University of Washington, CSE 142

Lab 2: Expressions, Variables, and Loops

Except where otherwise noted, the contents of this document are Copyright 2013 Stuart Reges and Marty Stepp.

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.

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...

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.");

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");

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:

1. initialization :: int i = 1; :: start a counter at 1
2. test :: i <= 3; :: continue as long as the counter i is less than 3
3. execute the statements :: { System.out.println("We're number one!"); }
4. update :: i++ :: add 1 to the counter
5. 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");

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!