University of Washington, CSE 142 (190)

Lab 3: Parameters

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

Today's lab

Goals for today:

• learn the basics of declaring and passing parameters to methods
• use Strings to represent and manipulate text data
• Learn to set a break with the jGRASP debugger to find out what is going on inside a program
• use the built-in Math library to perform mathematical computations
• Where you see this icon, you can click it to check the problem in Practice-It!

Exercise : Parameters

• The following code passes some parameters to a method. What output is produced by the two method calls? Write the output of each call in the box provided.

  public class MysteryNums {
      public static void main(String[] args) {
          int x = 15;
          sentence(x, 42);      // 15 42
  
          int y = x - 7;
          sentence(y, x + y);   // 8 23
      }
      
      public static void sentence(int num1, int num2) {
          System.out.println(num1 + " " + num2);
      }
  }
  

Exercise : String values

• The String constants we have been using since chapter 1 can be stored in variables of type String. What output is produced by the following code?

  String first = "James";
  String last = "Kirk";
  String middle = "T."
  System.out.println(last);                               // Kirk
  System.out.println("My name is " + first);              // My name is James
  System.out.println(first + " " + last);                 // James Kirk
  System.out.println(last + ", " + first + " " + middle); // Kirk, James T.
  System.out.println(middle + " is for Tiberius");        // T. is for Tiberius
  

Exercise : Parameter Mystery

• Fill in the boxes with the output that each method call would produce:

  public class MysterySoda {
      public static void main(String[] args) {
          String soda = "coke";
          String pop = "pepsi";
          String coke = "pop";
          String pepsi = "soda";
          String say = pop;
  
          carbonated(coke, soda, pop);         // say coke not pepsi or pop
          carbonated(pop, pepsi, pepsi);       // say soda not soda or pepsi
          carbonated("pop", pop, "koolaid");   // say pepsi not koolaid or pop
          carbonated(say, "say", pop);         // say say not pepsi or pepsi
      }
      public static void carbonated(String coke, String soda, String pop) {
          System.out.println("say " + soda + " not " + pop + " or " + coke);
      }
  }
  

Exercise : Parameter Mystery

• Fill in the boxes with the output that each method call would produce:

  public class MysteryNumbers {
      public static void main(String[] args) {
          String one = "two";
          String two = "three";
          String three = "1";
          int number = 20;
  
          sentence(one, two, 3);                  // three times two = 6
          sentence(two, three, 14);               // 1 times three = 28
          sentence(three, three, number + 1);     // 1 times 1 = 42
          sentence(three, two, 1);                // three times 1 = 2
          sentence("eight", three, number / 2);   // 1 times eight = 20
      }
      public static void sentence(String three, String one, int number) {
          System.out.println(one + " times " + three + " = " + (number * 2));
      }
  }
  

Exercise : Syntax errors

• The following Java program has 9 errors. Can you find all of them?

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

  public class Parameters { public static void main() { double bubble = 867.5309; double x = 10.01; printer(double x, double y); printer(x); printer("barack", "obama"); System.out.println("z = " + z); } public static void printer(x, y double) { int z = 5; System.out.println("x = " + double x + " and y = " + y); System.out.println("The value from main is: " + bubble); } }

• Copy and paste the code into jGrasp and see if you can fix the errors.

Exercise - answer

1. line 5: cannot use variable y without declaring and initializing it
2. line 5: cannot declare the type of y in the method call
3. line 6: cannot call printer without the correct number of parameters (2, in this case)
4. line 7: cannot call printer by passing the correct type of parameters (double, in this case)
5. line 8: cannot refer to the variable z: it is in scope inside printer, not main
6. line 11: must provide a type for x
7. line 11: must provide a variable name for the second parameter
8. line 12: must refer to the parameters using the exact same spelling
9. line 13: cannot refer to variables in main that were not passed into printer as a parameter

Exercise - Corrected version

• Here is a corrected version of the program:

  public class Parameters {
      public static void main() {
          double bubble = 867.5309;
          double x = 10.01;
          double y = 5.3;
          printer(double x, double y);
          printer(x, 0.0);
          printer("barack", "obama");
          int z = 5;
          System.out.println("z = " + z);
      }
  
      public static void printer(double x, double y) {
          System.out.println("x = " + x + " and y = " + y);
          System.out.println("The value from main is: " + bubble);
          int z = 5;
      }
  }
  

Exercise : jGRASP Debugger

• You will find it useful to be able to set a breakpoint to debug your programs. Copy and paste the following code into jGRASP:

  1 2 3 4 5 6 7 8 9 10

  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). You 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 instead of the run icon (it is to the right of the run icon and looks like a ladybug). It will stop every time it hits line 5. Then you can examine the values of the local variables i and number. When you want it to continue, you can hit the button that looks like the standard "play" icon of a right-pointing arrow. 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		42
  i = 2		9
  i = 3		24
  i = 4		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 stop on line number 7. This will allow you to find out the value of number after the loop is done executing. Again hit the "play" button to start the programming running again. What value does number have at line 7?

