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 );

Lowe

strchr() – Balkin

possible duplicate of String.indexOf function in C – Farina

Answer 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
}

Voltameter

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

You did not tell me what to #include – Chally

@turmuka then it will only return the first occurance – Sandler

Answer 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");

Tybi

Answer 3

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

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

Mangle

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

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

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)). – Inbreeding

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 ;) – Mangle

Answer 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;
}

Inbreeding

Answer 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.

Bluey

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

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. – Bluey

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

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. – Bluey

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

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

+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. – Sexism

Answer 6

You can use strcspn() to get the index of a char in a string, or use my lousy implementation:

// Returns the index of the first occurrence of a char
int string_indexof(char ch, char *everything) {
    int everythingLength = strlen(everything);
    for (int i = 0; i < everythingLength; i++) {
        if (ch == everything[i]) {
            return i;
        }
    }

    return -1;
}

Hexateuch

Answer 7

You can use strchr to get a pointer to the first occurrence and the subtract that (if not null) from the original char* to get the position.

Dermatome