University of Washington, CSE 142

Lab 3: Return, if/else, and Scanner

lab document created by Marty Stepp, Stuart Reges, Whitaker Brand and Hélène Martin

Basic lab instructions

Talk to your classmates for help. You can even work on the lab with a partner if you like.
You may want to bring your textbook to future labs to look up syntax and examples.
Stuck? Confused? Have a question? Ask a TA for help, or look at the book or past lecture slides.
Complete as many problems as you can within the allotted time. You don't need to keep working on these exercises after you leave the lab.
Feel free to complete problems in any order.
Before you leave today, make sure to check in with one of the TAs in the lab to get credit for your work.

Today's lab

Goals for today:

practice passing parameters in the context of drawing graphics
use the instructor-provided DrawingPanel and Java's Graphics and Color classes
use return values to send data between methods
Where you see this icon, you can click it to check the problem in Practice-It!

Exercise : Check MyUW scores

The scores you receive on homework and labs are available for you to view in MyUW. Follow the instructions on our MyUW page to check your scores.
You should see scores for labs 1 and 2. If not, contact your TA or a TA from your lab period.
Using the information on your MyUW page, fill in the values below.

class mean for a1 (i.e., average score for assignment 1)	/ 10	9.07
class mean for lab1 (i.e., average score for lab 1)	/ 2	2.0

Graphics

Now we'll explore several exercises related to drawing graphics.

We'll use a provided class DrawingPanel that works with Java classes Graphics (a "pen" for drawing) and Color.
Download DrawingPanel.java to your program directory.
To check your output in the DrawingPanel, click File, Compare to URL....

Exercise : Syntax errors

The following program contains 12 mistakes! What are they? Once you think you've found them all, compile/run your corrected code.

import Java.*;

public class PrettyPicture {
    public static void main(String[] args) {
        DrawingPanel panel = DrawingPanel(220, 150);
        setBackgroundColor(Color.YELLOW);
        
        Graphics g = panel.Graphics();
        panel.setColor(new Color.BLUE);
        g.drawRectangle(50, 25);
        g.setColor("RED");
        g.fillEllipse(130, 25, 42.1, 40.5);
    }
}

answer on next slide...

Exercise - answer

line 1: incorrect import statement; should import java.awt.*
line 5: missing word new before 2nd occurrence of DrawingPanel
line 6: method name should be setBackground
line 6: missing panel. before setBackground
line 8: method name should be getGraphics
line 9: the setColor method is part of object g, not panel
line 9: should not write new before Color.BLUE
line 10: method name should be drawRect
line 10: missing two parameters from drawRect (width and height)
line 11: color should be specified as Color.RED, not "RED"
line 12: method name should be fillOval
line 12: wrong parameter types; width/height must be integers

Exercise - corrected version

Here is a corrected version of the program:

import java.awt.*;

public class PrettyPicture {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(220, 150);
        panel.setBackground(Color.YELLOW);
        
        Graphics g = panel.getGraphics();
        g.setColor(Color.BLUE);
        g.drawRect(50, 25, 10, 10);
        g.setColor(Color.RED);
        g.fillOval(130, 25, 42, 40);
    }
}

Exercise : Face

Write a complete Java program that draws the following output:

window size: 220 x 150 px
overall face circle: 100 px diameter; top-left corner at (10, 30)
eyes: blue circles, 20 px diameter; top-left at (30, 60) and (70, 60)
mouth: red line from (40, 100) to (80, 100)

Exercise - answer

import java.awt.*;

public class Face1 {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(220, 150);
        Graphics g = panel.getGraphics();
        
        g.drawOval(10, 30, 100, 100);   // face outline
        g.setColor(Color.BLUE);
        g.fillOval(30, 60, 20, 20);     // eyes
        g.fillOval(70, 60, 20, 20);
        g.setColor(Color.RED);          // mouth
        g.drawLine(40, 100, 80, 100);
    }
}

Exercise : Stairs loop table

Consider the output at right. The first stair's top-left corner is at position (5, 5). The first stair is 10x10 px in size. Each stair is 10px wider than the one above it.

Fill in the table below with the coordinates and sizes of the first five stairs. Note which values change and which ones stay the same.

stair	x	y	width	height
`1`	5	5	10	10
`2`	5	15	20	10
`3`	5	25	30	10
`4`	5	35	40	10
`5`	5	45	50	10

Exercise : Stairs

Write a complete Java program to draw the stairs. Copy/paste the code template below into jGRASP and fill in your own expressions or values for each stair's x, y, width, and height.

Use your table from the previous slide to help you find the correct expressions. The values that change for each stair should become expressions in terms of the loop counter variable, i.

import java.awt.*;

public class Stairs1 {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(110, 110);
        Graphics g = panel.getGraphics();
        for (int i = 0; i < 10; i++) {
            g.drawRect(x, y, width, height);
        }
    }
}

Exercise : Stairs 2

Modify your stairs program to draw one (or all) of the following outputs. Modify only the body in your for loop. (You may want to make a new table to find the expressions for x, y, width, and height.)

