Building Java Programs, 3rd edition

Lab: For loops

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

Lab goals

Goals for this problem set:

trace and write for loops for repeating lines of code
draw patterned figures of text using loops
use class constants to represent common unchanging values

`for` loops

A for loop repeats a group of statements a given number of times.

for (initialization; test; update) {
    statement(s) to repeat;
}

Example:

for (int i = 1; i <= 10; i++) {
    System.out.println("We're number one!");
}

Exercise : Bottles of beer

Write a for loop that produces the song Bottles of Beer on the Wall:

10 bottles of beer on the wall, 10 bottles of beer
Take one down, pass it around, 9 bottles of beer on the wall

9 bottles of beer on the wall, 9 bottles of beer
Take one down, pass it around, 8 bottles of beer on the wall

... (output continues in the same pattern) ...

1 bottles of beer on the wall, 1 bottles of beer
Take one down, pass it around, 0 bottles of beer on the wall

Write and test your code in Practice-It! by clicking the checkmark icon at the top of the slide.

Exercise : `for` loop table practice

Create a table of the number of stars on each line:

*******
*****
***
*

line	stars
`1`	7
`2`	5
`3`	3
`4`	1

multiplier: How much does the # of stars change between lines? -2

shift: Given your multiplier, what must be added to get that many stars on line 1? 9

Test your loop expression in Practice-It! using the checkmark icon above. Use the form:

for (int stars = 1; stars <= multiplier * line + shift; stars++) {

Exercise : simple `for` loop

Copy/paste the following code into jGRASP.

public class Count2 {
    public static void main(String[] args) {
        for ( fill me in! ) {
            System.out.println( fill me in! );
        }
    }
}

Modify the code to produce the following output:

2 times 1 = 2
2 times 2 = 4
2 times 3 = 6
2 times 4 = 8

Exercise : Verify solution in Practice-It!

screenshot Our Practice-It! system lets you solve Java problems online.

Go to the Practice-It! web site.
Create an account if you don't have one, and log in.
Under the "CS1 Labs", "Lab 2" category, select the "simple for loop" problem. (direct link)
Copy/paste your program from jGRASP into Practice-It, and submit. If it does not pass the test, modify your code and re-submit it.

Exercise : Sequence of numbers

Write a complete Java program that produces this sequence of numbers using a for loop:
```
8
11
14
17
20
23
```
Hint: You want to come up with an expression with a multiplier and a constant. First pick a multiplier (if your loop variable goes up by 1 each time, how much do these numbers go up by each time?). Then pick a constant (when your loop variable is 1, this is supposed to be 8, what do you have to add to get this value?).

Exercise : Sequence of characters

Modify the code from the previous exercise to produce the following output:

********
***********
**************
*****************
********************
***********************

Use the Output Comparison Tool to check your work.
(Hint: Note the number of stars in each row. Instead of printing the number 8, now you're printing 8 stars. You will now need a nested loop.)

Exercise : What's the output?

What output is produced by the following Java program? Write the output in the box on the right side.

public class OddStuff {
    public static void main(String[] args) {
        int number = 32;
        for (int count = 1; count <= number; count++) {
            System.out.println(number);
            number = number / 2;
        }
    }
}

Output:

Exercise : Number lines, part 1

Write some nested for loops to produce the following output:

000111222333444555666777888999
000111222333444555666777888999
000111222333444555666777888999

You can use the Output Comparison Tool to check your work.

Exercise : Number lines, part 2

Modify the previous code to have it do something a little different:

99999888887777766666555554444433333222221111100000
99999888887777766666555554444433333222221111100000
99999888887777766666555554444433333222221111100000
99999888887777766666555554444433333222221111100000
99999888887777766666555554444433333222221111100000

You can use the Output Comparison Tool to check your work.

Exercise : Number lines, part 3

Modify the code to have it produce this output:

999999999888888887777777666666555554444333221
999999999888888887777777666666555554444333221
999999999888888887777777666666555554444333221
999999999888888887777777666666555554444333221

You can use the Output Comparison Tool to check your work.

Exercise : What's the output?

