr/include/stdio.h:638:37: note: 'gets' has been explicitly marked deprecated here
extern char *gets (char *__s) __wur __attribute_deprecated__;
^
/usr/include/x86_64-linux-gnu/sys/cdefs.h:254:51: note: expanded from macro '__attribute_deprecated__'
# define __attribute_deprecated__ __attribute__ ((__deprecated__))
^
/tmp/613814454/Main.cpp:16:26: error: implicit instantiation of undefined template 'Name<char *(char *)>'
Name<decltype(gets)> n;
^
/tmp/613814454/Main.cpp:12:25: note: template is declared here
template <class> struct Name;
^
1 warning and 1 error generated.
gets
is marked as deprecated and has the signature char *(char *)
. But then how is FirstFactorial(gets(stdin));
compiling?
Let's try something else:
int main()
Name<decltype(gets(stdin))> n;
// keep this function call here
cout << FirstFactorial(gets(stdin));
return 0;
Which gives us:
/tmp/286775780/Main.cpp:15:21: error: implicit instantiation of undefined template 'Name<int>'
Name<decltype(8)> n;
^
Finally we are getting something: decltype(8)
. So the entire gets(stdin)
was textually replaced with the input (8
).
And the things get weirder. The compiler error continues:
/tmp/596773533/Main.cpp:18:26: error: no matching function for call to 'gets'
cout << FirstFactorial(gets(stdin));
^~~~
/usr/include/stdio.h:638:14: note: candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument
extern char *gets (char *__s) __wur __attribute_deprecated__;
So now we get the expected error for cout << FirstFactorial(gets(stdin));
I checked for a macro and since #undef gets
seems to do nothing it looks like it isn't a macro.
But
std::integral_constant<int, gets(stdin)> n;
It compiles.
But
std::integral_constant<int, gets(stdin)> n; // OK
std::integral_constant<int, gets(stdin)> n2; // ERROR wtf??
Doesn't with the expected error at the n2
line.
And again, almost any modification to main
makes the line cout << FirstFactorial(gets(stdin));
spit out the expected error.
Moreover the stdin
actually seems to be empty.
So I can only conclude and speculate they have a little program that parses the source and tries (poorly) to replace gets(stdin)
with the test case input value before actually feeding it into the compiler. If anybody has a better theory or actually knows what they are doing please share!
This is obviously a very bad practice. While researching this I found there is at least a question here (example) about this and because people have no idea that there is a site out there who does this their answer is "don't use gets
use ... instead" which is indeed a good advice but only confuses the OP more since any attempt at a valid read from stdin will fail on this site.
TLDR
gets(stdin)
is invalid C++. It's a gimmick this particular site uses (for what reasons I cannot figure out). If you want to continue to submit on the site (I am neither endorsing it neither not endorsing it) you have to use this construct that otherwise would not make sense, but be aware that it is brittle. Almost any modifications to main
will spit out an error. Outside of this site use normal input reading methods.
edited 2 days ago
scohe001
8,15212442
8,15212442
answered 2 days ago
bolovbolov
32.6k676140
32.6k676140
24
I'm genuinely amazed. Maybe this Q/A can be a canonical post on why not to learn from coding challenge sites.
– alter igel
2 days ago
1
The "8" comes from the left box at the bottom of the screen. Try typing a text string in there (I tried "maplemaple") and see the result...
– Stobor
2 days ago
24
Something really evil is happening, and I think it's at the level of text replacement in the source code outside of the compiler. Try this:std::cout << "gets(stdin)";
and the output is8
(or whatever you type into the 'input' field. This is a disgraceful abuse of the language.
– alter igel
2 days ago
11
@Stobor note the quotes around"gets(stdin)"
. That's a string literal that even the preprocessor wouldn't touch
– alter igel
2 days ago
1
To quote James Kirk: "This is damn peculiar."
– ApproachingDarknessFish
yesterday
|
show 6 more comments
24
I'm genuinely amazed. Maybe this Q/A can be a canonical post on why not to learn from coding challenge sites.
– alter igel
2 days ago
1
The "8" comes from the left box at the bottom of the screen. Try typing a text string in there (I tried "maplemaple") and see the result...
