[c] How do I find the index of a character within a string in C?

Suppose I have a string "qwerty" and I wish to find the index position of the e character in it. (In this case the index would be 2)

How do I do it in C?

I found the strchr function but it returns a pointer to a character and not the index.

void myFunc(char* str, char c)
{
    char* ptr;
    int index;

    ptr = strchr(str, c);
    if (ptr == NULL)
    {
        printf("Character not found\n");
        return;
    }

    index = ptr - str;

    printf("The index is %d\n", index);
    ASSERT(str[index] == c);  // Verify that the character at index is the one we want.
}

This code is currently untested, but it demonstrates the proper concept.

You can also use strcspn(string, "e") but this may be much slower since it's able to handle searching for multiple possible characters. Using strchr and subtracting the pointer is the best way.

What about:

char *string = "qwerty";
char *e = string;
int idx = 0;
while (*e++ != 'e') idx++;

copying to e to preserve the original string, I suppose if you don't care you could just operate over *string

This should do it:

//Returns the index of the first occurence of char c in char* string. If not found -1 is returned.
int get_index(char* string, char c) {
    char *e = strchr(string, c);
    if (e == NULL) {
        return -1;
    }
    return (int)(e - string);
}