University of Washington, CSE 142

Lab 3: Return, if/else, and Scanner

Except where otherwise noted, the contents of this document are Copyright 2012 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:

• use return values to send data between methods
• use if/else statements to select between multiple code actions
• use Scanner to create interactive programs that read user input
• Where you see this icon, you can click it to check the problem in Practice-It!

Exercise : Check MyUW scores

• The scores you receive on homework and labs are available for you to view in MyUW. Follow the instructions on our MyUW page to check your scores.
• You should see scores for labs 1 and 2. If not, contact your TA or a TA from your lab period.
• Using the information on your MyUW page, fill in the values below.

9.14

1.92

Returning Values

A return value is when a method sends a value back to the code that called it.

public static type name(parameters) {      // declare
    ...
    return expression;
}

variableName = methodName(parameters);     // call

Example:

public static double fToC(double tempF) {
    return (tempF - 32) * 5.0 / 9.0;
}
...
double bodyTemp = fToC(98.6);          // bodyTemp stores 37.0
double freezing = fToC(32);            // freezing stores  0.0

Math expression syntax

Exercise - Math expressions

Write the results of each expression. Use the proper type (such as .0 for a double). Note that a variable's value changes only if you re-assign it using the = operator. Discuss any errors you make with your neighbor.

double grade = 2.7;
Math.round(grade);                               // grade = 2.7
grade = Math.round(grade);                       // grade = 3.0

double min = Math.min(grade, Math.floor(2.9));   //   min = 2.0

double x = Math.pow(2, 4);                       //     x = 16.0
x = Math.sqrt(64);                               //     x = 8.0

int count = 25;
Math.sqrt(count);                                // count = 25
count = (int) Math.sqrt(count);                  // count = 5

int a = Math.abs(Math.min(-1, -3));              //     a = 3

Exercise : area

Consider the following method for converting milliseconds into days:

// converts milliseconds to days
public static double toDays(double millis) {
    return millis / 1000.0 / 60.0 / 60.0 / 24.0;
}

Write a similar method named area that takes as a parameter the radius of a circle and that returns the area of the circle. For example, the call area(2.0) should return 12.566370614359172. Recall that area can be computed as π times the radius squared and that Java has a constant called Math.PI.

• (Try solving this problem in Practice-It by clicking the icon above.)

Exercise : pay

Write a method named pay that accepts two parameters: a real number for a TA's salary, and an integer for the number of hours the TA worked this week. The method should return how much money to pay the TA. For example, the call pay(5.50, 6) should return 33.0.

The TA should receive "overtime" pay of 1 ½ normal salary for any hours above 8. For example, the call pay(4.00, 11) should return (4.00 * 8) + (6.00 * 3) or 50.0.

• (Try solving this problem in Practice-It by clicking the icon above.)

if/else Statements

An if/else statement lets your program choose between 2 or more options.

if (test) {
    statement(s);
} else {
    statement(s);
}

Example:

if (gpa >= 2.0) {
    System.out.println("Welcome to Mars University!");
} else {
    System.out.println("Please apply again soon.");
}

Exercise : if/else mystery

Consider the following Java code. Fill in the boxes with the output produced by each of the method calls.

public static void mystery(int n) {
    System.out.print(n + " ");
    if (n > 10) {
        n = n / 2;
    } else {
        n = n + 7;
    }
    if (n * 2 < 25) {
        n = n + 10;
    }
    System.out.println(n);
}

40 20

8 15

0 17

12 16

20 20

Exercise : numUnique

Write a method named numUnique that accepts three integers as parameters and that returns the number of unique integers among the three. For example, the call numUnique(18, 3, 4) should return 3 because the parameters have 3 different values. By contrast, the call numUnique(6, 7, 6) would return 2 because there are only 2 unique numbers among the three parameters: 6 and 7.

Compare your solution to your neighbors'. Did you all solve it the same way?

• (Try solving this problem in Practice-It by clicking the icon above.)

User input and Scanner

Example:

import java.util.*;   // so you can use Scanner
...
Scanner console = new Scanner(System.in);
System.out.print("How old are you? ");   // prompt
int age = console.nextInt();
System.out.println("You typed " + age);

Exercise : DevryAdmit

Write a complete program DevryAdmit with the behavior shown below. Use the Scanner to read user input for a student's grade point average and SAT exam score. A GPA below 1.8 will cause the student to be rejected; an SAT score below 900 will also cause a rejection. Otherwise the student is accepted.

Devry University admission program
What is your GPA? 3.2
What is your SAT score? 1280
You were accepted!

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

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 : Scanner sum

Copy and paste the following code into jGrasp.

public class SumNumbers {
    public static void main(String[] args) {
        int low = 1;
        int high = 1000;
        int sum = 0;
        for (int i = low; i <= high; i++) {
            sum += i;
        }
        System.out.println("sum = " + sum);
    }
}

continued on next slide...

Exercise : Scanner sum

Modify the code to use a Scanner to prompt the user for the values of low and high. Below is a sample execution in which the user asks for the same values as in the original program (1 through 1000):

low? 1
high? 1000
sum = 500500

Below is an execution with different values for low and high:

low? 300
high? 5297
sum = 13986903

You should exactly reproduce this format.

Use the Output Comparison Tool to check your work.

Exercise : pow

Write a method named pow that accepts a base and an exponent as parameters and returns the base raised to the given power. For example, the call pow(3, 4) returns 3 * 3 * 3 * 3 or 81. Do not use Math.pow in your solution; use a cumulative algorithm instead. Assume that the base and exponent are non-negative. See ch4 lecture slides on cumulative sums for a hint.