– Stobor
2 days ago
24
Something really evil is happening, and I think it's at the level of text replacement in the source code outside of the compiler. Try this:std::cout << "gets(stdin)";
and the output is8
(or whatever you type into the 'input' field. This is a disgraceful abuse of the language.
– alter igel
2 days ago
11
@Stobor note the quotes around"gets(stdin)"
. That's a string literal that even the preprocessor wouldn't touch
– alter igel
2 days ago
1
To quote James Kirk: "This is damn peculiar."
– ApproachingDarknessFish
yesterday
24
24
I'm genuinely amazed. Maybe this Q/A can be a canonical post on why not to learn from coding challenge sites.
– alter igel
2 days ago
I'm genuinely amazed. Maybe this Q/A can be a canonical post on why not to learn from coding challenge sites.
– alter igel
2 days ago
1
1
The "8" comes from the left box at the bottom of the screen. Try typing a text string in there (I tried "maplemaple") and see the result...
– Stobor
2 days ago
The "8" comes from the left box at the bottom of the screen. Try typing a text string in there (I tried "maplemaple") and see the result...
– Stobor
2 days ago
24
24
Something really evil is happening, and I think it's at the level of text replacement in the source code outside of the compiler. Try this:
std::cout << "gets(stdin)";
and the output is 8
(or whatever you type into the 'input' field. This is a disgraceful abuse of the language.– alter igel
2 days ago
Something really evil is happening, and I think it's at the level of text replacement in the source code outside of the compiler. Try this:
std::cout << "gets(stdin)";
and the output is 8
(or whatever you type into the 'input' field. This is a disgraceful abuse of the language.– alter igel
2 days ago
11
11
@Stobor note the quotes around
"gets(stdin)"
. That's a string literal that even the preprocessor wouldn't touch– alter igel
2 days ago
@Stobor note the quotes around
"gets(stdin)"
. That's a string literal that even the preprocessor wouldn't touch– alter igel
2 days ago
1
1
To quote James Kirk: "This is damn peculiar."
– ApproachingDarknessFish
yesterday
To quote James Kirk: "This is damn peculiar."
– ApproachingDarknessFish
yesterday
|
show 6 more comments
I tried the following addition to main
in the Coderbyte editor:
std::cout << "gets(stdin)";
Where the mysterious and enigmatic snippet gets(stdin)
appears inside a string literal. This shouldn't possibly be transformed by anything, not even the preprocessor, and any C++ programmer should expect this code to print the exact string gets(stdin)
to the standard output. And yet we see the following output, when compiled and run on coderbyte:
8
Where the value 8
is taken straight from the convenient 'input' field under the editor.
From this, it's clear that this online editor is performing blind find-and-replace operations on the source code, substitution appearances of gets(stdin)
with the user's 'input'. I would personally call this a misuse of the language that's worse than careless preprocessor macros.
In the context of an online coding challenge website, I'm worried by this because it teaches unconventional, non-standard, meaningless, and at least unsafe practices like gets(stdin)
, and in a manner that can't be repeated on other platforms.
I'm sure it can't be this hard to just use std::cin
and just stream input to a program.
and it's not even a blind "find and replace" because sometimes it replaces it sometimes it does not.
– bolov
2 days ago
3
@bolov could it be just the first occurrence ofgets(stdin)
that is replaced? I meant 'blind' in the sense that it appears to be unaware of the language's syntax or grammar.
– alter igel
2 days ago
yes, you are right. It replaces the first occurence. I tried putting one before main and that's what I got indeed.
– bolov
2 days ago
1
Further research suggests that that site does it for all languages, not just C++ - python/ruby it uses the function call ("raw_input()" or "STDIN.gets") which would typically return a string from stdin, but ends up doing a string substitution of that string instead. I guess finding a regex match for the getline function was too hard, so they went with gets(stdin) for C/C++.
– Stobor
2 days ago
3
@Stobor dang, you're right. I can confirm this happens for Java too, the lineSystem.out.print(FirstFactorial(s.nextLine()9));
prints89
even whens
is undefined.
– alter igel
2 days ago
add a comment |
I tried the following addition to main
in the Coderbyte editor:
std::cout << "gets(stdin)";
Where the mysterious and enigmatic snippet gets(stdin)
appears inside a string literal. This shouldn't possibly be transformed by anything, not even the preprocessor, and any C++ programmer should expect this code to print the exact string gets(stdin)
to the standard output. And yet we see the following output, when compiled and run on coderbyte:
8
Where the value 8
is taken straight from the convenient 'input' field under the editor.
