Determine if char is a num or letter

chars are just integers, so you can actually do a straight comparison of your character against literals:

if( c >= '0' && c <= '9' ){

This applies to all characters. See your ascii table.

ctype.h also provides functions to do this for you.

You'll want to use the isalpha() and isdigit() standard functions in <ctype.h>.

char c = 'a'; // or whatever

if (isalpha(c)) {
    puts("it's a letter");
} else if (isdigit(c)) {
    puts("it's a digit");
} else {
    puts("something else?");
}

Neither of these does anything useful. Use isalpha() or isdigit() from the standard library. They're in <ctype.h>.

<ctype.h> includes a range of functions for determining if a char represents a letter or a number, such as isalpha, isdigit and isalnum.

The reason why int a = (int)theChar won't do what you want is because a will simply hold the integer value that represents a specific character. For example the ASCII number for '9' is 57, and for 'a' it's 97.

Also for ASCII:

• Numeric - if (theChar >= '0' && theChar <= '9')
• Alphabetic -
  if (theChar >= 'A' && theChar <= 'Z' || theChar >= 'a' && theChar <= 'z')

Take a look at an ASCII table to see for yourself.