tolower() Function not Working in C99

Question

I am using the CS50 appliance from Harvard and trying to make a character lowercase. I am trying to use the tolower() function but when I try to use it I get the message implicit declaration of function 'tolower' is invalid in C99. Anyone care to elaborate on why I would be getting this message. I have included stdio.h as well as string.h.

Answer 1

To use tolower in C99, use #include <ctype.h>

It is not an I/O function and it doesn't operate on strings (it operates on characters), so it's not in stdio or string.

Answer 2

tolower is defined in ctype.h. That is the file you should be including:

#include <ctype.h>

will solve your problem.