There are a few different ways to check what the user has entered; if it's a character or a number.
1. Use the isalpha() function in the ctype.h system header file
Note that the isalpha() function checks the input is in the alphabet A-Za-z. Therefore non-alphabetic characters such as + will return false.
2. Use the isdigit() function in the ctype.h system header file
To bypass how isalpha() checks the input is in the alphabet meaning characters such as + return false, why not check it's not a digit (0-9) and if returns false, it means it's a character that's either in alphabet or non-alphabetic.
3. Validate the ASCII value is within a character range