4 ms·

That is one of the most confusing pieces of C code I have seen lately. And it fails on an empty string: I would expect it to set b to 1 for an empty string, but

That is one of the most confusing pieces of C code I have seen lately. And it fails on an empty string: I would expect it to set b to 1 for an empty string, but it sets it to 0. Of course that could easily be fixed by setting b to 1 at the top. Also, code like this should always be put inside a function that returns a value, not just written inline. Making it a function allows simpler and more understandable code too. The funniest part is that it is not necessary to call strlen() at all! The whole thing can be written in a single pass over the string. Here is how I would code it in C: int OnlyDigits( char str[] ) { for( int i = 0; str[i] != '\0'; ++i ) { if( str[i] < '0' || str[i] > '9' ) return 0; } return 1; } Try it here: https://replit.com/@geary/OnlyDigitsC https://replit.com/@geary/OnlyDigitsC

pvtmert 5y ago
You do realize it returns 1 for an empty string right? I mean it doesn't have any digits in it... What about adding a check of str[0] == 0 -> return 0 Also, giving char str[] will make it char* str. Which can be null. This may cause reading a random memory location (possibly segfault or use-after-free) edit: I get the comments but empty string still contains no digits. Given the regex would be ^[0-9]+ (+ instead of *) What I want to say is string has a numerical value or not. Empty string is NaN.
- zeven7 5y ago
  Empty string contains no non-digits. Therefore it is only digits.
- Someone 5y ago
  “contains only digits” isn’t synonymous with “not (doesn’t have any digits)” It is synonymous with “none of the characters is a non-digit”, which is true for the empty string.
- Stratoscope 5y ago
  > You do realize it returns 1 for an empty string right? I mean it doesn't have any digits in it... Yes, and that was deliberate on my part, as it meets my expectation of what such a function should do in this edge case. The problem statement was "Set boolean b to true if string s contains only characters in range '0'..'9', false otherwise." To my mind, the question "is every character in the string a digit" should be equivalent to "are there any non-digits in the string" (with the answer inverted, of course). Returning 0 (false) for the empty string makes those questions not equivalent. It makes the empty string a special case. Of course the real problem is that the problem is under-specified. It should call out specifically what should happen for an empty string, because as illustrated here, this is something where reasonable people may disagree. > Also giving char str[] will make it char* str. Which can be null. This may cause reading a random memory location (possibly segfault or use-after-free) Well yes, of course. The point of my comment wasn't to write bullet-proof library-ready code, it was only to illustrate two things: code like this should always go in a function, and the entire task can be accomplished in a single pass through the string. Thanks for keeping me on my toes!
- anonymoushn 5y ago
  Where did your choice of regex come from? The problem statement does not have any regex, and if you wanted to derive a regex from the problem statement you would probably need to get ^[0-9]*$ if you wanted to be correct.

pvtmert 5y ago

You do realize it returns 1 for an empty string right? I mean it doesn't have any digits in it... What about adding a check of str[0] == 0 -> return 0 Also, giving char str[] will make it char* str. Which can be null. This may cause reading a random memory location (possibly segfault or use-after-free) edit: I get the comments but empty string still contains no digits. Given the regex would be ^[0-9]+ (+ instead of *) What I want to say is string has a numerical value or not. Empty string is NaN.

zeven7 5y ago
Empty string contains no non-digits. Therefore it is only digits.
Someone 5y ago
“contains only digits” isn’t synonymous with “not (doesn’t have any digits)” It is synonymous with “none of the characters is a non-digit”, which is true for the empty string.
Stratoscope 5y ago
> You do realize it returns 1 for an empty string right? I mean it doesn't have any digits in it... Yes, and that was deliberate on my part, as it meets my expectation of what such a function should do in this edge case. The problem statement was "Set boolean b to true if string s contains only characters in range '0'..'9', false otherwise." To my mind, the question "is every character in the string a digit" should be equivalent to "are there any non-digits in the string" (with the answer inverted, of course). Returning 0 (false) for the empty string makes those questions not equivalent. It makes the empty string a special case. Of course the real problem is that the problem is under-specified. It should call out specifically what should happen for an empty string, because as illustrated here, this is something where reasonable people may disagree. > Also giving char str[] will make it char* str. Which can be null. This may cause reading a random memory location (possibly segfault or use-after-free) Well yes, of course. The point of my comment wasn't to write bullet-proof library-ready code, it was only to illustrate two things: code like this should always go in a function, and the entire task can be accomplished in a single pass through the string. Thanks for keeping me on my toes!
anonymoushn 5y ago
Where did your choice of regex come from? The problem statement does not have any regex, and if you wanted to derive a regex from the problem statement you would probably need to get ^[0-9]*$ if you wanted to be correct.