C Library Function isalpha() Usage and Example

C Standard Library <ctype.h>

The isalpha() function checks if a character is an alphabet.

In C programming, the isalpha() function checks if a character is an alphabet (a-z and A-Z).

If the character passed to isalpha() is an alphabet, it returns a non-zero integer, otherwise it returns 0.

The isalpha() function is in<ctype.h>Defined in header files.

C isalpha() Function Prototype

int isalpha(int argument);

The function isalpha() accepts a single parameter in integer form and returns an integer value.

Even if isalpha() is passed an integer as a parameter, the character is passed to the isalpha() function.

Internally, the character is converted to its corresponding integer value based on its ASCII value when passed.

isalpha() Return Value

Example: C Language isalpha() Function

#include <stdio.h>
#include <ctype.h>
int main()
{
　　　　char c;
　　　　c = 'Q';
　　　　printf("\nResult when passing an uppercase letter: %d", isalpha(c));
　　　　c = 'q';
　　　　printf("\nResult when passing a lowercase letter: %d", isalpha(c));
　　　　c='+;
　　　　printf("\nResult when passing a non-alphabetic character: %d", isalpha(c));
　　　　return 0;
}

Output Result

Result when passing an uppercase letter:　1
Result when passing a lowercase letter:　2
Result when passing a non-alphabetic character: 0

Note:When an alphabetic character is passed to the isalpha() on the system, you get a different non-zero integer. However, when you pass a non-alphabetic character to isalpha(), it always returns 0.

Example: C program to check if a user input character is an alphabet

#include <stdio.h>
#include <ctype.h>
int main()
{
　　　　char c;
　　　　printf("Enter a character: ");
　　　　scanf("%c", &c);
　　　　if (isalpha(c) == 0)
　　　　　　　　　printf("%c is not a letter.", c);
　　　　else
　　　　　　　　　printf("%c is a letter.", c);
　　　　return 0;
}

Output Result

Enter a character:　5
5　Not a letter.

C Standard Library <ctype.h>