How to use isdigit() for a pin while loop
<p>This program accepts any four digit number as the pin , so i used a <code>strlen()</code> to find out whether the pin is has four characters, but I need to make sure that user enters four numbers, how do I use <code>isdigit()</code> before the loop and for the loop condition?</p>

<pre><code>#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;
#include &lt;ctype.h&gt;

char pin [4];

printf("Please enter your pin :    \n") ;
scanf("%4s" , &amp;pin) ;
system("cls") ;

while (strlen(pin) != 4) {
    count = count + 1 ;
    if(count &lt; 3){
        printf("Invalid Pin \n") ;
        printf("Please enter your pin again \n") ;
        scanf("%4s", &amp;pin) ;
        system("cls");
    }
    else if (count == 3){
        printf("Sorry you can't continue , Please contact your bank for assistance !") ;
        return 0 ;
    }
}
</code></pre>