Implementation of strspn( )

Question

The definition of library function strspn is:

size_t strspn(const char *str, const char *chars)

/* Return number of leading characters at the beginning of the string `str`
   which are all members of string `chars`. */

e.g. if str is "fecxdy" and chars is "abcdef" then the function would return 3, since f, e and c all appear somewhere in chars, giving 3 leading characters of str, and x is the first character of str which is not a member of chars.

Could someone help me write an implementation of strspn in C. The only library function I am allowed to call from the implementation is strlen?

Answer 1

The basic idea is to step through the string, one character at a time, and test if it's in the character set. If it's not, stop and return the answer. In pseudocode, that would look like:

count = 0
for each character c in str
    if c is not in chars
        break
    count++
return count

The if c is not in chars test can be implemented by iterating through all of the characters of chars and testing if c matches any of the characters. Note that this is not the fastest implementation, since it involves stepping through the chars string for each character in str. A faster implementation would use a lookup table to test if c is not in chars.