University of Washington, CSE 142 (190)

Lab 2: Expressions, Variables, and Loops

Except where otherwise noted, the contents of this document are Copyright 2010 Stuart Reges and Marty Stepp.

lab document created by Whitaker Brand and Marty Stepp

Basic lab instructions

• We encourage you to 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.
• You probably won't finish all of the exercises. Just do as much as you can within the allotted time. You don't need to keep working on these exercises after you leave the lab.
• 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
• draw patterned figures of text using loops

Exercise : Expressions (2.1)

Write the results of each of the following expressions.

4

19

-16

3

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

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++; 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
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 : simple for loop

• Copy and 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
  

• Use the Output Comparison Tool to check your work.

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.

7

5

3

1

continued on the next slide...

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 : 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 : for loop practice

• Copy and paste the following code into jGrasp.

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

continued on the next slide...

Exercise - expected output

• Modify the code to produce the song 99 Bottles of Beer on the Wall:

  99 bottles of beer on the wall, 99 bottles of beer
  Take one down, pass it around, 98 bottles of beer on the wall
  
  98 bottles of beer on the wall, 98 bottles of beer
  Take one down, pass it around, 97 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
  

• Use the Output Comparison Tool to check your work. You can set your program to count down from 9 to make it easier to copy the output.

Exercise : More Expressions (2.1)

Write the results of each of the following expressions.

-10

7

9.0

9.0

1.75

2.0

10.5

Exercise : i, j, and k

What are the values of i, j, and k after the following statements?
Write your answers on the right. (What is the code really doing?)

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

(answer on next slide)

Exercise - answer

• i : 4
• j : 2
• k : 3
• The code is "rotating" the values of the three variables, swapping each variable's value for the previous variable's. i takes k's value; j takes i's value; and k takes j's value.

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.

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;

Exercise : What's the output?

• What output is produced by the following Java program?

  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;
          }
      }
  }
  

• Answer:

  32
  16
  8
  4
  

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 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 : 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 : printing a design

• Write a complete Java program to produce the following output:

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

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

Exercise : SlashFigure

• Write a complete Java program to produce the following output with nested for loops:

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

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

Exercise : SlashFigure2

• Add a class constant so your program can be resized from its default of 6:

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

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 Chapter 2 and try to solve them! We suggest doing some Exercises related to loops.