→

Exercise - answer

To get each output, change the for loop body to the following:

// output 2
g.drawRect(5, 5 + 10*i, 100 - 10*i, 10);

// output 3
g.drawRect(95 - 10*i, 5 + 10*i, 10 + 10*i, 10);

// output 4
g.drawRect(5 + 10*i, 5 + 10*i, 100 - 10*i, 10);

Parameterized methods and Graphics

When you want to divide a graphical program into multiple drawing methods, you must pass Graphics g as a parameter in addition to any other parameters. Example:

public static void main(String[] args) {
    DrawingPanel panel = new DrawingPanel(400, 300);
    Graphics g = panel.getGraphics();
    ...
    drawStuff(g, 13, 52, 7);
}

public static void drawStuff(Graphics g, int a, int b, int c) {
    g.drawLine(a, 45, b, c);
    ...
}

Exercise : Face 2

Modify your previous Face program to draw the following output. Write a parameterized method that allows you to draw a face at different positions.

window size: 320 x 180 px
faces' top-left corners at (10, 30) and (150, 50)

Exercise - answer

import java.awt.*;

public class Face2 {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(320, 180);
        Graphics g = panel.getGraphics();
        drawFace(g, 10, 30);
        drawFace(g, 150, 50);
    }
    
    public static void drawFace(Graphics g, int x, int y) {
        g.setColor(Color.BLACK);
        g.drawOval(x, y, 100, 100);
        g.setColor(Color.BLUE);
        g.fillOval(x + 20, y + 30, 20, 20);
        g.fillOval(x + 60, y + 30, 20, 20);
        g.setColor(Color.RED);
        g.drawLine(x + 30, y + 70, x + 70, y + 70);
    }
}

Exercise : Face 3

Modify your previous Java program to draw the following output. Use a for loop with your parameterized method to draw faces at different positions.

window size: 520 x 180 px
faces' top-left at (10, 30), (110, 30), (210, 30), (310, 30), and (410, 30)

Exercise - answer

import java.awt.*;

public class Face3 {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(520, 180);
        Graphics g = panel.getGraphics();
        for (int i = 0; i < 5; i++) {
            drawFace(g, 10 + i * 100, 30);
        }
    }
    
    public static void drawFace(Graphics g, int x, int y) {
        g.setColor(Color.BLACK);
        g.drawOval(x, y, 100, 100);
        g.setColor(Color.BLUE);
        g.fillOval(x + 20, y + 30, 20, 20);
        g.fillOval(x + 60, y + 30, 20, 20);
        g.setColor(Color.RED);
        g.drawLine(x + 30, y + 70, x + 70, y + 70);
    }
}

Returning Values

A return value is when a method sends a value back to the code that called it.

public static type name(parameters) {      // declare
    ...
    return expression;
}

variableName = methodName(parameters);     // call

Example:

public static double fToC(double tempF) {
    return (tempF - 32) * 5.0 / 9.0;
}
...
double bodyTemp = fToC(98.6);          // bodyTemp stores 37.0
double freezing = fToC(32);            // freezing stores  0.0

Math expression syntax

Method	Description	Example
`Math.abs`	absolute value	`Math.abs(-308)` returns `308`
`Math.ceil`	ceiling (rounds upward)	`Math.ceil(2.13)` returns `3.0`
`Math.floor`	floor (rounds downward)	`Math.floor(2.93)` returns `2.0`
`Math.max`	max of two values	`Math.max(45, 207)` returns `207`
`Math.min`	min of two values	`Math.min(3.8, 2.75)` returns `2.75`
`Math.pow`	power	`Math.pow(3, 4)` returns `81.0`
`Math.round`	round to nearest integer	`Math.round(2.718)` returns `3`
`Math.sqrt`	square root	`Math.sqrt(81)` returns `9.0`

Exercise - Math expressions

Write the results of each expression. Use the proper type (.0 for a double).

`Math.ceil(9.17)`		10.0
`Math.pow(2, 4)`		16.0
`Math.sqrt(64)`		8.0
`Math.floor(12.73) + Math.max(8, 5)`		20.0
`Math.abs(Math.min(-1, -3))`		3
`Math.ceil(Math.random())`		1.0
`-Math.pow(2, 2) + Math.pow(-2, 3) + Math.pow(2, -2)`		-11.75
`Math.round(4.25) + Math.round(5.38) + Math.round(6.49)`		15

Exercise - Math expressions

Write the results of each expression. Use the proper type (such as .0 for a double). Note that a variable's value changes only if you re-assign it using the = operator. Discuss any errors you make with your neighbor.

double grade = 2.7;
Math.round(grade);                               // grade = 2.7
grade = Math.round(grade);                       // grade = 3.0

double min = Math.min(grade, Math.floor(2.9));   //   min = 2.0

double x = Math.pow(2, 4);                       //     x = 16.0
x = Math.sqrt(64);                               //     x = 8.0

int count = 25;
Math.sqrt(count);                                // count = 25
count = (int) Math.sqrt(count);                  // count = 5

int a = Math.abs(Math.min(-1, -3));              //     a = 3

Exercise : lastDigit

Write a method named lastDigit that returns the last digit of an integer. For example, lastDigit(3572) should return 2. It should work for negative numbers as well. For example, lastDigit(-947) should return 7.
Try solving this problem in Practice-It! using the link above.
(Hint: This is a short method.)

