University of Washington, CSE 142 (190)

Lab 4: if/else, Scanner, and return

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:

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

Exercise : Math Expressions

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

10.0

16.0

8.0

20.0

3

1.0

-11.75

15.0

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 yet
          message(myAge);
      }
  
      // Displays a single message about driving to the user based on the given age
      public static void message(int age) {
          if (myAge >= 16) {
              System.out.println("I'm old enough to have a driver's license!");
          }
          if (myAge <= 16) {
              System.out.println("I'm 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 yet
          message(myAge);
      }
  
      // Displays a single message about driving to the user based on the given age
      public static void message(int age) {
          if (age >= 16) {
              System.out.println("I'm old enough to have a driver's license!");
          } else {
              System.out.println("I'm 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 : 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 : area

Write a method named area that accepts the radius of a circle as a parameter and returns the area of a circle with that radius. For example, the call area(2.0) should return 12.566370614359172.

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

Exercise : WazzuAdmit

Write a complete program WazzuAdmit 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 of 1.8 or an SAT score of 900 or above (or both) will cause the student to be accepted; anything less will cause him/her to be rejected.

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

Exercise - answer

import java.util.*;   // for Scanner

public class WazzuAdmit {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.println("Washington State University admission program");

        System.out.print("What is your GPA? ");
        double gpa = console.nextDouble();
        System.out.print("What is your SAT score? ");
        int sat = console.nextInt();

        if (gpa >= 1.8 || sat >= 900) {
            System.out.println("You were accepted!");
        } else {
            System.out.println("You were rejected!");
        }
    }
}

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 : pay

Write a method named pay that accepts a real number for a TA's salary and an integer for the number of hours the TA worked this week, and returns 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.)

Exercise : seeMovie

You're thinking about going with your friends to a movie. Write a Java method seeMovie that accepts two parameters: the cost of a ticket in dollars, and the rating number of stars the movie received out of 5. The method should print how interested you are (very, sort-of, or not). Use the following criteria:

• You like bargains. Any movie that costs less than $5.00 is one that you want to see very much.
• You dislike expensive movies. You are not interested in seeing any movie that costs $12.00 or more, unless it got 5 stars (and even then, you are only sort-of interested).
• You like quality. You are very interested in seeing 5-star movies that cost under $12.00.
• You are sort-of interested in seeing movies costing between $5.00 - $11.99 that also got between 2-4 stars inclusive.
• You are not interested in seeing any other movies not described previously.

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 : 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.

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

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.

• (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.

Exercise : smallestLargest

Write a method named smallestLargest that prompts the user to enter numbers, reads them, then prints the smallest and largest of all the numbers typed in by the user. For example:

How many numbers do you want to enter? 4
Number 1: 5
Number 2: 11
Number 3: -2
Number 4: 3
Smallest = -2
Largest = 11

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 4 and try to solve them!