C library function isdigit() usage and example

C Standard Library <ctype.h>

isdigit() function checks if a character is a digit character (0-9）。

isdigit() function prototype

int isdigit(int arg);

The function isdigit() accepts a single parameter in integer form and returns a value of type int.

Even if isdigit() takes an integer as a parameter, the character is passed to the function. Internally, the character is converted to its ASCII value for checking.

It is<ctype.h>Defined in header file.

C isdigit() return value

Example: C isdigit() function

#include <stdio.h>
#include <ctype.h>
int main()
{
　　　　char c;
　　　　c='5';
　　　　printf("Result when passing a digit character: %d", isdigit(c));
　　　　c='+';
　　　　printf("\nResult when passing a non-digit character: %d", isdigit(c));
　　　　return 0;
}

Output Result

Result when passing a digit character:　1
Result when passing a non-digit character: 0

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

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

Output Result

Enter a character:　8
8It is a number.

C Standard Library <ctype.h>