Java program to solve simple cryptarithmetic puzzle

Question

Edit: I got my program working now, but still need some clarification for the else if (400*T + 40*O + 4*O == 1000*G + 100*O + 10*O + D) part that is key to solving the puzzle. I just want to fully understand every bit of the program, thank you.

This is for review purposes only and I have spent a couple hours trying to figure it out. I'm either getting all zeroes for the variables or it's an infinite loop. Here is the question, as it's written in the book:

"In cryptarithmetic puzzles, mathematical equations are written using letters. Each letter can be a digit from 0 to 9, but no two letters can be the same. Here is a sample problem: SEND + MORE = MONEY A solution to the puzzle is S = 9, R = 8, O = 0, M = 1, Y = 2, E = 5, N = 6, D = 7. Write a program that finds a solution to the cryptarithmetic puzzle of the following: TOO + TOO + TOO + TOO = GOOD The simplest technique is to use a nested loop for each unique letter (in this case T, O, G, D). The loops would systematically assign the digits from 0 to 9 to each letter. For example, it might first try T = 0, O = 0, G = 0, D = 0 then T = 0, O = 0, G = 0, D = 1 then T = 0, O = 0, G = 0, D = 2, etc., up to T = 9, O = 9, G = 9, D = 9. In the loop body, test that each variable is unique and that the equation is satisfied. Output the values for the letters that satisfy the equation."

public class PracticeProjectEight
{
    public static void main(String[] args)
    {
        int T = 0 , O = 0 , G = 0 , D = 0;
        boolean keepGoing = true;
        //boolean againT = true , againO = true , againG = true , againD = true ;

        // while (keepGoing)   
        //{
            for (T = 0 ;  T > 10 ; T++)  
            {    
                for (O = 0 ; O > 10 ; O++)  
                {    
                    for (G = 0 ; G > 10 ; G++) 
                    {    
                        for (D = 0 ; D > 10 ; D++)
                        {    
                            if ((D == G) || (D == O) || (D == T) || (G == O) || (G == T) || (O == T))
                            {
                                //keepGoing = true;
                                continue;
                            }

                            else if (400*T + 40*O + 4*O == 1000*G + 100*O + 10*O + D)
                            {      
                                //keepGoing = false;
                                System.out.println("T = " + T);
                                System.out.println("O = " + O);
                                System.out.println("G = " + G);
                                System.out.println("D = " + D);
                                System.exit(0);
                            }
                        }
                    }
                }      
            }
       //}
    }
}

As you can see, I tried to put all the for loops in a while loop to control everything as one of my many attempts to fix. It didn't work (at all), so I commented it out.

Answer 1

All of those >s should be <s. You want < 10, not > 10.

In other words, it should be:

for (T = 0; T < 10; T++) {
    for (O = 0; O < 10; O++) { 
        for (G = 0; G < 10; G++) {
            for (D = 0; D < 10; D++) {
                // your logic as before
            }
        }
    }
}