从strchr获取int而不是指针

Question

如何获取字符串中字符首次出现的索引作为int而不是指向其位置的指针？

Answer 1

If you have two pointers to an array in C, you can simply do:

index = later_pointer - base_address;

where base_address is the array itself.

For example:

#include <stdio.h>
int main (void) {
    int xyzzy[] = {3,1,4,1,5,9};       // Dummy array for testing.

    int *addrOf4 = &(xyzzy[2]);        // Emulate strchr-type operation.

    int index = addrOf4 - xyzzy;       // Figure out and print index.
    printf ("Index is %d\n", index);   //   Or use ptrdiff_t (see footnote a).

    return 0;
}

which outputs:

Index is 2

As you can see, it scales correctly to give you the index regardless of the underlying type (not that it matters for char but it's useful to know in the general case).

So, for your specific case, if your string is mystring and the return value from strchr is chpos , just use chpos - mystring to get the index (assuming you found the character of course, ie, chpos != NULL ).

---

^(a) As rightly pointed out in a comment, the type of a pointer subtraction is ptrdiff_t which, may have a different range to int . To be perfectly correct, the calculation and printing of the index would be better done as:

    ptrdiff_t index = addrOf4 - xyzzy;       // Figure out and print index.
    printf ("Index is %td\n", index);

Note that this will only become an issue if your arrays are large enough that the difference won't fit in an int . That's possible since the ranges of the two types are not directly related so, if you value portable code highly, you should use the ptrdiff_t variant.

Answer 2

Use pointer arithmetic:

char * pos = strchr( str, c );
int npos = (pos == NULL) ? -1 : (pos - str);

Answer 3

if you're dealing with std::string and not plain c-strings then you can use std::string::find_first_of

http://www.cplusplus.com/reference/string/string/find_first_of/