Is there a function in c that will return the index of a char in a char array?

Question

Is there a function in c that will return the index of a char in a char array?

For example something like:

char values[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
char find = 'E';

int index = findIndexOf( values, find );

Solution 1

strchr returns the pointer to the first occurrence, so to find the index, just take the offset with the starting pointer. For example:

char values[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
char find = 'E';

const char *ptr = strchr(values, find);
if(ptr) {
   int index = ptr - values;
   // do something
}

Solution 2

There's also size_t strcspn(const char *str, const char *set); it returns the index of the first occurence of the character in s that is included in set:

size_t index = strcspn(values, "E");

Solution 3

int index = strchr(values,find)-values;

Note, that if there's no find found, then strchr returns NULL, so index will be negative.

Solution 4

Safe index_of() function that works even when it finds nothing (returns -1 in such case).

#include <stddef.h>
#include <string.h>
ptrdiff_t index_of(const char *string, char search) {
    const char *moved_string = strchr(string, search);
    /* If not null, return the difference. */
    if (moved_string) {
        return moved_string - string;
    }
    /* Character not found. */
    return -1;
}

Solution 5

What about strpos?

#include <string.h>

int index;
...
index = strpos(values, find);

Note that strpos expects a zero-terminated string, which means you should add a '\0' at the end. If you can't do that, you're left with a manual loop and search.

View more solutions

Comments

[chaos]

strchr()

[Ciro Santilli OurBigBook.com]

possible duplicate of String.indexOf function in C

[Ed S.]

I don't think that exists in the C standard library, though I could be wrong.

[Lasse V. Karlsen]

Personally I have more confidence in the strchr answers, I'd really like one of those be marked as the answer instead. I only found one really old piece of source code using that function, and I don't know how standard it is when I consider it.

[jesup]

strpos() is not a standard C (or even C++) function. Quite common in PHP, etc...

[Lasse V. Karlsen]

Odd, but I do have a piece of C code that uses it. But as I said, I don't know how standard it is.

[Martin Beckett]

strpos isn't part of the C std lib - but it is probably present on most systems.

[Josh Curren]

after looking through some other code I have found some c that uses strpos()

[Konrad Borowski]

Pointer arithmetic on NULL is undefined behavior. It doesn't have to return any particular value.

[Michael Krelin - hacker]

That's true, it doesn't have to, but it does usually :). My point was that one should expect NULL pointer here.

[Konrad Borowski]

Just because it appears to work for you, it doesn't mean that's right. Your code may randomly fail when pointer is high enough (I actually checked that, see gist.github.com/GlitchMr/8056175 (should be an infinite loop, if this would work correctly)).

[Michael Krelin - hacker]

I didn't say it's right. And yes, that's very correct what you point out, but it is only because the behaviour is de facto defined you could do that ;)

[Jumabek Alikhanov]

+1 for reminding that we can do the manual loop. Cuz in my case i need something like lfind not the rfind which strchr will return.

[turmuka]

my question, how do I find indexes in a char. If I had 2 E s in the string?

[ctrl-alt-delor]

You did not tell me what to #include

[Ratul Hasan]

@turmuka then it will only return the first occurance