University of Washington, CSE 142

Lab 2: Expressions, Variables, and Loops

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

Basic lab instructions

Mouse over highlighted words if you're not sure what they mean!
Talk to your classmates for help.
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 much of the lab as you can within the allotted time. You don't need to keep working on these exercises after you leave.
Feel free to complete problems in any order.
Make sure you've signed in on the sign-in sheet before you leave!

Today's lab

Goals for today:

write and evaluate expressions to compute numeric values
use variables to store results of a computation into memory
trace and write for loops for repeating lines of code
draw patterned figures of text using loops

Expressions

Recall that Java has expressions to represent math and other computations. Expressions may use operators, which are evaluated according to rules of precedence. Every expression produces a value of a given type.

Type	Description	Expression Example	Result
`int`	integers (up to 2³¹ - 1)	`3 + 4 * 5`	`23`
`double`	real numbers (up to 10³⁰⁸)	`3.0 / 2.0 + 4.1`	`5.6`
`String`	text characters	`"hi" + (1 + 1) + "u"`	`"hi2u"`

Operators

Java has operators for the standard mathematical operations. Note that + works a little differently depending on the type of the operands.

Operator	Description
`+`	Addition for `int`s and `double`s, concatenation for `String`s
`-`	Subtraction for `int`s and `double`s, doesn't compile for `String`s
*``**	Multiplication for `int`s and `double`s, doesn't compile for `String`s
`/`	Division for `int`s and `double`s, doesn't compile for `String`s Note that `int`s will follow integer (truncated) division rules
`%`	Modulus for `int`s and `double`s, doesn't compile for `String`s

Exercise : Expressions (2.1)

Write the results of each of the following expressions. If you're stuck, ask a TA or neighbor.

`12 / 5`		2
`12.0 / 5`		2.4
`12 / 5 + 8 / 4`		4
`3 * 4 + 15 / 2`		19
`42 % 5 + 3 % 16***`		5

***Hint: Suppose you have 3 apples, and you distribute them among 16 people so that everyone gets the same number of (whole) apples. How many (whole) apples does each person get? How many apples do you have left over?

String Concatenation

The + operator does addition on numbers, but on Strings, it instead appends the two Strings together.

   System.out.println("Hello, World!");         // prints: Hello, World!   
   System.out.println("Hello, " + "World!");    // prints: Hello, World!         
 
   System.out.println("You " + "can" + " add" + 
       "many" +   "   Strings together");       // prints: You can addmany   Strings together
   System.out.println("Numbers " + 2 + "!");    // prints: Numbers 2!

Note that only the whitespace inside the quotation marks is preserved.

Using quotation marks tells Java to keep track of every character from the opening " to the closing ", including the whitespaces like spaces, tabs, and newlines.

Outside the quotes, we're back in the Java program itself, where Java doesn't notice or care about the newline we've used to format the long line of source code.

String Concatenation with numbers

We do String concatenation instead of addition if either of the operands are Strings.

We only add the numbers together if both of them are numbers (not Strings). Java keeps track of the type of pieces of data in your program. It treats "4" (a String) differently than it treats 4 (a numerical value).

Having the distinction is crucial for some tasks: if the computer always treated numerical Strings as numbers, then we wouldn't have a way to prepend an area code to the beginning of a phone number.

   // All are using the + on a String -- all perform concatenation.
   System.out.println("1" + "2");      // prints 12
   System.out.println("1" + 2);        // same, prints 12
   System.out.println(1 + "2");        // same, prints 12

   // Since both operators here are numbers, we do addition:
   System.out.println(1 + 2);          // does addition before printing -- prints 3

Note: Even though the + operator works differently on Strings, it still follows the same precedence rules.

Exercise : More expressions (2.1)

Write the results of each of the following expressions.

Some of these expressions include String concatenation. When the result of the expression evaluates to a String, put "quotation marks" around the result:

`"x" + 2`	"x2"	`// Add the other quote to get the correct answer`
`2 + 6 + "cse 142"`	"8cse 142"
`"cse 142" + 2 + 6`	"cse 14226"
`1 + 9 / 2 * 2.0`	9.0
`46 / 3 / 2.0 / 3 * 4/5`	2.0

jGRASP Interactions Pane

jGRASP has a useful feature called the Interactions Pane that allows you to type in Java expressions or statements one at a time and instantly see their results.
To use it, run jGRASP and then click the "Interactions" tab near the bottom.

continued on the next slide...

Exercise : Using jGRASP Interactions Pane

In this exercise, you'll use the Interactions Pane to quickly discover the result of some expressions that would be difficult to evaluate by hand. Copy/paste each expression below into the Interactions Pane to evaluate it, then input the answer into this slide.

`123 * 456 - 789`		55299
`3.14 + 1.59 * 2.65`		7.3535
`2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2`		1024
`2 + 2 + "xyz" + 3 + 3`		"4xyz33"

(For the last expression, the Interactions Pane doesn't put "" quotes around Strings when displaying results, so you must add those yourself if needed. For example, if the Interactions Pane gives you a result of abc123, it should be written here as "abc123".)

