Does your filename or path contain a space? If so, make sure to pass it inside double quotes...

you can do this:

char temp[10];
int num = 3;
sprintf( temp, "String%d", num);

gebbit, you can also look into sprintf, which gives you a lot of options.

OK, but why the for loop? You only want the user input once correct?

A couple of things:

First, you need to include "stdlib.h" for atoi. (Note, I think atoi is not standard, consider sprintf)

Second, you are trying to get the length of an integer (...