From this, it's clear that this online editor is performing blind find-and-replace operations on the source code, substitution appearances of gets(stdin)
with the user's 'input'. I would personally call this a misuse of the language that's worse than careless preprocessor macros.
In the context of an online coding challenge website, I'm worried by this because it teaches unconventional, non-standard, meaningless, and at least unsafe practices like gets(stdin)
, and in a manner that can't be repeated on other platforms.
I'm sure it can't be this hard to just use std::cin
and just stream input to a program.
and it's not even a blind "find and replace" because sometimes it replaces it sometimes it does not.
– bolov
2 days ago
3
@bolov could it be just the first occurrence ofgets(stdin)
that is replaced? I meant 'blind' in the sense that it appears to be unaware of the language's syntax or grammar.
– alter igel
2 days ago
yes, you are right. It replaces the first occurence. I tried putting one before main and that's what I got indeed.
– bolov
2 days ago
1
Further research suggests that that site does it for all languages, not just C++ - python/ruby it uses the function call ("raw_input()" or "STDIN.gets") which would typically return a string from stdin, but ends up doing a string substitution of that string instead. I guess finding a regex match for the getline function was too hard, so they went with gets(stdin) for C/C++.
– Stobor
2 days ago
3
@Stobor dang, you're right. I can confirm this happens for Java too, the lineSystem.out.print(FirstFactorial(s.nextLine()9));
prints89
even whens
is undefined.
– alter igel
2 days ago
add a comment |
I tried the following addition to main
in the Coderbyte editor:
std::cout << "gets(stdin)";
Where the mysterious and enigmatic snippet gets(stdin)
appears inside a string literal. This shouldn't possibly be transformed by anything, not even the preprocessor, and any C++ programmer should expect this code to print the exact string gets(stdin)
to the standard output. And yet we see the following output, when compiled and run on coderbyte:
8
Where the value 8
is taken straight from the convenient 'input' field under the editor.
From this, it's clear that this online editor is performing blind find-and-replace operations on the source code, substitution appearances of gets(stdin)
with the user's 'input'. I would personally call this a misuse of the language that's worse than careless preprocessor macros.
In the context of an online coding challenge website, I'm worried by this because it teaches unconventional, non-standard, meaningless, and at least unsafe practices like gets(stdin)
, and in a manner that can't be repeated on other platforms.
I'm sure it can't be this hard to just use std::cin
and just stream input to a program.
I tried the following addition to main
in the Coderbyte editor:
std::cout << "gets(stdin)";
Where the mysterious and enigmatic snippet gets(stdin)
appears inside a string literal. This shouldn't possibly be transformed by anything, not even the preprocessor, and any C++ programmer should expect this code to print the exact string gets(stdin)
to the standard output. And yet we see the following output, when compiled and run on coderbyte:
8
Where the value 8
is taken straight from the convenient 'input' field under the editor.
From this, it's clear that this online editor is performing blind find-and-replace operations on the source code, substitution appearances of gets(stdin)
with the user's 'input'. I would personally call this a misuse of the language that's worse than careless preprocessor macros.
In the context of an online coding challenge website, I'm worried by this because it teaches unconventional, non-standard, meaningless, and at least unsafe practices like gets(stdin)
, and in a manner that can't be repeated on other platforms.
I'm sure it can't be this hard to just use std::cin
and just stream input to a program.
edited 15 hours ago
Peter Mortensen
13.8k1987113
13.8k1987113
answered 2 days ago
alter igelalter igel
3,27811230
3,27811230
and it's not even a blind "find and replace" because sometimes it replaces it sometimes it does not.
– bolov
2 days ago
3
@bolov could it be just the first occurrence ofgets(stdin)
that is replaced? I meant 'blind' in the sense that it appears to be unaware of the language's syntax or grammar.
– alter igel
2 days ago
yes, you are right. It replaces the first occurence. I tried putting one before main and that's what I got indeed.
– bolov
2 days ago
1
Further research suggests that that site does it for all languages, not just C++ - python/ruby it uses the function call ("raw_input()" or "STDIN.gets") which would typically return a string from stdin, but ends up doing a string substitution of that string instead. I guess finding a regex match for the getline function was too hard, so they went with gets(stdin) for C/C++.