Variables

Recall that you can use a variable to store the results of an expression in memory and use them later in the program.

type name;                       // declare
name = value or expression;        // assign a value
...
type name = value or expression;   // declare-and-initialize together

Examples:

double iPhonePrice;
iPhonePrice = 299.95;

int siblings = 3;
System.out.println("I have " + siblings + " brothers/sisters.");

Exercise : Variable declaration syntax

Which of the following choices is the correct syntax for declaring a real number variable named grade and initializing its value to 4.0?

grade : 4.0;
int grade = 4.0;
4.0 = grade;
grade = 4.0;
double grade = 4.0;
grade = double 4.0;

Exercise : Variable assignment syntax

Suppose you have a variable named grade, set to 1.6:

double grade = 1.6;   // uh-oh

Suppose later in the program's code, we want to change the value of grade to 4.0. Which is the correct syntax to do this?

grade : 4.0;
double grade = 4.0;
grade = 4.0;
4.0 = grade;
set grade = 4.0;

Variable Mutation

Assume that we have executed the following line of code:

    int x = 5;

How could we add 6 to the value currently stored in x? A naive approach might be to try this line of code:

    x + 6;

However, this line of code is an expression that results in a value: we have not altered the value of x.

    // Remember, x is a variable that is holding the value 5
    x + 6;      // evaluates to:
    5 + 6;      // and then to:
    11;
    
    // But 11; isn't a statement that Java understands, 
    // so the compiler throws an error when it sees:  x + 6;

To increase the value of x by 6, we need to actually reassign the value of x to be the result of x + 6:

    x = x + 6;       // evaluates to:
    x = 5 + 6;       // and then to:
    x = 11;

Here, we've used the value of x to calculate and store a new value into the variable x; in this case, 11.

Exercise : Variable mutation

Suppose you have a variable named balance, set to 463.23:

double balance = 463.23

Suppose later in the program's code, we want to add 5 to the account balance. Which is a correct statement to do this?

balance + 5;
balance = 5;
balance = balance + 5;
5 + balance = balance;
balance <-- 5;

Exercise : `a`, `b`, and `c`

What are the values of a, b, and c after the following statements? Write your answers in the boxes on the right.

int a = 5;
int b = 10;
int c = b;

a = a + 1;            // a? 6
b = b - 1;            // b? 9
c = c + a;            // c? 16

`print` vs `println`

There is a method called System.out.print that is similar to System.out.println.

They are different in that print doesn't add a "new line" (line break, '\n') character to the end of your output.

    System.out.print("hello,");
    System.out.print("helloooo");
    // prints: hello,helloooo

    System.out.println("hello,");
    System.out.println("helloooo");
    // prints:
    // hello,
    // helloooo

Every time you call System.out.println() Java will add a "new line" character to your output.

`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 <= 3; i++) {
    System.out.println("We're number one!");
}
System.out.println("/cheering");

Output:

We're number one!
We're number one!
We're number one!
/cheering

`for` loops

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

for (int i = 1; i <= 3; i++) {
    System.out.println("We're number one!");
}
System.out.println("/cheering");

The for loop keeps executing the println as long as the test condition is met:

initialization :: int i = 1; :: start a counter at 1
test :: i <= 3; :: continue as long as the counter i is less than 3
execute the statements :: { System.out.println("We're number one!"); }
update :: i++ :: add 1 to the counter
go back to step 2 and check the test condition again: i is 1 bigger than it was last time through the loop

Once the test isn't true anymore, Java exits the for loop :: System.out.println("/cheering");

Exercise : `for` loop repeating

For each of the following for loops, write the output that would be produced by executing them.

for(int i = 1; i <= 3; i++) {
    System.out.print("*");
}

output:

***

for(int i = 1; i <= 4; i++) {
    System.out.print("!-!"); 
}

output:

!-!!-!!-!!-!

// Note the number/String concatenation here in the print().
// i is an int variable that holds onto a numerical value.
//
// For which numerical values does i hold as the for loop executes?
for(int i = 1; i <= 5; i++) {
    System.out.print(i + "~");
}

output:

1~2~3~4~5~

Exercise : `print`, `println`, and `for` loops

Write a Java class called PrintLoops that produces the following output:

>>>>
CSE142<<<<

To accomplish this:

first, write a for loop that prints out 4 ">" characters
next, write a blank line using the statement System.out.println() after the for loop
then, use System.out.print to print "CSE142"
finally, use another for loop to print out 4 "<" characters

`PrintLoops` Solution

One possible solution:

public class PrintLoops {
    public static void main(String[] args) {
        // Step 1 -- print the > characters
        for (int i = 1; i <= 4; i++) {
            System.out.print(">");
        }
       
        // Step 2 -- print the blank line
        System.out.println();

        // Step 3 -- careful to use print here, not println
        System.out.print("CSE142");

        // Step 4 -- print the < characters:
        for (int i = 1; i <= 4; i++) {
            System.out.print("<");
        }
    }
}

Exercise : Writing a `for` loop!

