isdigit() function in C

This topic will discuss the isdigit() function in the C language. The isdigit() function is a predefined function of the C library, which is used to check whether the character is a numeric character from 0 to 9 or not. And if the given character is a numeric number or digit, it returns a true Boolean value or non-zero; else, it returns zero or false value. The isdigit function is declared inside the ctype.h header file, so we must add to the C program.

For example, suppose we enter character 'h' to the isdigit() function; the function checks whether the input character is a digit or not. Here the character is not a digit. So the isdigit() function returns zero (0) or false value. Similarly, we again enter numeric character '5' to the isdigit() function. This time, the function returns a true or non-zero value because '5' is a numeric characte from 0 to 9 digits.

Syntax of the isdigit() function

Following is the syntax of the isdigit() function in the C language.

Parameters:

Ch - It defines the numeric character to be passed in the isdigit() function.

Return Value:

The isdigit() function checks the 'ch' argument, and if the passed character is a digit, it returns a non-zero value. Otherwise, it shows a zero or false Boolean value.

Example 1: Program to check whether the given character is digit or not

Let's create a program to check the given characters are digit or not using the isdigit() function in the C programming.

/* Check whether the given characters are digits or not in C. */
#include <stdio.h>
#include <ctype.h> 

int main ()
{

	// Use the isdigit() function to check the character is digit or not. */
	if (isdigit ( 'P' ) == 0)  //check 'P' is digit 
	{
		printf (" \n The given character 'P' is not digit'. ");
	}
	else
	{
		printf ("\n The given character 'P' is a digit. ");
	}
	
	
	if (isdigit ( '3' ) == 0)  //check for '3'  
	{
		printf (" \n The given character '3' is not digit'. ");
	}
	else
	{
		printf ("\n The given character '3' is a digit. ");
	}

	
	if (isdigit ( '!' ) == 0)  //check for '!' character 
	{
		printf (" \n The given character '!' is not digit'. ");
	}
	else
	{
		printf ("\n The given character '!' is a digit. ");
	}


	if (isdigit ( '$4' ) == 0)  //check for '44'  
	{
		printf (" \n The given character '$4' is not digit'. ");
	}
	else
	{
		printf ("\n The given character '$4' is a digit. ");
	}
	
	
	if (isdigit ( '0' ) == 0)  //check for '0' 
	{
		printf (" \n The given character '0' is not a digit. ");
	}
	else
	{
		printf ("\n The given character '0' is a digit. ");
	}
	
	return 0;
}

Output:

The given character 'P' is not a digit.
The given character '3' is a digit.
The given character '!' is not a digit.
The given character '$4' is not a digit.
The given character '0' is not a digit.

In the above program, we defined different characters like 'P', '3', '!', '$4', 0 to check whether these are valid characters or not using the isdigit() function. And then, we use the isdigit() function that checks and returns the characters 'P', '1', $4 are not digits.

Example 2: Program to check whether the character entered by the user is a digit or not

Let's write a program to find whether the input character is valid or not using the isdigit() function in C.

/* Check whether the given characters are digits or not in C. */
#include <stdio.h>
#include <ctype.h> 

int main ()
{

	char n; // declare an integer type variable n
	
	printf (" Enter a number to check for valid digits: ");
	scanf ("%c", &n);
	
	// use the isdigit() function to check the digit
	if (isdigit (n))                                              
	{
		printf (" \n It is a digit. ");
	}
	else
	{
		printf (" \n It is not a digit. ");
	}
	return 0;
} 

Output:

Enter a number to check for valid digits: 5
It is a digit.

In the above program, we input the '5' character from the user and then use the isdigit function to check whether the passed argument is a digit. Here, the passed character is a digit, so the isdigit() function returns the statement "It is a digit."

2^nd execution

Enter a number to check for valid digits: A
It is not a digit.

Similarly, when we enter character 'A' to the isdigit() function, the function checks for the valid character, and we can see character 'A' is not a digit. So, the function returns the statement 'It is not a digit.'

Example 3: Program to print zero and non-zero numbers for passed character in C

Let's write a program to check all given characters and returns zeroes and non-zeroes values based on the passed characters to the isdigit() function in C.

/* create a simple program to return zeroes and non-zeroes values based on the given characters. */
#include <stdio.h>
#include <ctype.h>

int main ()
{
	char num; // declare num as the character type variable
	
	
	num = '0';
	// check character 0 using the isdigit() function
	printf (" \n The isdigit() function returns result based on character (0) passed: %d ", isdigit (num));
	
	num = 'o';
	// check character o using the isdigit() function
	printf (" \n The isdigit() function returns result based on character (o) passed: %d ", isdigit (num));
	
	num = '08';
	// check character 08 using the isdigit() function
	printf (" \n The isdigit() function returns result based on character (08) passed: %d ", isdigit (num));
	
	num = '+';
	// check character + using the isdigit() function
	printf (" \n The isdigit() function returns result based on character (+) passed: %d ", isdigit (num));
	
	num = '%';
	// check character % using the isdigit() function
	printf (" \n The isdigit() function returns result based on character ('%') passed: %d ", isdigit (num));
		
	return 0;
} 

Output:

The isdigit() function returns result based on character (0) passed: 1 
The isdigit() function returns result based on character (o) passed: 0
The isdigit() function returns result based on character (08) passed: 1
The isdigit() function returns result based on character (+) passed: 0
The isdigit() function returns result based on character (%) passed: 0

In the above program, we tests for valid given digit characters using the isdigit() function. The isdigit() function returns 1 to the numeric characters and 0 to the alphabetic or special symbols defined in the program that are not digits.

Example 4: Program to check all the array elements using the isdigit() function

Let's write a program to find all the valid digits of the array element using the isdigit() function in C.

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

int main ()
{
	int i;
	// declare and initialize the character type array
	char arr[8] = {'E', '08', '@', '-', '3', '/', '007', '$'};
	
	int arr2[8] = {0, 0, 0, 0, 0, 0, 0, 0}; // initialize arr2 with containing all zeroes element
	
	// use for loop to iterate all elements
	for (i = 0; i < 8; i++)
	{
		// check and assign digits to the arr2[i] using the isdigit() function
		arr2[i] = isdigit (arr[i]); 
	}
	
	for (i = 0; i < 8; i++)
	{
	// display array characters
	printf (" \n '%c'", arr[i]);
	
	// check array elements
	if (arr2[i] != 0)
	{
		printf (" is a digit character. ");
	}
	else
	{
		printf (" is not a valid digit character. ");
	}
}
return 0;
}

Output:

'E' is not a valid digit character.
'8' is a digit character.
'@' is not a valid digit character.
'-' is not a valid digit character.
'3' is a digit character.
'/' is not a valid digit character.
'7' is a digit character.
'$' is not a valid digit character.

We declared and initialized the arr[] variable with some elements in the above program. And then, we create another array arr2[] that contains zero (0) to assign the result to those elements that are not valid digits. The isdigit() function checks all the arr[] array elements and returns non-zero values to the valid digit elements. Else it returns zeroes (0) to non-digit elements.

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