void condense_by_removing(char* s, char c) { char* d = s; while (*d = *s++) d += *d != c; } It always does the copy, then evaluates *d!=c. I

void condense_by_removing(char* s, char c) { char* d = s; while (*d = *s++) d += *d != c; } It always does the copy, then evaluates *d!=c. If what it just copied is equal to the forbidden character this evaluates as false, which in C is 0, and hence d is left unchanged. If what it copied is not equal then this evaluates as true, which is 1, and hence d is incremented.