University of Washington, CSE 142

Lab 1: Expressions and Variables; 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

• 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:

• 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

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.

Exercise : Expressions (2.1)

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

4

19

-16

3

9.0

1.75

Exercise : More expressions (2.1)

Write the results of each of the following expressions.

"8cse 142"

"cse 14226"

9.0

2.0

10.5

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.

55299

7.3535

1024

"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?

1. grade : 4.0;
2. int grade = 4.0;
3. 4.0 = grade;
4. grade = 4.0;
5. double grade = 4.0;
6. 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?

1. grade : 4.0;
2. double grade = 4.0;
3. grade = 4.0;
4. 4.0 = grade;
5. set grade = 4.0;

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?

1. balance + 5;
2. balance = 5;
3. balance = balance + 5;
4. 5 + balance = balance;
5. 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

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 : 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!

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;
        }
    }
}

32
16
8
4

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 : ComputePay

The following program redundantly repeats the same expressions many times. Download it and open it in jGRASP, then modify the program to remove the redundancy using variables. Use an appropriate type for each variable.

• ComputePay.java

The program's output should be the same after your modifications. No expression should be computed more than once in the code.

Exercise : Syntax errors

• The following program contains 9 mistakes! What are they? Copy and paste the following code into jGRASP and correct the various mistakes. The answer is on the next 2 slides if you need some help.

  1 2 3 4 5 6 7 8 9 10 11 12 13

  public class Oops { public static void main(String[] args) { int x; System.out.println("x is" x); int x = 15.2; // set x to 15.2 System.out.println("x is now + x"); int y; // set y to 1 more than x y = int x + 1; System.out.println("x and y are " + x + and + y); } }

answer on next slide...

Exercise - answer

1. line 4: missing + between "x is" and x
2. line 4: cannot print the value of x before assigning it a value
3. line 6: cannot assign 15.2 into a variable of type int
4. line 6: should not redeclare the variable's type
5. line 7: " mark should be between now and +
6. line 10: should not write the word int here
7. line 10: variable y should be same type as x
8. line 10: does not properly set y to be 1 more than x (should not write the word int here)
9. line 11: and should be in quotes with surrounding spaces

Exercise - corrected version

• Here is a corrected version of the program:

  public class Oops {
      public static void main(String[] args) {
          double x = 0.0;
          System.out.println("x is" + x);
  
          x = 15.2;       // set x to 15.2
          System.out.println("x is now " + x);
  
          double y;       // set y to 1 more than x
          y = x + 1;
          System.out.println("x and y are " + x + " and " + y);
      }
  }
  

Exercise : i, j, and k

What are the values of i, j, and k after the following statements?

int i = 2;
int j = 3;
int k = 4;
int x = i + j + k;

i = x - i - j;            // i? 4
j = x - j - k;            // j? 2
k = x - i - k;            // k? 1

Exercise : Equation

Suppose you have a real number variable x. Write a Java expression that computes a variable named y storing the following value:

y = 12.3x⁴ - 9.1x³ + 19.3x² - 4.6x + 34.2

(We haven't learned a way to do exponents yet, but you can simulate them using several multiplications.)

Use the example program on the next slide to test your code.

Exercise - Example code

Copy/paste this program into jGRASP to test your solution.

// expected output:
// y is 7043.7

public class EquationY {
    public static void main(String[] args) {
        double x = 5;

        double y = put your expression for y here ;

        System.out.println("y is " + y);
    }
}

(answer on next slide)

Exercise - answer

double y = 12.3*x*x*x*x - 9.1*x*x*x + 19.3*x*x - 4.6*x + 34.2;

If you want an added challenge, try to come up with a way to compute the above value while using the * operator no more than 4 times.

(click Next → for answer)

double y = (((12.3 * x - 9.1) * x + 19.3) * x - 4.6) * x + 34.2;

Static methods

Recall the syntax for writing a static method. Methods are useful for representing a program's structure and capturing common code to avoid redundancy:

public static void name() {
    statements;
}

Example:

public static void song() {
    System.out.println("This is the song that never ends,");
    System.out.println("Yes, it goes on and on, my friends.");
}

Exercise : FightSong

Go, team, go!
You can do it.

Go, team, go!
You can do it.
You're the best,
In the West.
Go, team, go!
You can do it.

Go, team, go!
You can do it.
You're the best,
in the West.
Go, team, go!
You can do it.

Go, team, go!
You can do it.

The following program produces the output at left, but it has poor structure and redundancy. Download it and open it in jGRASP, then add at least two static methods.

• FightSong.java

continued on the next slide...

Exercise : FightSong, cont'd

Go, team, go!
You can do it.

Go, team, go! 
You can do it.
You're the best,
In the West.
Go, team, go!
You can do it.

Go, team, go! 
You can do it.
You're the best,
In the West.
Go, team, go!
You can do it.

Go, team, go!
You can do it.

Did you choose your methods well? Avoid the following pitfalls:

• Does the same println statement occur more than once in your code?
• Are there any println statements in the main method?
• Is your main method a good summary of the overall program? (If main is only 1-2 lines long, it is not a good summary.)
• Do you have any patterns of method calls that are repeated in the code?

Compare your method choice with your neighbor's. Discuss any different decisions you made and ask a TA if you have questions about them.

Exercise : Lanterns

• Write a complete program named Lanterns that produces the following output. Use static methods to capture structure and remove redundancy. (Check your output on the Comparison Tool.)

      *****
    *********
  *************
  
      *****
    *********
  *************
  * | | | | | *
  *************
      *****
    *********
  *************
  
      *****
    *********
  *************
      *****
  * | | | | | *
  * | | | | | *
      *****
      *****
  

Exercise : Birthday variables

• Create a complete Java program in a class named Bday that declares four variables and assigns appropriate values to them.
  • your birthday month (1-12)
  • your birthday day (1-31)
  • the birthday month of another student sitting near you today (1-12)
  • the birthday day of that same student near you (1-31)
• Ask your neighbor for their name and for the proper numbers to store in the variables for his/her birthday. Then produce output in this format using your four variables:

  My birthday is 9/19, and Suzy's is 6/14.
  

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!