Write a Java class called Stars that, using a for loop, produces the following output:

*****

Then modify your program to instead produce the following output:

********

`Stars` Solution

One possible solution for the first part of Stars:

public class Stars {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
	    System.out.print("*");
        }
    }
}

One possible solution for the second part of Stars:

public class Stars {
    public static void main(String[] args) {
        for (int i = 1; i <= 8; i++) {
            System.out.print("*");
        }
    }
}

Checkpoint: Congratulations!

Nice job making it this far -- this lab is tough! Feel free to work with the person next to you for this next portion of the lab. Labs are a unique opportunity (unlike homework) to collaborate on ideas, and practice peer programming.

These next problems get a little more challenging as we explore for loops a little more.

We put a lot of problems in here so that you have plenty to refer back to later when working on homework. Don't feel bad if you don't finish all of them -- Whitaker can't finish them all in a 50-minute lab, either! :)

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 : 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 : loopSquares

Write a for loop that produces the following output:

1 4 9 16 25 36 49 64 81 100

Check your answer using PracticeIt!

Exercise : Writing a nested `for` loop

Write a Java program called StarSquared that uses nested for loops (one inner, one outer) to produce the following output:

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

Then, modify the program to produce the following output:

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

`StarSquared` Part 1 Solution

One possible solution for the first problem:

public class StarSquared {
    public static void main(String[] args) {
        for (int i = 1; i <= 4; i++) {
            for (int j = 1; j <= 4; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

`StarSquared` Part 2 Solution

One possible solution for the second problem:

public class StarSquared {
    public static void main(String[] args) {
        for (int i = 1; i <= 4; i++) {
            for (int j = 1; j <= 10; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

Exercise : `for` loop practice

Copy and paste the following code into jGrasp.

public class Triangle {
    public static void main(String[] args) {
        for (int line = 1; line <= 4; line++) {
            for (int stars = 1; stars <=  expression ; stars++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

continued on the next slide...

Exercise - fill in a table

We want to produce the following output:

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

Fill in the table below indicating how many stars appear on each line of output.

Line	Stars
`1`	7
`2`	5
`3`	3
`4`	1

Exercise - complete the expression

We need an expression for the number of stars on each line of this form:

multiplier * line + constant

First pick a multiplier. Looking at the table, the line numbers go up by 1 each time, and the number of stars changes by how much each time? -2
Now pick a constant. Look at the table to see how many stars are on line 1. Given your multiplier, what constant do you need to use in the expression to get that number of stars for line 1? 9
Using your multiplier and constant, fill in the expression for this line of code in jGRASP:
```
for (int stars = 1; stars <=  expression ; stars++) {
```
Your program should now produce the correct output. You can verify with the Output Comparison Tool.

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 : 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

Class constants are variables that are assigned and unchangeable throughout a program. They make it simpler to improve the flexibility of code.

Example:

public class Birthday {
    
    public static final int MY_AGE = 18;
    
    public static void main(String[] args) {
        System.out.println("I am " + MY_AGE + "years old!");
    }
}

Exercise : SlashFigure2

Make a table of "\", "!", and "/" counts in the size 4 figure. Then, write a for loop to produce the size 4 figure. Finally, compare the loop tests for the size 4 and 7 figures, and write a for loop with a class constant that will produce a figure of any SIZE.

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

size 4

size 7

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

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

Test your code in the Output Comparison Tool or in PracticeIt!

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 2: Expressions, Variables, and Loops

University of Washington, CSE 142

Lab 2: Expressions, Variables, and Loops

Basic lab instructions

Today's lab

Expressions

Operators

Exercise : Expressions (2.1)

String Concatenation

String Concatenation with numbers

Exercise : More expressions (2.1)

jGRASP Interactions Pane

Exercise : Using jGRASP Interactions Pane

Variables

Exercise : Variable declaration syntax

Exercise : Variable assignment syntax

Variable Mutation

Exercise : Variable mutation

Exercise : a, b, and c

print vs println

for loops

for loops

Exercise : for loop repeating

Exercise : print, println, and for loops

PrintLoops Solution

Exercise : Writing a for loop!

Stars Solution

Checkpoint: Congratulations!

Exercise : simple for loop

Exercise : Verify solution in Practice-It!

Exercise : What's the output?

Exercise : loopSquares

Exercise : Writing a nested for loop

StarSquared Part 1 Solution

StarSquared Part 2 Solution

Exercise : for loop practice

Exercise - fill in a table

Exercise - complete the expression

Exercise : Number lines, part 1

Exercise : Number lines, part 2

Exercise : SlashFigure

Class Constants

Exercise : SlashFigure2

If you finish them all...

Exercise : `a`, `b`, and `c`

`print` vs `println`

`for` loops

`for` loops

Exercise : `for` loop repeating

Exercise : `print`, `println`, and `for` loops

`PrintLoops` Solution

Exercise : Writing a `for` loop!

`Stars` Solution

Exercise : simple `for` loop

Exercise : Writing a nested `for` loop

`StarSquared` Part 1 Solution

`StarSquared` Part 2 Solution

Exercise : `for` loop practice