  69

Exercise : Math Expressions

continued on the next slide...

Exercise - Math Expressions

Write the results of each of the following expressions. Make sure to use the proper type (.0 for a double).

10.0

16.0

8.0

20.0

3

1.0

-11.75

15

Exercise : Printing Strings

• Write a method called printStrings that accepts a String and a number of repetitions as parameters and prints that String the given number of times. For example, the call:

  printStrings("abc", 5);
  

  will print the following output:

  abcabcabcabcabc
  

• Complete the program with a main method. Try calling your method several times from main and see what happens when you pass different values.

Cumulative algorithms

• A cumulative algorithm is one where you incrementally accumulate a larger value by repeatedly adding, multiplying, etc., and storing the result into a variable over and over.
• Key aspect of a cumulative algorithm: A loop, and a variable declared outside the loop whose value is modified inside the loop.
• Example: Cumulative algorithm to sum the numbers 1-100:

  int sum = 0;
  for (int i = 1; i <= 100; i++) {
      sum = sum + i;
  }
  System.out.println(sum);    // 5050
  

• Some of the following problems ask you to write cumulative algorithms.

Cumulative algorithms

• A cumulative algorithm is one where you incrementally accumulate a larger value by repeatedly adding, multiplying, etc., and storing the result into a variable over and over.
• Key aspect of a cumulative algorithm: A loop, and a variable declared outside the loop whose value is modified inside the loop.
• Example: Cumulative algorithm to sum the numbers 1-100:

  int sum = 0;
  for (int i = 1; i <= 100; i++) {
      sum = sum + i;
  }
  System.out.println(sum);    // 5050
  

• Some of the following problems ask you to write cumulative algorithms.

Exercise : Powers of 2

• Write a Java method printPowersOf2 that uses a cumulative algorithm to print out the powers of 2 up to 2^x. It should take a parameter that determines x; that is, the exponent of the highest power of 2 to be printed. For example, the following call:

  printPowersOf2(10);
  

  should produce the following output, because 2¹⁰ = 1024:

  1 2 4 8 16 32 64 128 256 512 1024
  

  (For this problem it's okay if your numbers have a .0 at the end of them.)
• Complete the program by writing a main method. Call your method several times in main to ensure that it works. Also check your answer using Practice-It by clicking the check-mark icon above.

Exercise : Powers of n

• Modify your program from the previous exercise to have a method printPowersOfN that prints the powers of any given number n up to n^x. For example, the following call:

  printPowersOfN(3, 5);
  

  should produce the following output, because 3⁵ = 243:

  1 3 9 27 81 243
  

  and the call:

  printPowersOfN(2, 10);
  

  should produce the same output as printPowersOf2(10); :

  1 2 4 8 16 32 64 128 256 512 1024
  

Exercise : printSquare

• Write a method printSquare that accepts min and max integers and prints a square of lines of increasing numbers. The first line should start with the minimum; each line that follows should start with the next-higher number. The numbers on a line wrap back to the minimum after it hits the maximum. For example, the call:

  printSquare(3, 6);
  

  should produce the following output:

  3456
  4563
  5634
  6345
  

• If the maximum passed is less than the minimum, the method produces no output. Check your answer using Practice-It from the check-mark icon above.

Exercise : printGrid

• Write a method called printGrid that accepts two integers representing a number of rows and columns and prints a grid of integers from 1 to (rows*columns) in column-major order.
• For example, the call:

  printGrid(4, 6);
  

  should produce the following output:

  1 5 9 13 17 21
  2 6 10 14 18 22
  3 7 11 15 19 23
  4 8 12 16 20 24
  

• Check your answer using Practice-It from the check-mark icon above.

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 3 and try to solve them! Note that you won't be able to do every problem from Chapter 3; some involve concepts we have not learned yet, such as return and Scanner.