Determine if char is a num or letter

Question

How do I determine if a char in C such as a or 9 is a number or a letter?

Is it better to use:

int a = Asc(theChar);

or this?

int a = (int)theChar

Answer 1

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?");
}

Answer 2

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.

Answer 3

<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.

Answer 4

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

Answer 5

If (theChar >= '0' && theChar <='9') it's a digit. You get the idea.

Answer 6

You can normally check for letters or numbers using simple conditions

if((ch>='a' && ch<='z') || (ch>='A' && ch<='Z))
{
    /*This is an alphabet*/
}

For digits you can use

if(ch>='0' && ch<='9')
{
    /*It is a digit*/
}

But since characters in C are internally treated as ASCII values you can also use ASCII values to check the same.

How to check if a character is number or letter

Answer 7

C99 standard on c >= '0' && c <= '9'

c >= '0' && c <= '9' (mentioned in another answer) works because C99 N1256 standard draft 5.2.1 "Character sets" says:

In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous.

ASCII is not guaranteed however.