How to check a character value in C

When working in C, we can use the ctype.h standard library set of functions to check the value of a char type variable. You include the header, call the function you need, and it tells you if the character is a letter, a digit, whitespace, and so on.

We have access to several useful checks:

• isalnum() checks if a character is alphanumeric
• isalpha() checks if a character is alphabetic
• iscntrl() checks if a character is a control character
• isdigit() checks if a character is a digit
• isgraph() checks if a character is a printable ASCII character (but not a space)
• islower() checks if a character is lowercase
• isprint() checks if a character is a printable ASCII character
• ispunct() checks if a character is a punctuation character (a printable char, not a space, not alphanumeric)
• isspace() checks if a character is a whitespace character (see more later)
• isupper() checks if a character is uppercase
• isxdigit() checks if a character is an hexadecimal digit (0-F)

How to use them

Each function takes the character and returns a nonzero value when the check passes, and 0 when it fails. That makes them perfect inside an if:

#include <stdio.h>
#include <ctype.h>

int main(void) {
  char c = 'a';

  if (isalpha(c)) {
    printf("%c is a letter\n", c);
  }

  if (islower(c)) {
    printf("%c is lowercase\n", c);
  }

  return 0;
}

Running this prints:

a is a letter
a is lowercase

Notice that “nonzero” does not mean 1. Never compare the result to 1 directly, write if (isalpha(c)) instead of if (isalpha(c) == 1).

A common use case is validating user input. Here we check if a character is a digit before converting it to a number:

#include <stdio.h>
#include <ctype.h>

int main(void) {
  char c = '7';

  if (isdigit(c)) {
    int n = c - '0';
    printf("the digit is %d\n", n);
  }

  return 0;
}

This prints the digit is 7. Subtracting '0' works because digit characters are consecutive in the ASCII table.

What counts as whitespace?

I mentioned that isspace() checks if a character is a whitespace character. What is a whitespace character?

• Horizontal tab (HT), '\t', character 9 of the ASCII table
• Vertical tab (VT), '\v', character 11 of the ASCII table
• Form Feed (FF), '\f', character 12 of the ASCII table
• Carriage Return (CR), '\r', character 13 of the ASCII table
• Space, ' ', character 32 of the ASCII table
• New line, '\n'

One thing to be careful with

These functions take an int, and the value must be representable as an unsigned char (or be EOF). Passing a negative value is undefined behavior.

This bites you when a plain char holds a byte outside the ASCII range, like accented characters, because on many platforms char is signed. The fix is a cast before the call:

if (isalpha((unsigned char)c)) {
  //...
}

For plain ASCII input you won’t notice the difference, but the cast makes the code correct for any byte.