• (Try solving this problem in Practice-It by clicking the icon above.)
• For added challenge, try turning your solution into a second version pow2 that works with real number bases and negative exponents, as in book Exercise 4.11.

if/else factoring

• It's easy to introduce redundancy in the branches of an if/else. For example:

  if (x < 30) {
      a = 2;
      x++;
      System.out.println("CSE 142 TAs are awesome! " + x);
  } else {
      a = 2;
      System.out.println("CSE 142 TAs are awesome! " + x);
  }
  

• The code can be factored out to become: (notice that the else went away!)

  a = 2;
  if (x < 30) {
      x++;
  }
  System.out.println("CSE 142 TAs are awesome! " + x);
  

Exercise : if/else Factoring

• Download the following program FactorExample.java to your machine and open it with jGrasp.
• The program's method is redundant. Factor the method, restructuring the code to eliminate unnecessary statements while retaining the same behavior.
• Insert some test calls of the method in main and run it to make sure it works properly.

Exercise : Syntax errors

• The following Java program has 7 errors. Can you find all of them?

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

  public class IfOops { public static void main(String[] args) { int a = 7, b = 42; minimum(a, b); if {smaller = a} { System.out.println("a is the smallest!"); } } public static void minimum(int a, int b) { // returns which int is smaller if (a < b) { int smaller = a; } else (a => b) { int smaller = b; } return int smaller; } }

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

Exercise - answer

1. line 5: if statement should use () parentheses, not {} brackets
2. line 5: = should be ==
3. line 5: smaller is out of scope here
4. line 10: void should be int
5. line 13: => should be >= (or better yet, no if test is needed)
6. line 16: should not write variable's type of int when returning it
7. line 16: int smaller is out of scope here (declare outside if or return directly)

Exercise - Corrected version

• public class IfOops {
      public static void main(String[] args) {
          int a = 7, b = 42;
          int smaller = minimum(a, b);
          if (smaller == a) {
              System.out.println("a is the smallest!");
          }
      }
  
      public static void int minimum(int a, int b) {  // returns which int is smaller
          int smaller;
          if (a < b) {
              int smaller = a;
          } else if (a >= b) {
              int smaller = b;
          }
          return int smaller;
      }
  }
  

Exercise : AgeCheck

• Copy/paste and save the following program in jGRASP, then see the instructions on the next slide.

  public class AgeCheck {
      public static void main(String[] args) {
          int myAge = 19;   // I am 19; let me see if I can drive
          message(myAge);
      }
  
      // Displays message about driving to user based on given age
      public static void message(int age) {
          if (myAge >= 16) {
              System.out.println("I'm old enough to drive!");
          }
          if (myAge <= 16) {
              System.out.println("Not old enough yet... :*(");
          }
      }
  }
  

Exercise - things to fix

• The program has a few syntax errors. Fix them until it compiles.
• The code has a logic problem. (For some value(s), it prints the wrong answer.) Find any such problems and fix them. (You may need to run the program a few times and try different values to see which ones fail.)
• The program uses if and else in a clumsy way. Improve the style of the code.

Exercise - answer

• The following is a corrected version of the program:

  public class AgeCheck {
      public static void main(String[] args) {
          int myAge = 19;   // I am 19; let me see if I can drive
          message(myAge);
      }
  
      // Displays a message about driving to user based on given age
      public static void message(int age) {
          if (age >= 16) {
              System.out.println("I'm old enough to drive!");
          } else {
              System.out.println("Not old enough yet... :*(");
          }
      }
  }
  

Exercise : AgeCheck2

• Now replace your previous AgeCheck program's message method with:

      // Possibly prints some message(s) to the user based on the given age
      public static void message(int age) {
          if (age >= 21) {
              System.out.println("I can legally purchase alcohol!");
          } else if (age >= 17) {
              System.out.println("I can purchase a ticket to an R-rated movie.");
          } else if (age >= 16) {
              System.out.println("I can get my driver's license!");
          }
      }
  

• For some age(s), the message(s) printed are not accurate. (In some cases, it should print no messages; in some cases, one message; and in others, many messages.) Improve the usage of ifs and elses in this method to behave properly.

Exercise : Spiral

Write a Java program that draws the following output using a for loop.

• window size: 170 x 170 px
• "spiral" lines starting from (0, 10), going right 160, down 150, left 150, up 140, right 140, down 130, left 130, up 120, ...

Exercise - answer

import java.awt.*;

public class Spiral {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(170, 170);
        Graphics g = panel.getGraphics();
        for (int i = 0; i < 8; i++) {
            g.drawLine(      10*i,   10 + 10*i, 160 - 10*i,  10 + 10*i);  // top
            g.drawLine(160 - 10*i,   10 + 10*i, 160 - 10*i, 160 - 10*i);  // right
            g.drawLine( 10 + 10*i,  160 - 10*i, 160 - 10*i, 160 - 10*i);  // bottom
            g.drawLine( 10 + 10*i,   20 + 10*i,  10 + 10*i, 160 - 10*i);  // left
        }
    }
}

Exercise - answer, alternative version

import java.awt.*;

public class Spiral {
    public static void main(String[] args) {
        DrawingPanel panel = new DrawingPanel(170, 170);
        Graphics g = panel.getGraphics();
        int x = 0, y = 10;
        int len = 160;
        for (int i = 0; i < 8; i++) {
            g.drawLine(x, y, x + len, y);  // right
            x = x + len;
            len = len - 10;
            g.drawLine(x, y, x, y + len);  // down
            y = y + len;
            g.drawLine(x, y, x - len, y);  // left
            x = x - len;
            len = len - 10;
            g.drawLine(x, y, x, y - len);  // up
            y = y - len;
        }
    }
}

Exercise : Sierpinski Triangle

Write a Java program that draws the following output.... Just kidding!

If you finish them all...

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