Exercise : area

Consider the following method for converting milliseconds into days:

// converts milliseconds to days
public static double toDays(double millis) {
    return millis / 1000.0 / 60.0 / 60.0 / 24.0;
}

Write a similar method named area that takes as a parameter the radius of a circle and that returns the area of the circle. For example, the call area(2.0) should return 12.566370614359172. Recall that area can be computed as π times the radius squared and that Java has a constant called Math.PI.

(Try solving this problem in Practice-It by clicking the icon above.)

Exercise : pay

Write a method named pay that accepts two parameters: a real number for a TA's salary, and an integer for the number of hours the TA worked this week. The method should return how much money to pay the TA. For example, the call pay(5.50, 6) should return 33.0.

The TA should receive "overtime" pay of 1 ½ normal salary for any hours above 8. For example, the call pay(4.00, 11) should return (4.00 * 8) + (6.00 * 3) or 50.0.

(Try solving this problem in Practice-It by clicking the icon above.)
Hint: The practice-it problem is in the if/else section (which we haven't covered yet), but you can do this problem with clever use of the Math.max and Math.min methods.

Exercise : Powers of 2

Write a Java method printPowersOf2 to print out the powers of 2 up to 2^x. It should take a parameter that determines x; that is, the exponent of the highest power of 2 to be printed. For example, the following call:
```
printPowersOf2(10);
```
should produce the following output, because 2¹⁰ = 1024:
```
1 2 4 8 16 32 64 128 256 512 1024
```
Complete the program by writing a main method. Call your method several times in main to ensure that it works. Also check your answer using Practice-It by clicking the check-mark icon above.

Exercise : Powers of `n`

Modify your program from the previous exercise to have a method printPowersOfN that prints the powers of any given number n up to n^x. For example, the following call:
```
printPowersOfN(3, 5);
```
should produce the following output, because 3⁵ = 243:
```
1 3 9 27 81 243
```
and the call:
```
printPowersOfN(2, 10);
```
should produce the same output as printPowersOf2(10); :
```
1 2 4 8 16 32 64 128 256 512 1024
```

Exercise : Spiral

Write a Java program that draws the following output using a for loop.

window size: 170 x 170 px
"spiral" lines starting from (0, 10), going right 160, down 150, left 150, up 140, right 140, down 130, left 130, up 120, ...

Exercise - answer

import java.awt.*;

public class Spiral {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(170, 170);
        Graphics g = panel.getGraphics();
        for (int i = 0; i < 8; i++) {
            g.drawLine(      10*i,   10 + 10*i, 160 - 10*i,  10 + 10*i);  // top
            g.drawLine(160 - 10*i,   10 + 10*i, 160 - 10*i, 160 - 10*i);  // right
            g.drawLine( 10 + 10*i,  160 - 10*i, 160 - 10*i, 160 - 10*i);  // bottom
            g.drawLine( 10 + 10*i,   20 + 10*i,  10 + 10*i, 160 - 10*i);  // left
        }
    }
}

Exercise - answer, alternative version

import java.awt.*;

public class Spiral {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(170, 170);
        Graphics g = panel.getGraphics();
        int x = 0, y = 10;
        int len = 160;
        for (int i = 0; i < 8; i++) {
            g.drawLine(x, y, x + len, y);  // right
            x = x + len;
            len = len - 10;
            g.drawLine(x, y, x, y + len);  // down
            y = y + len;
            g.drawLine(x, y, x - len, y);  // left
            x = x - len;
            len = len - 10;
            g.drawLine(x, y, x, y - len);  // up
            y = y - len;
        }
    }
}

If you finish them all...

If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.

You can view an exercise, type a solution, and submit it to see if you have solved it correctly.

Choose some problems from the book and try to solve them!

University of Washington, CSE 142

Lab 3: Return, if/else, and Scanner

University of Washington, CSE 142

Lab 3: Return, if/else, and Scanner

Basic lab instructions

Today's lab

Exercise : Check MyUW scores

Graphics

Exercise : Syntax errors

Exercise - answer

Exercise - corrected version

Exercise : Face

Exercise - answer

Exercise : Stairs loop table

Exercise : Stairs

Exercise : Stairs 2

Exercise - answer

Parameterized methods and Graphics

Exercise : Face 2

Exercise - answer

Exercise : Face 3

Exercise - answer

Returning Values

Math expression syntax

Exercise - Math expressions

Exercise - Math expressions

Exercise : lastDigit

Exercise : area

Exercise : pay

Exercise : Powers of 2

Exercise : Powers of n

Exercise : Spiral

Exercise - answer

Exercise - answer, alternative version

If you finish them all...

Exercise : Powers of `n`