What output is produced by the following Java method?

public static void stars() {
    for (int i = 1; i <= 10; i++) {
        for (int j = 1; j <= i; j++) {
            System.out.print("*");
        }
        for (int j = 1; j <= 20 - 2 * i; j++) {
            System.out.print(" ");
        }
        for (int j = 1; j <= i; j++) {
            System.out.print("*");
        }
        System.out.println();
    }
}

(Try to figure it out without running the code...
If you give up, copy and paste it into jGrasp and run it!)
Hint: Think of each inner loop from 1 to X as a loop that prints X copies of the given character.

Exercise - answer

Answer:

*                  *
**                **
***              ***
****            ****
*****          *****
******        ******
*******      *******
********    ********
*********  *********
********************

Exercise : What's the output?

What's the output of the following Java method?

public static void mystery() {
    for (int i = 1; i <= 10; i++) {
        for (int j = 1; j <= 10; j++) {
            System.out.print(i * j + "\t");
        }
        System.out.println();
    }
}

(Try to figure it out without running the code...
If you give up, copy and paste it into jGrasp and run it!)
Recall: \t represents a tab character.

Exercise - answer

Answer:

1	2	3	4	5	6	7	8	9	10
2	4	6	8	10	12	14	16	18	20
3	6	9	12	15	18	21	24	27	30
4	8	12	16	20	24	28	32	36	40
5	10	15	20	25	30	35	40	45	50
6	12	18	24	30	36	42	48	54	60
7	14	21	28	35	42	49	56	63	70
8	16	24	32	40	48	56	64	72	80
9	18	27	36	45	54	63	72	81	90
10	20	30	40	50	60	70	80	90	100

Exercise : printing a design

Write a program to produce the following output using nested for loops. Use a table to help you figure out the patterns of characters on each line.

-----1-----
----333----
---55555---
--7777777--
-999999999-

Line	Dashes	Numbers
`1`	5	1
`2`	4	3
`3`	3	5
`4`	2	7
`5`	1	9

dashes expression
-1 `* line +` 6

numbers expression
2 `* line +` -1

Test your loop expressions in Practice-It! using the checkmark icon above. Use your expressions in the loop tests of the inner loops of your code.

Exercise : SlashFigure

Write a Java program in a class named SlashFigure to produce the following output with nested for loops. Use a loop table if necessary to figure out the expressions.

!!!!!!!!!!!!!!!!!!!!!!
\\!!!!!!!!!!!!!!!!!!//
\\\\!!!!!!!!!!!!!!////
\\\\\\!!!!!!!!!!//////
\\\\\\\\!!!!!!////////
\\\\\\\\\\!!//////////

Line	`\`	`!`	`/`
`1`	0	22	0
`2`	2	18	2
`3`	4	14	4
`4`	6	10	6
`5`	8	6	8
`6`	10	2	10
multiplier	2	-4	2
shift	-2	26	-2

Test your code in Practice-It! or the Output Comparison Tool.

Class constants

A class constant is a global value that cannot be changed.

public static final type name = expression;

Example:

public static final int DAYS_PER_WEEK = 7;
public static final double TAX_RATE = 0.10;

`for` loop expressions w/ constant

When adding a class constant to a loop expression, it affects the constant that must be added in the expression. Suppose we have the two loop expressions below for figure sizes of 5 and 9. The third line of the table shows the general formula that would be used if we turned our figure's size into a constant named SIZE.

size	expression	relationship
`5`	`8 * line + 16`	`16 = 3 * 5 + 1`
`9`	`8 * line + 28`	`28 = 3 * 9 + 1`
`SIZE`	`8 * line + (3 * SIZE + 1)`

continued on the next slide ...

Exercise : `for` loop table w/ constant

You already found loop expressions for the slash figure at size 6. Now make a table at size 4 and use the two to generalize the loop expression in terms of a constant for the figure size.

