isdigit() function in C

The isdigit() function is part of the C Standard Library and is used to determine if a given character is a decimal digit or not. It checks whether a character represents one of the decimal digits from ‘0’ to ‘9’. The function returns a non-zero value (true) if the character is a digit, and it returns 0 (false) otherwise. The isdigit() function is defined in the ctype.h header.

Here’s the prototype of the isdigit() function:

#include <ctype.h>

int isdigit(int c);

• c: An integer that represents a character (or the result of getc() or fgetc()).

The isdigit() function is useful for checking whether a character is a numeric digit before attempting to perform numeric operations on it. Here’s an example of how to use it:

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

int main() {
    char ch;

    printf("Enter a character: ");
    scanf(" %c", &ch);  // Note the space before %c to skip leading whitespace

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

    return 0;
}

In this example:

• We use scanf to read a character from the user, and the character is stored in the variable ch.
• We then use the isdigit() function to check whether the entered character is a decimal digit.
• Depending on the result, we print a message indicating whether the character is a decimal digit or not.

This function is particularly helpful when you need to validate or filter input to ensure that it consists of numeric digits.