The solution this month’s Exercise isn’t that complex, but it was handy in disproving a theory.

As a recap, the code simulates two opponents in a number game, a player and the computer.

The player goes first, guessing a value, 1 to 3, which is added to a running total. The total starts at zero.

The computer’s job is to also guess values 1 to 3, but with the running total result always a multiple of 4.

The guessing continues until the total surpasses 30.

Here is my solution:

2020_08-Exercise.c

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

int main()
{
    int total,four,player;

    /* initialize randomizer */
    srand((unsigned)time(NULL));

    total = 0;
    while( total<=30 )
    {
        /* get a value from 1 to 3 */
        player=rand() % 3 + 1;
        total+=player;
        /* output player's guess and total */
        printf("Player adds %d, total is %d\n",
                player,
                total
              );
        /* make the computer add a value to
           equal a multiple of 4 */
        four = 4 - total % 4;
        total += four;
        printf("Computer adds %d to make %d\n",
                four,
                total
              );
    }

    return(0);
}

Variable total represents the game’s running total. It’s initialized to zero at Line 12. The while loop spins as long as the total value is less than or equal to 30.

Variable player holds the player’s guess, which is obtained at Line 16: player=rand() % 3 + 1; This value is added to the running total at Line 17: total+=player;

After the player’s guess, the computer’s job is easy. Its guess is stored in variable four, which is calculated at Line 25: four = 4 - total % 4; The value in total is divided by 4 to obtain the remainder, which will be values 1, 2, or 3. It can’t ever be zero because the player went first; only the computer can reach a multiple of 4. The result of the modulo operation is subtracted from 4, which means the computer always guesses a multiple of 4.

Here’s a sample run:

Player adds 3, total is 3
Computer adds 1 to make 4
Player adds 2, total is 6
Computer adds 2 to make 8
Player adds 3, total is 11
Computer adds 1 to make 12
Player adds 2, total is 14
Computer adds 2 to make 16
Player adds 3, total is 19
Computer adds 1 to make 20
Player adds 2, total is 22
Computer adds 2 to make 24
Player adds 3, total is 27
Computer adds 1 to make 28
Player adds 3, total is 31
Computer adds 1 to make 32

If your solution yields similar results, great! Remember that your code need not match mine to successfully complete the Exercise.

I wrote this simulation to test the theory that going second and always guessing am multiple of 4 helps you to win the 21 number game. It failed. Regardless, in future Lesson I present my version of the 21 number game.