how to use isDigit?

[khaichiew85]

im having a program.I want it to check if it is a character or integer is entered
where shall i put the isDigit and how to use?i put it as highlighted below but got error.

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

int is_prime( int input, int z);

int main (void)
{
int input;
char on;
char option;

printf("Press <enter> to continue\n");
scanf("%c",&on);
system("cls");
fflush(stdin);

//if(isdigit(input))

printf("Please enter a number:");
scanf ("%d",&input);

fflush(stdin);

if( is_prime(input,2))
printf("\nThis number is PRIME\nThank you.\a\a\n\n");
else
printf("\nThis number is NOT PRIME\nThank you.\a\a\n\n");

do{
printf("Do you still want to continue?(Press 'Y' for YES or 'N' for No)\n");
scanf ("%c",&option);

if(option=='Y' || option=='y')
{
system("cls");
return main();
}
else if(option=='N' || option=='n')
return 0;
else
printf("Incorrect input.The program will quit now.\n\n");
}while(option=='Y' || option=='y' || option=='n' || option=='N');
}

int is_prime( int input,int n)
{
if( input < 2 )
return 0;

else if( input == 2 )
return 1;

else
if( !(input % n ))
return 0;
else if(n < input-1)
return is_prime(input, n+1);
else
return 1;
}

[khaichiew85]

Problem solved.
if(!(isdigit(input)))
fflush(stdin);
if( is_prime(input,2))
printf("\nThis number is PRIME\nThank you.\a\a\n\n");
else
printf("\nThis number is NOT PRIME\nThank you.\a\a\n\n");

do{
printf("Do you still want to continue?(Press 'Y' for YES or 'N' for No)\n");
scanf ("%c",&option);

fflush(stdin);

if(option=='Y' || option=='y')
{
system("cls");
return main();
}
else if(option=='N' || option=='n')
return 0;
else
{
printf("Incorrect input.The program will quit now.\n\n");
return 0;
}
}while(option=='Y' || option=='y' || option=='n' || option=='N');