University of Washington, CSE 142

Lab 4: Return, if/else, and Scanner

Except where otherwise noted, the contents of this document are Copyright 2013 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 if and else to have different branches of execution
• use Scanner to create interactive programs that read user input
• use Strings to represent and manipulate text data
• practice cumulative algorithms for complex computations
• Where you see this icon, you can click it to check the problem in Practice-It!

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.)

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);

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.

Exercise : repl

• Write a method named repl that accepts a String and a number of repetitions as parameters and returns the String concatenated that many times. For example, the call repl("hello", 3) returns "hellohellohello". If the number of repetitions is 0 or less, an empty string is returned.
• Try solving this problem in Practice-It! using the link above.
• (Hint: This is best solved with a cumulative algorithm. Start with an empty string and build it up piece by piece.)

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.

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.

13 21

5 6

6 5

7 11

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 : 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 : 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.)

Exercise : jGRASP Debugger

We are going to practice using the jGRASP debugger with Hailstone.java. This program computes a sequence of integers called a hailstone sequence. (This is related to an unsolved problem in mathematics known as the Collatz Conjecture.)

• Recall: To Set a breakpoint, move your cursor to the beginning of the desired line until you see a stop-sign icon and click.
• Recall: To run a program in debug mode, click the debug icon (looks like a ladybug).
• Recall: To resume a stopped program, press the top-left play or step buttons.

continued on the next slide...

Exercise - Values of value

• For this exercise you will trace the first call: printHailstoneMaxMin(7, 10);
• Set a breakpoint that will allow you to determine the first several distinct values of the variable value as the loop executes.

11

34

17

52

continued on the next slide...

Exercise - Values of min

• Now you will trace the second call: printHailstoneMaxMin(7, 20);
• Clear your previous breakpoint by clicking on it a second time, then set a new breakpoint for the second method call, and resume execution.
• Use your new breakpoint to determine the first several distinct values of min during the second call.

5

4

2

1

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 : season

• Write a method named season that takes two integers as parameters representing a month and day and that returns a String indicating the season for that month and day. Assume that months are specified as an integer between 1 and 12 (1 for January, 2 for February, and so on) and that the day of the month is a number between 1 and 31.
• If the date falls between 12/16 and 3/15, you should return "Winter". If the date falls between 3/16 and 6/15, you should return "Spring". If the date falls between 6/16 and 9/15, you should return "Summer". And if the date falls between 9/16 and 12/15, you should return "Fall".
• Try solving this problem in Practice-It! using the link above.

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.

0 3

6 7

-4 4

8 21

String methods

Exercise : String expressions

Write the results of each expression with Strings in "quotes" and characters in single quotes ('a')

//       index 0123456789012345
String str1 = "Frodo Baggins";
String str2 = "Gandalf the GRAY";

13

'a'

'G'

2

"GANDALF THE GRAY"

-1

"o Baggins"

"dalf the GR"

"Goondoolf the GRAY"

"Gandalf the GRAY"

"strange1"

Exercise : ProcessName

Copy/paste and save ProcessName.java in jGRASP, then go to the next slide.

import java.util.*;  // for Scanner

public class ProcessName {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        
        // your code goes here
        
        System.out.println("Your name is: " + name);
    }
}

continued on the next slide ...

Exercise - code to add

• Add code to the program so that it reads the user's first and last name (read an entire line as a single string), then prints the last name followed by a comma and the first initial. (Assume that the user types a valid name.) Example:

  Type your name: Jessica Miller
  Your name is: Miller, J.
  

• Notice that the program reads an entire line of user input, not just one word.
• Try solving this problem in Practice-It! using the link 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 the book and try to solve them!