isalpha() function in C

The isalpha() function is a library function declared in the <ctype.h> header. It is used to determine whether a given character is an alphabetic character, which means it is a letter of the alphabet (either uppercase or lowercase).

Here’s the function signature:

int isalpha(int c);

• c: The integer representation of the character to be tested.

The function returns a non-zero (true) value if the character is an alphabetic character (a letter), and it returns 0 (false) if it’s not.

Here’s an example of how to use isalpha():

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

int main() {
    char ch1 = 'A';
    char ch2 = '7';
    char ch3 = '$';

    if (isalpha(ch1)) {
        printf("%c is an alphabetic character.\n", ch1);
    } else {
        printf("%c is not an alphabetic character.\n", ch1);
    }

    if (isalpha(ch2)) {
        printf("%c is an alphabetic character.\n", ch2);
    } else {
        printf("%c is not an alphabetic character.\n", ch2);
    }

    if (isalpha(ch3)) {
        printf("%c is an alphabetic character.\n", ch3);
    } else {
        printf("%c is not an alphabetic character.\n", ch3);
    }

    return 0;
}

In this example, isalpha() is used to check whether three characters (ch1, ch2, and ch3) are alphabetic characters. The function returns true for 'A' since it’s an alphabetic character, and it returns false for '7' and '$' since they are not alphabetic characters.

Output:

A is an alphabetic character.
7 is not an alphabetic character.
$ is not an alphabetic character.

You can use isalpha() to validate and process input where you expect alphabetic characters or to filter out characters that don’t represent letters.