Question

I was having a headache understanding why certain nodes on a lesson example were being printed as a result of alpha-beta pruning, so I implemented Peter Norvig’s version of alpha beta pruning in Java.

With a tree like this:

                     max
                  /    |     \
    min               min            min
  / / |              / | \          / | \
 3 12 8             2  3  9        14 1  8

The textbook says that the only terminal nodes that should be expanded are

3, 12, 8, 2, 14, 1

in that order.

My algorithm prints:

visited leaf with value 3
visited leaf with value 12
visited leaf with value 2
visited leaf with value 3
visited leaf with value 14
visited leaf with value 1
visited leaf with value 8
Root value 3

The root is the correct value for the minimax root. I’ve debugged for a couple of hours and can’t seem to find fault. Have I overlooked something or is the textbook incorrect?

package alphabeta;

import java.util.ArrayList;

class Node {

    boolean isMin;
    boolean isMax;
    boolean isTerminal;
    int value;
    int depth;
    ArrayList <Node> children = new ArrayList <Node>();

}
public class AlphaBeta {


    static Node alphaBetaSearch(Node state){

        state.value = max_value(state,-99999,99999);

        System.out.println(state.value);

    return null;
    } 


    static int max_value(Node state, int alpha, int beta){


        if (state.isTerminal){
            System.out.println("visited leaf with value " + state.value);
            return state.value;
        }

        state.value = -99999;

        for (Node a: state.children){

            state.value = Math.max(state.value , min_value(a, alpha, beta));
            if (state.value >= beta){

                return state.value;                
            }
            alpha = Math.max(alpha, state.value);
        }
        return state.value;
    }

    static int min_value(Node state, int alpha, int beta){


        if (state.isTerminal) 
            return state.value;

        state.value = 99999;

        for (Node a: state.children){

            state.value = Math.min(state.value, max_value(a, alpha, beta));
            if (state.value >= beta){

                return state.value;

            }
            beta = Math.min(beta, state.value);
        }
        return state.value;
    }


    public static void main(String[] args) {

        Node t1 = new Node();
//            t1.value = 4; 
            t1.value = 3;
            t1.depth =2;
            t1.isTerminal = true;

        Node t2 = new Node();
//             t2.value = 8;
             t2.value = 12;
             t2.depth=2;
             t2.isTerminal= true;

        Node t3 = new Node();
            // t3.value = 7;
             t3.value = 8;
             t3.depth=2;
             t3.isTerminal= true;

        Node min1 = new Node();
             min1.isTerminal=false;
             min1.depth=1;
             min1.children.add(t1);
             min1.children.add(t2);
             min1.children.add(t3);

        Node t4 = new Node();
//            t4.value = 5;
            t4.value = 2;
            t4.depth =2;
            t4.isTerminal = true;

        Node t5 = new Node();
             //t5.value = 2;
             t5.value =3;
             t5.depth=2;
             t5.isTerminal= true;

        Node t6 = new Node();
//             t6.value = 1;
             t6.value=9;
             t6.depth=2;
             t6.isTerminal= true;

        Node min2 = new Node();
             min2.isMin=true;
             min2.isTerminal=false;
             min2.depth=1;
             min2.children.add(t4);
             min2.children.add(t5);
             min2.children.add(t6);  


        Node t7 = new Node();
//             t7.value = 1; 
             t7.value=14;
             t7.depth =2;
             t7.isTerminal = true;

        Node t8 = new Node();
//             t8.value = 6;
             t8.value=1;
             t8.depth=2;
             t8.isTerminal= true;

        Node t9 = new Node();
//             t9.value = 0;
             t9.value=8;
             t9.depth=2;
             t9.isTerminal= true;

        Node min3 = new Node();
             min3.isMin=true;
             min3.isTerminal=false;
             min3.depth=1;
             min3.children.add(t7);
             min3.children.add(t8);
             min3.children.add(t9);

         Node max1 = new Node();
              max1.isMax=true;
              max1.isTerminal=false;
              max1.depth=0;
              max1.children.add(min1);
              max1.children.add(min2);
              max1.children.add(min3);



              alphaBetaSearch (max1);



    }
}

Answer 1

You have two lines:

if (state.value >= beta){

One of those lines should have alpha instead of beta.