University of Washington, CSE 142 (190)

Lab 4: `if/else`, `Scanner`, and `return`

lab document created by Whitaker Brand and Marty Stepp

Today's lab

Goals for today:

practice checking scores on myuw, downloading textbook program examples, using the jGRASP debugger
use return values to send data between methods
use Scanner to create interactive programs that read user input
understand if/else statements to select between multiple code actions
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 lab assignments are available for you to view in MyUW. Follow the instructions on this page to check your scores. For each lab, there is an attendance score and a homework score. Because this is lab 4, you shouldn't expect to see scores yet for lab 3, but you should see scores for labs 1 and 2. You need to get credit for both attendance and homework to get credit for a lab. Make sure that you have scores for lab 1 and lab 2. If you don't have credit and you think you should, contact the head TA managing the labs, Will Beebe (nedrager@cs.washington.edu).

Using the information on your MyUW page, fill in the values below.

class mean for a1 (i.e., average score for assignment 1)		8.81
class mean for lab1 (i.e., average score for lab 1)		0.98

Exercise : area

In lecture we wrote 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;
}

You are going to write a similar method for performing a particular mathematical computation. Write a method called 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 computes as pi 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 : `Scanner`

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`

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 : `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);
}

`mystery(40);`		40 20
`mystery(8);`		8 15
`mystery(0);`		0 17
`mystery(12);`		12 16
`mystery(20);`		20 20

Exercise : `if`/`else` mystery

Consider the following Java code.

public static void mystery2(int a, int b) {
    if (a < b) {
        a = a * 2;
    }
    if (a > b) {
        a = a - 10;
    } else {
        b++;
    }
    System.out.println(a + " " + b);
}

Fill in the boxes with the output produced by each of the method calls.

`mystery2(10, 3);`		0 3
`mystery2(6, 6);`		6 7
`mystery2(3, 4);`		-4 4
`mystery2(4, 20);`		8 21

Exercise : `if`/`else` mystery

Consider the following Java code.

public static void mystery3(int x, int y) {
    int z = 4;
    if (z <= x) {
        z = x + 1;
    } else {
        z = z + 9;
    }
    if (z <= y) {
        y++;
    }
    System.out.println(z + " " + y);
}

Fill in the boxes with the output produced by each of the method calls.

`mystery3(3, 20);`		13 21
`mystery3(4, 5);`		5 6
`mystery3(5, 5);`		6 5
`mystery3(6, 10);`		7 11

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.)

`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

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 : Syntax errors

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

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

line 5: if statement should use () parentheses, not {} brackets
line 5: = should be ==
line 5: smaller is out of scope here
line 10: void should be int
line 13: => should be >= (or better yet, no if test is needed)
line 16: should not write variable's type of int when returning it
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 : 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 : 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 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!

University of Washington, CSE 142 (190)

Lab 4: if/else, Scanner, and return

University of Washington, CSE 142 (190)

Lab 4: if/else, Scanner, and return

Today's lab

Exercise : Check MyUW scores

Exercise : area

Exercise : Scanner

Exercise : Scanner

Exercise : if/else mystery

Exercise : if/else mystery

Exercise : if/else mystery

Exercise : pay

if/else Factoring

Exercise : if/else Factoring

Exercise : numUnique

Exercise : pow

Exercise : smallestLargest

Exercise : AgeCheck

Exercise - things to fix

Exercise - answer

Exercise : AgeCheck2

Exercise : Syntax errors

Exercise - answer

Exercise - Corrected version

Exercise : WazzuAdmit

Exercise - answer

Exercise : seeMovie

If you finish them all...

Lab 4: `if/else`, `Scanner`, and `return`

Lab 4: `if/else`, `Scanner`, and `return`

Exercise : `Scanner`

Exercise : `Scanner`

Exercise : `if`/`else` mystery

Exercise : `if`/`else` mystery

Exercise : `if`/`else` mystery

`if`/`else` Factoring

Exercise : `if`/`else` Factoring