!!!!!!!!!!!!!!
\\!!!!!!!!!!//
\\\\!!!!!!////
\\\\\\!!//////

Line	`\`	`!`	`/`
`1`	0	14	0
`2`	2	10	2
`3`	4	6	4
`4`	6	2	6

`\` and `/`
size 6	`2 * line +`	-2
size 4	`2 * line +`	-2
size SIZE	`2 * line +`	-2

`!`
size 6	`-4 * line +`	26
size 4	`-4 * line +`	18
size SIZE	`-4 * line +`	`(` 4 `* SIZE +` 2 `)`

Exercise : SlashFigure2

Add a class constant to your slash figure program so that it can be resized from its default of 6:

size 4	size 7
!!!!!!!!!!!!!! \\!!!!!!!!!!// \\\\!!!!!!//// \\\\\\!!//////	!!!!!!!!!!!!!!!!!!!!!!!!!! \\!!!!!!!!!!!!!!!!!!!!!!// \\\\!!!!!!!!!!!!!!!!!!//// \\\\\\!!!!!!!!!!!!!!////// \\\\\\\\!!!!!!!!!!//////// \\\\\\\\\\!!!!!!////////// \\\\\\\\\\\\!!////////////

size 4

size 7

!!!!!!!!!!!!!!
\\!!!!!!!!!!//
\\\\!!!!!!////
\\\\\\!!//////

!!!!!!!!!!!!!!!!!!!!!!!!!!
\\!!!!!!!!!!!!!!!!!!!!!!//
\\\\!!!!!!!!!!!!!!!!!!////
\\\\\\!!!!!!!!!!!!!!//////
\\\\\\\\!!!!!!!!!!////////
\\\\\\\\\\!!!!!!//////////
\\\\\\\\\\\\!!////////////

Test your code in the Output Comparison Tool.

Exercise : jGRASP Debugger

Breakpoints help you debug programs (). Copy/paste this code into jGRASP:

public class Numbers {
    public static void main(String[] args) {
        int number = 42;
        for (int i = 1; i <= 1000; i++) {
            number = number * 37 % 103;
        }
        int number2 = number * number;
        System.out.println("result = " + number2);
    }
}

Set a breakpoint on the statement inside the for loop (line 5). Do this by moving your cursor to the beginning of that line until you see a stop-sign icon and then clicking.

continued on the next slide...

Exercise - jGRASP Debugger

Now you can debug the program by clicking on the debug icon (looks like a ladybug).
It will stop every time it hits line 5. Then you can see the values of variables i and number.
To continue running, press the top-left play or step buttons.
Using this approach, fill in the table below indicating what value number has when i has the given value. Keep in mind that you are figuring out what value number has just before it executes this line of code.
i = 1, number =
42
i = 2, number =
9
i = 3, number =
24
i = 4, number =
64

continued on the next slide...

Exercise - jGRASP Debugger

Click on the stop-sign a second time to get rid of it and then set a new stop point on line number 7. This will allow you to find out the value of number after the loop is done executing.
Again press "play" to start the programming running again.
At line 7, what is the value of number? 69

Variable Scope

A variable's scope is the part of a program in which it exists. In Java, the scope of a variable starts when it is declared and ends when the closing curly brace for the block that contains it is reached. A variable is said to be in scope where it is accessible.

public class Example {
    public static void main(String[] args) {
        performTest();
    }
	
    public static void performTest() {
        int count = 12;
        for (int i = 1; i <= 12; i++) {
            runSample();
            System.out.print(count);   
        }
    }

    public static void runSample() {
	    System.out.print("sample");
    }
}

In which of these two blocks is the variable count in scope?

main method
for loop
performTest method
runSample method

`i = 1, number =`		42
`i = 2, number =`		9
`i = 3, number =`		24
`i = 4, number =`		64

Building Java Programs, 3rd edition

Lab 2: For loops

Building Java Programs, 3rd edition

Lab: For loops

Lab goals

for loops

Exercise : Bottles of beer

Exercise : for loop table practice

Exercise : simple for loop

Exercise : Verify solution in Practice-It!

Exercise : Sequence of numbers

Exercise : Sequence of characters

Exercise : What's the output?

Exercise : Number lines, part 1

Exercise : Number lines, part 2

Exercise : Number lines, part 3

Exercise : What's the output?

Exercise - answer

Exercise : What's the output?

Exercise - answer

Exercise : printing a design

Exercise : SlashFigure

Class constants

for loop expressions w/ constant

Exercise : for loop table w/ constant

Exercise : SlashFigure2

Exercise : jGRASP Debugger

Exercise - jGRASP Debugger

Exercise - jGRASP Debugger

Variable Scope

`for` loops

Exercise : `for` loop table practice

Exercise : simple `for` loop

`for` loop expressions w/ constant

Exercise : `for` loop table w/ constant