Checking the first letter of a string in c

Question

I am writing a very simple function in C to check if a string is an absolute path or relative path. No matter what I try it is always returning false.

Here is what I have tried:

int isAbsolute(char *str){
    if(strcmp(str,"/")){
        return 1;
    }
    return 0;
}

and I call it like:

printf("%d\n", isAbsolute("/"));

which is returning false every time. Clearly I am missing something obvious but I haven't been able to figure it out...

`strcmp` compares the whole string, so it will only return true if the string you're passing is "/". You can look at `strncmp` instead, or compare only one character (`if (str[0] == '/')`), instead of a string. — AntonH, Sep 06 '17 at 16:41
@alk I was only going to comment why it wasn't working, but edited a solution in after. Put it as an answer now ... — AntonH, Sep 06 '17 at 16:46
Ah wow ok thank you. I had tried this as well but I was using "/" instead of '/'. Thank you. Not sure why when I use "/" it tries to compare str[0] as a pointer and "/" as an int? — CodySig, Sep 06 '17 at 16:59

Billy Ferguson · Accepted Answer · 2017-09-06T16:52:29.927

6

Don't have access to a compiler, but I think this will work because C-style strings are just arrays with a termination character:

int isAbsolute(const char *str){
    return (str[0] == '/');
}

edited Sep 06 '17 at 16:52

answered Sep 06 '17 at 16:45

Billy Ferguson

1,429
11
23

2

You might want to use `const char * str`. – alk Sep 06 '17 at 16:48
Don't know what OS/shell you're using, but `cd /../` leaves me in the root directory, not in the parent of the cwd. – pat Sep 06 '17 at 16:51

score 3 · Answer 2 · answered Sep 06 '17 at 16:47

As was pointed out, strcmp only matches if the strings being compared are of the same length.

To compare a single character at the front of the string, you can just do:

int isAbsolute(const char *str) {
  return (str[0] == '/');
}

If the prefix you are looking for is longer than one character, then this might help. I like Fred Foo's answer better than the one that got accepted (as did a majority of voters).

AntonH · Answer 3 · 2017-09-11T20:12:28.530

3

strcmp compares the whole string, so your function will only return true if the string you're passing is "/".

You can look at strncmp instead:

if(strncmp(str,"/", 1)) ...

or compare only one character:

(if (str[0] == '/')) ...

edited Sep 11 '17 at 20:12

answered Sep 06 '17 at 16:48

AntonH

6,359
2
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Henno Brandsma · Answer 4 · 2017-09-07T04:15:03.707

2

Similarly to strncmp you can use memcmp which has the number of bytes to compare as an argument:

int isAbsolute(const char *str){
    if (0 == memcmp(str, "/", 1){
         return 1;
    } else {
         return 0;
    }
}

Don't forget that return value $0$ means equality. In your code you return 0 in that case which is probably not as you intended.

edited Sep 07 '17 at 04:15

answered Sep 06 '17 at 18:01

Henno Brandsma

2,116
11
12

score 0 · Answer 5 · answered Sep 06 '17 at 19:33

0

Strcmp return value is zero on success case ,that's why it is not going to true

answered Sep 06 '17 at 19:33

rohit

39
1
8

Checking the first letter of a string in c

5 Answers5