CS 10: Introduction to Computer Programming

In-Class Examination 2: Solutions

(Although one solution is given for each question, it is not necessarily the only, or best, solution.) Open-Book, 75 minutes, Spring 1997

General Instructions

Read each question carefully.
Make sure that you clearly understand a question before answering it.
Please put your answer to each question on its page.
You may wish to work out an answer on scratch paper before writing it on your answer page; answers that are difficult to read may lose points for that reason.

Name:

Perm:

Question	Value	Score
1	10
2	15
3	10
4	15
5	15
6	10
7	10
8	15
Total	100

1. Write a boolean method, called xyz, that returns true if and only if its int argument is divisible by 3, 5, 7, and 11.

    Solution:

        boolean xyz(int n) {
            return n % 3 == 0 && n % 5 == 0 && n % 7 == 0 && n % 11 == 0;
       }

2. Write a method, called magic, that has 3 boolean arguments and behaves according to the truth table below.


arg1	arg2	arg3	returns
true	true	true	false
true	true	false	true
true	false	true	true
true	false	false	true
false	true	true	false
false	true	false	true
false	false	true	false
false	false	false	false

Solution:

boolean magic(boolean x, boolean y, boolean z) {

return ( x && y && !z) ||

( x && !y && z) ||

( x && !y && !z) ||

(!x && y && !z);

}

3. What is the output of the applet below (approximate the String positions, based on the coordinates given)?

import java.awt.*;

import java.applet.*;

public class Temp extends Applet {

int i = -1, j;

       public void paint( Graphics g ) {
          int i = 7;
          g.drawString( "i = " + i, 50, j += 15);
          xyz(g);
          abc(g);
          xyz(g);
          abc(g);
          g.drawString( "i = " + i, 50, j += 15);
       }

        void xyz(Graphics g) {
          g.drawString( "i = " + i, 50, j += 15);
          i *= 9;
          g.drawString( "i = " + i, 50, j += 15);
       }

       void abc(Graphics g) {
          int i = 25;
          g.drawString( "i = " + i, 50, j += 15);
          ++i;
          g.drawString( "i = " + i, 50, j += 15);
       }
}

Solution:

i = 7

i = -1

i = -9

i = 25

i = 26

i = -9

i = -81

i = 25

i = 26

i = 7

4. Write a Java expression that evaluates to a "random" int value in the set {17, 18, 19, ..., 100}. If it cannot be done, say "It cannot be done."

Solution:

(int) (84 * Math.random()) + 17

5. Convert the following if statement to equivalent Java statements without using boolean operators. If it cannot be done, say "It cannot be done."

if ( x != 10 && y <= 53 || y > 60 && x < 0 )

i = 10;

else

i = -1;

Solution 1:

Solution 2:

i = -1; if (x != 10)

if (x != 10) if ( y <= 53)

if (y <= 53) i = 10;

i = 10; else // x != 10 && y > 53

if (y > 60) if (y > 60)

if (x < 0) if (x < 0)

i = 10; i = 10;

else // x!= 0 && y > 60 && x >= 0

i = -1

else // x != 10 && y > 53 && y <= 60

i = -1;

else // x == 10

i = -1;

Solution 3:

    i = xyz(x, y);
    ...
    int xyz(int x, int y) {
        if (x != 10)
            if (y <= 53)
                return 10;
        if (y > 60)
            if (x < 0)
                return 10;
        return -1;
    }

6. Write a method or pair of methods called magic that returns true, if its argument is a boolean and false if its argument is a double. If it cannot be done, say "It cannot be done."

import java.awt.*;

import java.applet.*;

public class Temp extends Applet {

       public void paint( Graphics g ) {
           g.drawString(magic(true) + " | " + magic(1.0), 20, 20);
       }

// your code would go here
}

Your resulting applet would produce the output:

true | false

Solution:

boolean magic(boolean x) {

return true;

}

    boolean magic(double x) {
        return false;
    }

7. Write a method whose argument is an int array, and that returns an int that is the sum of the elements up to the first element whose value is 0. If the array has no element whose value is 0, then the method returns the sum of the elements of the entire array.

Solution:

int summer(int a[]) {

int sum = 0;

        for (int i = 0; i < a.length; i++)
            if (a[i] == 0)
                return sum;
            else
                sum += a[i];
        return sum;
       }

8. You have the following applet:

import java.awt.*;
import java.applet.*;
public class Temp extends Applet {
        int potSize = 20;                   // size of color squares
        int paletteX = 0, paletteY = 30;    // origin of palette
        Color pots[][] = new Color[4][4], active = Color.black;

        public void init() {
            for (int i = 0; i < 4; i++)
                for (int j = 0; j < 4; j++)
                    pots[i][j] = new Color(i*64, 0, j*64);
        }

        public void paint(Graphics g) {
            // paint palette
            for (int i = 0; i < 4; i++)
            for (int j = 0; j < 4; j++) {
                g.setColor(pots[i][j]);
                g.fillRect(paletteX + j*potSize, paletteY + i*potSize, potSize, potSize);
            }
            // paint active color
            g.setColor(active);
            g.fillRect(0, 0, potSize, potSize);
        }

        public boolean mouseUp(Event event, int x, int y) {
            if (x >= paletteX && x <= paletteX + 4*potSize &&
                y >= paletteY && y <= paletteY + 4*potSize)
                active = select(x, y);
            repaint();
            return true;
        }

public Color select(int x, int y) {

            //your code goes here
        }
    }

Write a method, public Color select(int x, int y), that takes the cursor position (i.e., 2 int arguments), and returns the Color of the square that has been clicked.

Solution:

public Color select(int x, int y) {

return pots[(y - paletteY) / potSize][(x - paletteX) / potSize];

}