Building Java Programs

Lab 4: Ch. 4: If/Else, Scanner, Cumulative Algorithms

Except where otherwise noted, the contents of this document are Copyright 2012 Stuart Reges and Marty Stepp.

lab document created by Marty Stepp and Stuart Reges

Today's lab

Goals for today:

• use if/else statements to select between alternate code paths
• use Scanner to 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!

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 : if/else mystery

Consider the following Java code.

public static void ifElseMystery(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

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 : 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?

  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 : 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.

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.

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.

Exercise : Syntax errors

• The following Java program has 11 errors. Can you find them all?

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

  public class StringOops { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("Type your name: "); String name = console.nextString(); process(name); } public static void process(string "name") { if (name == Whitaker) { System.out.println("You must be really awesome."); } replace("a", "e"); toUppercase(name); name.substring(0, 3); System.out.println(name + " has " + name.length + " letters"); } }

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

Exercise - answer

1. line 5: nextString should be next
2. line 9: string should be String
3. line 9: name should not be in quotes
4. line 10: Whitaker should be in quotes
5. line 10: cannot compare strings with ==; must use .equals
6. line 13: cannot call replace without specifying a string object (name)
7. line 14: toUppercase should be toUpperCase
8. line 14: name. should come before toUpperCase, not passed as a parameter to it
9. line 14: must say name = to store the result of toUpperCase
10. line 15: must say name = to store the result of substring
11. line 16: must use parentheses () when calling length

Exercise - Corrected version

public class StringOops {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);
        System.out.print("Type your name: ");
        String name = console.next();
        process(name);
    }

    public static void process(String "name") {
        if (name.equals("Whitaker")) {
            System.out.println("You must be really awesome.");
        }
        name = name.replace("a", "e");
        name = name.toUpperCase();
        name = name.substring(0, 3);
        System.out.println(name + " has " + name.length() + " letters");
    }
}

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 : 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.)

Exercise : longestName

Write a method named longestName that reads names typed by the user and prints the longest name (the name that contains the most characters) in the format shown below. Your method should accept a console Scanner and an integer n as parameters and should then prompt for n names.

A sample execution of the call longestName(console, 4) might look like the following:

name #1? roy
name #2? DANE
name #3? sTeFaNiE
name #4? Erik
Stefanie's name is longest

Try to solve this problem in Practice-It: click on the check-mark above!

Exercise : swapPairs

• Write a method named swapPairs that accepts a String as a parameter and returns that String with each pair of adjacent letters reversed. If the String has an odd number of letters, the last letter is unchanged. For example, the call swapPairs("forget") should return "ofgrte" and the call swapPairs("hello there") should return "ehll ohtree".
• Try solving this problem in Practice-It! using the link 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

Exercise : hopscotch

Write a method named hopscotch that accepts an integer parameter for a number of "hops" and prints a hopscotch board of that many hops.

For example, the call hopscotch(3); would produce the following output:

   1
2     3
   4
5     6
   7
8     9
   10

Try to solve this problem in Practice-It: click on the check-mark 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!