C Library - isdigit() function

The C ctype library isdigit() function checks if the passed character is a decimal digit character. This function is useful in various scenarios such as parsing numerical input from a user or processing data where digit validation is required.

Decimal digits are (numbers) 0 1 2 3 4 5 6 7 8 9.

Syntax

Following is the C library syntax of the isdigit() function −

int isdigit(int c);

Parameters

This function accepts a single parameter −

• c − This is the character to be checked, passed as an integer.This is an ASCII value of a character.

Return Value

Following is the return value −

• The function returns a non-zero value (true) if the character c is a decimal digit (i.e., '0' through '9').
• It returns 0 (false) if the character c is not a decimal digit.

Example 1: Simple Digit Check

The main function checks if the digits/character passed to it is a digit/ non-digit.

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

int main() {
   char ch = '5';
   if (isdigit(ch)) {
      printf("'%c' is a digit.\n", ch);
   } else {
      printf("'%c' is not a digit.\n", ch);
   }
   return 0;
}

Output

The above code produces following result−

'5' is a digit.

Example 2: Digit Check in a String

The below code demonstrates the use of isdigit in processing strings and identifying digits within them.

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

int main() {
   char str[] = "Hello123";
   for (int i = 0; str[i] != '\0'; i++) {
      if (isdigit(str[i])) {
         printf("'%c' is a digit.\n", str[i]);
      } else {
         printf("'%c' is not a digit.\n", str[i]);
      }
   }
   return 0;
}

Output

After execution of above code, we get the following result

'H' is not a digit.
'e' is not a digit.
'l' is not a digit.
'l' is not a digit.
'o' is not a digit.
'1' is a digit.
'2' is a digit.
'3' is a digit.