CS 1713/1711 Introduction to Computer Science Lecture 26

CS 1713 Lecture 26: Nested Loops and Classes
October 29, 1999

Craps design - collect designs and go over the strategy.
Nested loop - loops within loops:
The inner loop(s) is executed completely for each iteration of an enclosing loop.

Example:

  for(int j = 1; j <= 3; j++) {
     for(int k = 1; k <= 2; k++) {
          System.out.println("j = ",+ j + ",  k = " + k); 
     } 
  }

has output:

  j = 1,  k = 1 
  j = 1,  k = 2
  j = 2,  k = 1
  j = 2,  k = 2
  j = 3,  k = 1
  j = 3,  k = 2

Note: The scopes of j and k are confined to the loops in which they are defined.

Example:

  for(int j = 1; j <= 5; j++) { 
     for(int k = 1; k <= j; k++) { 
        System.out.print( (j*k) + " "   ); 
     } 
     System.out.println(); 
  }

has output:

 
   1 
   2  4 
   3  6  9 
   4  8 12 16 
   5 10 15 20 25

Example: Write code to perform Experiment I 10 times. Experiment I consists of rolling three 6-sided dice 100 times and counting the total number of times that all three dice agree and the number of times exactly two of the three dice agree. Output the averages of these values over the 10 trials.

   Dice myDice1 = new Dice(sides);
   Dice myDice2 = new Dice(sides);
   Dice myDice3 = new Dice(sides);
   int totalTwoAgree = 0;
   int totalThreeAgree = 0;
  
   for (int i = 0; i < 10; i ++) {
       int threeAgree = 0;
       int twoAgree = 0;
       for (int j = 0; j < 100; j++) {
          int d1 = myDice1.Roll();
          int d2 = myDice2.Roll();
          int d3 = myDice3.Roll();
          if (d1 == d2 && d1 == d3)
             threeAgree++;
          else if (d1 == d2 || d2 == d3 || d1 == d3)
             twoAgree++;
       }
       totalThreeAgree += threeAgree;
       totalTwoAgree += twoAgree;
   }
   System.out.println("Average number of times all three agree: " +
                       (double)totalThreeAgree/10);
   System.out.println("Average number of times exactly two agree: " +
                       (double)totalTwoAgree/10);

Example: Adding a class to make the code more flexible and clearer.

public class ExperimentI {
   private int myTrials;
   private int mySides;
   private int myThreeAgree;
   private int myTwoAgree;

   // constructor
   public ExperimentI (int sides, int trials) {
      Dice dice1 = new Dice(sides);
      Dice dice2 = new Dice(sides);
      Dice dice3 = new Dice(sides);
      mySides = sides;
      myTrials = trials;
      myThreeAgree = 0;
      myTwoAgree = 0;
      for (int i = 0; i < myTrials; i++) {
          int d1 = dice1.Roll();
          int d2 = dice2.Roll();
          int d3 = dice3.Roll();
          if (d1 == d2 && d1 == d3)
             myThreeAgree++;
          else if (d1 == d2 || d2 == d3 || d1 == d3)
             myTwoAgree++;
       }
    }

    // accessors
    public int getSides() {
       return mySides;
    }

    public int getTrials() {
       return myTrials;
    }
  
    public int getTwoAgree() {
       return myTwoAgree;
    }

    public int getThreeAgree() {
       return myThreeAgree;
    }

    public String toString() {
       return "EXP I: Sides(" + mySides + ")" 
               + " Trials(" + myTrials +")" 
               + " Three agreed " + myThree Agree 
               + " Two agreed " + myTwoAgree;
    }
}

Example: Write client code that uses the ExperimentI class to Roll three 6-sided dice, 100 times and output the total number of times that all three dice agree. Also output the number of times that exactly two of the three dice agree.
```
    ExperimentI exp = new ExperimentI (6, 100);
    System.out.println("Number of times all three agree is " +  exp.getThreeAgree());
    System.out.println("Number of times only two agree is " + exp.getTwoAgree());
```

Example: Write client code to perform Experiment I 10 times and output the average number of times that all three dice agreed. (Use the ExperimentI class.)

    int totalThreeAgreed = 0;
    for (int i = 0; i < 10; i++) {
        ExperimentI exp = new ExperimentI(6, 100);
        totalThreeAgreed += exp.getThreeAgree();
    }
    System.out.println("The average over 10 trials that three agreed is " +
                       (double)totalThreeAgreed/10);

Example: Write a main class program that prompts for the number of trials and the number of sides and performs Experiment I:

import simpleIO.*;

public class ExperimentIMain {

    public static void main(String args[ ]) {
       SimpleGUI wind = new SimpleGUI();
       int numberSides = wind.getInt("Please enter number of sides:");
       int numberTrials = wind.GetInt("Please enter number of trials:");
       ExperimentI exp = newExperimentI(numberSides, numberTrials);
       System.out.println(exp.toString());
       wind.DisplayResult(exp.toString());
    }
}

Objective: Understand nested loops and how to eliminate them using classes.

Last revision: October 27, 1999 at 2:30 am