Please help me to develop algorithm

Applications Programming
1

Hi guys , in my study book from which I re-learn C is task to generate all possible characters combination from numbers entered by the user. I know this algorithm must use combinatorics to calculate all permutations. Problem is how to implement algortihm.

// This program reads the four numbers from the user
// and saves them into the array. Once all numbers entered
// program will translate input numbers to letters 
// like we have on phone keys, zero and and one will 
// be translated into the space
// Author: solaris_user
// Date: 08.05.2011

#include <stdio.h>

#define ARRAY_SIZE 4

int main(void)

{
    int phone_numbers[ARRAY_SIZE], counter, printer;

    for (counter = 1; counter <= ARRAY_SIZE; counter++) 
     {
        do {        
            printf("Enter %d phone number: ", counter);
            scanf("%d", &phone_numbers[counter]);
            if (phone_numbers[counter] < 0 || phone_numbers[counter] > 9) 
                {
                    printf("Incorrenct data input\n");
                    printf("Please try again\n");
                }
            } while (phone_numbers[counter] < 0 || phone_numbers[counter] > 9);
    }    
    
    for (printer = 1; printer <= counter; printer++) 
    {
        switch(phone_numbers[printer]) 
        {
            case 0: case 1: 
                {
                    printf("<space>\n");
                    break;
                }
            case 2: 
                {
                    printf("abc bac cab\n");
                    break;
                }
            case 3: 
                {
                    printf("cde dec ced\n");
                    break;
                }
           ....
        }
    }
        
    return 0;
}

Author says I must have four nested loops. So as a beginner I must ask for help from experienced programmers.

Thanks for reading and for your time :wink: