C ctype.h isdigit() Function

isdigit() Function

The isdigit() function in C checks whether a given character is a decimal digit. This function is useful when validating input or parsing strings where numeric characters are expected. It determines if a character represents one of the digits 0 through 9.

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Syntax of isdigit()

int isdigit(int c);

Parameters

Parameter	Description
c	An integer representing the character to be checked, typically cast from a char or EOF.

The isdigit() function examines if the character falls within the range of the decimal digits, i.e., from ‘0’ to ‘9’.

Return Value

The function returns a nonzero value (true) if the character is a decimal digit; otherwise, it returns zero (false).

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Examples for isdigit()

Example 1: Checking a Single Digit Character

This example demonstrates how to use isdigit() to check if a single character is a decimal digit.

Program

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

int main() {
    int ch = '5';

    if (isdigit(ch)) {
        printf("The character %c is a digit.\n", ch);
    } else {
        printf("The character %c is not a digit.\n", ch);
    }

    return 0;
}

Explanation:

1. A character variable is assigned the value '5'.
2. The isdigit() function checks whether '5' is a decimal digit.
3. Since '5' is within the range of 0 to 9, the condition is true and the program prints the appropriate message.

Program Output:

The character 5 is a digit.

Example 2: Testing a Non-Digit Character

This example illustrates how isdigit() behaves when the character checked is not a decimal digit.

Program

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

int main() {
    int ch = 'A';

    if (isdigit(ch)) {
        printf("The character %c is a digit.\n", ch);
    } else {
        printf("The character %c is not a digit.\n", ch);
    }

    return 0;
}

Explanation:

1. A character variable is assigned the value 'A'.
2. The isdigit() function checks whether 'A' is a decimal digit.
3. Since 'A' is not a digit, the function returns false and the program prints that the character is not a digit.

Program Output:

The character A is not a digit.

Example 3: Iterating Over a String to Identify Digits

This example demonstrates scanning through a string and using isdigit() to determine which characters are decimal digits.

Program

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

int main() {
    char str[] = "Room 101";
    int i = 0;

    while (str[i] != '\0') {
        if (isdigit(str[i])) {
            printf("Character '%c' is a digit.\n", str[i]);
        } else {
            printf("Character '%c' is not a digit.\n", str[i]);
        }
        i++;
    }

    return 0;
}

Explanation:

1. A string "Room 101" is defined.
2. The program iterates through each character of the string.
3. The isdigit() function is used to check if a character is a decimal digit.
4. A message is printed for each character indicating whether it is a digit or not.

Program Output:

Character 'R' is not a digit.
Character 'o' is not a digit.
Character 'o' is not a digit.
Character 'm' is not a digit.
Character ' ' is not a digit.
Character '1' is a digit.
Character '0' is a digit.
Character '1' is a digit.