#include <unistd.h>
void    triplet(void);

int     main(void)
{
        triplet();
}

void    triplet(void)
{
        char    i, j, k;

        i = '0';
        j = '1';
        k = '2';
        while (i < j)
        {
                j = i + 1;
                k = j + 1;
                while (j < k)
                {
                        k = j + 1;
                        while (k <= '9')
                        {
                                write(1, &i, 1);
                                write(1, &j, 1);
                                write(1, &k, 1);
                                if ( ( i != '7' ) && ( j != '8' ) && ( k != '9' ) )
                                {
                                        write(1, ", ", 2);
                                }
                                k++;
                        }
                        j++;
                }
                i++;
        }
}

The goal of the if statement is to control when to print the comma-space separator (", ") between the digit combinations. I want to print the separator after every combination except the last one (“789”).

The following conditional statement does not work as intended:

if ( ( i != '7' ) && ( j != '8' ) && ( k != '9' ) )

This one does:

if ( !( i == '7' && j == '8' && k == '9' ) )

What is wrong with the first conditional?