if (isalpha (*s++)) n++; meaning code example

Example 1: isalpha c

#include <stdio.h>
#include <ctype.h> //this is isalpha's library

int main(void)
{
  string variable0 = "a";
  if (isalpha(variable1))
  {
    printf("this will print");
  }
  
  string variable1 = 1;
  if (isalpha(variable1))
  {
    printf("this will NOT print");
  }
}

Example 2: isalpha c++

char test='a';
cout<<isalpha(test);