– Stobor
2 days ago
3
@Stobor dang, you're right. I can confirm this happens for Java too, the lineSystem.out.print(FirstFactorial(s.nextLine()9));
prints89
even whens
is undefined.
– alter igel
2 days ago
add a comment |
and it's not even a blind "find and replace" because sometimes it replaces it sometimes it does not.
– bolov
2 days ago
3
@bolov could it be just the first occurrence ofgets(stdin)
that is replaced? I meant 'blind' in the sense that it appears to be unaware of the language's syntax or grammar.
– alter igel
2 days ago
yes, you are right. It replaces the first occurence. I tried putting one before main and that's what I got indeed.
– bolov
2 days ago
1
Further research suggests that that site does it for all languages, not just C++ - python/ruby it uses the function call ("raw_input()" or "STDIN.gets") which would typically return a string from stdin, but ends up doing a string substitution of that string instead. I guess finding a regex match for the getline function was too hard, so they went with gets(stdin) for C/C++.
– Stobor
2 days ago
3
@Stobor dang, you're right. I can confirm this happens for Java too, the lineSystem.out.print(FirstFactorial(s.nextLine()9));
prints89
even whens
is undefined.
– alter igel
2 days ago
and it's not even a blind "find and replace" because sometimes it replaces it sometimes it does not.
– bolov
2 days ago
and it's not even a blind "find and replace" because sometimes it replaces it sometimes it does not.
– bolov
2 days ago
3
3
@bolov could it be just the first occurrence of
gets(stdin)
that is replaced? I meant 'blind' in the sense that it appears to be unaware of the language's syntax or grammar.– alter igel
2 days ago
@bolov could it be just the first occurrence of
gets(stdin)
that is replaced? I meant 'blind' in the sense that it appears to be unaware of the language's syntax or grammar.– alter igel
2 days ago
yes, you are right. It replaces the first occurence. I tried putting one before main and that's what I got indeed.
– bolov
2 days ago
yes, you are right. It replaces the first occurence. I tried putting one before main and that's what I got indeed.
– bolov
2 days ago
1
1
Further research suggests that that site does it for all languages, not just C++ - python/ruby it uses the function call ("raw_input()" or "STDIN.gets") which would typically return a string from stdin, but ends up doing a string substitution of that string instead. I guess finding a regex match for the getline function was too hard, so they went with gets(stdin) for C/C++.
– Stobor
2 days ago
Further research suggests that that site does it for all languages, not just C++ - python/ruby it uses the function call ("raw_input()" or "STDIN.gets") which would typically return a string from stdin, but ends up doing a string substitution of that string instead. I guess finding a regex match for the getline function was too hard, so they went with gets(stdin) for C/C++.
– Stobor
2 days ago
3
3
@Stobor dang, you're right. I can confirm this happens for Java too, the line
System.out.print(FirstFactorial(s.nextLine()9));
prints 89
even when s
is undefined.– alter igel
2 days ago
@Stobor dang, you're right. I can confirm this happens for Java too, the line
System.out.print(FirstFactorial(s.nextLine()9));
prints 89
even when s
is undefined.– alter igel
2 days ago
add a comment |
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Note that
stdin
in the standard library is aFILE*
, and a pointer to any type converts tochar*
, which is the type of the argument ofgets()
. However, you should never, ever, ever write that kind of code outside an obfuscated C contest. If your compiler even accepts it, add more warning flags, and if you’re trying to fix a codebase that has that construct in it, turn warnings into errors.– Davislor
2 days ago
@Davislor no it doesn't "candidate function not viable: no known conversion from 'struct _IO_FILE *' to 'char *' for 1st argument"
– bolov
yesterday
2
@Davislor huh, that might be true for ancient C, but definitely not for C++.
– Quentin
yesterday
@Quentin Yeah. That shouldn’t compile. The intended challenge might’ve been, “Take this broken code, read my mind about what it’s supposed to do, and fix it,” but in that case there should be a real specification. With test cases.
– Davislor
yesterday
4
I’m surprised no-one tried this, but
gets(stdin )
(with an extra space) produces the expected C++ error.– Roman Odaisky
yesterday