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

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");

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

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

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
}