chapter 5 | dr. hala | programming 2

8/2/2019 Chapter 5 | Dr. Hala | Programming 2

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Building Java Programs

Chapter 5:Program Logic and Indefinite Loops

These lecture notes are copyright (C) Marty Stepp and Stuart Reges, 2005. They may not be

rehosted, sold, or modified without expressed permission from the authors. All rights reserved.


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Chapter outline

while loops

special types of loops

fencepost loops

sentinel loops

boolean logic

boolean expressions and variables

logical operators testing for valid user input

generating random numbers

the Random class


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While loops

suggested reading: 5.1 - 5.2.1

self-checks: #1 - 10


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Types of loops

definite loop: A loop that executes a known numberof times. The for loops we have seen so far are definite loops. We often

use language like "repeat _ times" or "for each of these things".

Examples:

Repeat these statements 10 times. Repeat these statements ktimes.

Repeat these statements for each odd number between 5 and 27.

indefinite loop: A loop where it is not easilydetermined in advance how many times it will execute. Indefinite loops often keep looping as long as a condition is

true, or until a condition becomes false.

Examples:

Repeat these statements until the user types a valid integer.

Repeat these statements while the number n is not prime.

Repeat these statements until a factor ofn is found.


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The while loop statement

The while loop is a new loop statement that is wellsuited to writing indefinite loops.

The while loop, general syntax:while () {

;}

Example:

int number = 1;

while (number


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While loop flow chart

The execution of a while loop can be depicted as the following:

|

V

+---------------+

+------+

| +---------------+ || ^ V

V | +-----------------------------------+

| | | execute the controlled statements |

| | +-----------------------------------+

| ^ |

V | V| | |

| +---


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Example while loop

A loop that finds and prints the first factor of a number(other than 1):Scanner console = new Scanner(System.in);

System.out.print("Type a number: ");

int number = console.nextInt();

int factor = 2;

while (number % factor != 0) {

factor++;

}

System.out.println("First factor: " + factor);

OUTPUT:Type a number: 49

First factor: 7


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Equivalence of for,while loops

Any for loop of the following form:

for (; ; ) {

;}

can be replaced by a while loop of the following form:

;

while () {

;;

}


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for/while loop example

What while loop is essentially equivalent to thefollowing for loop?for (int i = 1; i


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While loop problem

Write a piece of Java code that uses a while loop torepeatedly prompt the user to type a number until theuser types a non-negative number, then square it.

Expected output:Type a non-negative integer: -5

Invalid number, try again: -1Invalid number, try again: 11

11 squared is 121

Solution:

System.out.print("Type a non-negative integer: ");int number = console.nextInt();while (number < 0) {

System.out.print("Invalid number, try again: ");number = console.nextInt();

}

int square = number * number;System.out.println(number + " squared is " + square);


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Fencepost loops

suggested reading: 5.2.1

self-checks: #11-12


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The fencepost problem

Problem: Write a static method named printNumbersthat prints each number from 1 to a given maximum,which is passed as a parameter, separated by commas.Assume that the maximum number passed in is greaterthan 0. For example, the method call:printNumbers(5)

should print:

1, 2, 3, 4, 5

Let's write a solution to this problem...


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Flawed solutions

public static void printNumbers(int max) {

for (int i = 1; i


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Fence posts

The problem here is that if we are printing n numbers,we only need n - 1 commas.

This problem is similar to the task of building a fence,where lengths of wire are separated by posts.

If we repeatedly place a post and place a length, wewill never have an end post.

A flawed algorithm:

for (length of fence):

place some post.

place some wire.


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Fencepost solution

The key to solving the fencepost problem is to add anextra statement outside the loop that places the initalpost. This is sometimes also called the "loop-and-a-half" solution.

We will encounter this concept many times in this chapter when

using indefinite while loops. The revised algorithm:

place a post.

for (length of fence - 1):

place some wire.place some post.


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Fencepost printNumbers

A version of printNumbers that works:

public static void printNumbers(int max) {

System.out.print(1);

for (int i = 2; i


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Fencepost practice problem

Write a Java program that reads a base and amaximum power and prints all of the powers of thegiven base up to that max, separated by commas.

Base: 2

Max exponent: 9

The first 9 powers of 2 are:

2, 4, 8, 16, 32, 64, 128, 256, 512


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Fencepost practice problem

Write a method named printFactors that, when given anumber, prints its factors in the following format (usingan example of 24 for the parameter value):

[1, 2, 3, 4, 6, 8, 12, 24]


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Sentinel loops


self-checks: #13-15


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Sentinel values

sentinel: A special input value that signals the end ofthe user's input.

Example: Write a program that repeatedly prompts the user fornumbers to add until the user types 0, then outputs their sum.

(In this case, the number 0 is our sentinel value.)

Expected output:

Enter a number (0 to quit): 95


Enter a number (0 to quit): 43Enter a number (0 to quit): 29


The total was 254

Let's write it...


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Flawed sentinel solution

What's wrong with this solution?

Scanner console = new Scanner(System.in);

int sum = 0;

int inputNumber = 1; // "dummy value", anything but 0

while (inputNumber != 0) {

System.out.print("Enter a number (0 to quit): ");

inputNumber = console.nextInt();

sum += inputNumber;}

System.out.println("The total was " + sum);


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Changing the sentinel value

To see the problem, change the sentinel's value to -1:Scanner console = new Scanner(System.in);

int sum = 0;

int inputNumber = 1; // "dummy value", anything but -1

while (inputNumber != -1) {

System.out.print("Enter a number (-1 to quit): ");


sum += inputNumber;

}


Now the solution produces the wrong output. Why?

The total was 253


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The problem with our code

Our code uses a pattern like this:sum = 0

while we have not seen the sentinel:

prompt for input, read input.

add input to the sum.

On the last pass through the loop, the sentinel is addedto the sum:

prompt for input, read sentinel of -1.

add -1 to the sum.

This is a fencepost problem! How can we solve it?


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A fencepost solution

We need the code to use a pattern like this:sum = 0

prompt for input, read inputn.

while n is not the sentinel:

add n to the sum.

prompt for input, read a value into n.

sentinel loop: A loop that successfully navigatesaround a sentinel value related fencepost issue.


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More correct code

This solution produces the correct output:System.out.print("Enter a number (-1 to quit): ");int inputNumber = console.nextInt();

while (inputNumber != -1) {

sum += inputNumber;

System.out.print("Enter a number (-1 to quit): ");


}



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Even better code

An even better solution creates a constant for the sentinel:public static final int SENTINEL = -1;

This solution uses the constant:System.out.print("Enter a number (" + SENTINEL +

" to quit): ");

int inputNumber = console.nextInt();

while (inputNumber != SENTINEL) {

sum += inputNumber;

System.out.print("Enter a number (" + SENTINEL +" to quit): ");


}



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Boolean logic

suggested reading: 5.3

self-checks: #16-19, 24


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Type boolean

boolean: Primitive type to represent logical values. A boolean variable can hold one of two values: true or false.

All the s we have used in our if statements andfor loops have been boolean literal values.

It is legal to create boolean variables, pass boolean

parameters, return boolean values from methods, ...

Example:int x = 7;

boolean test1 = true;

boolean test2 = (x < 10); // trueboolean test3 = (x % 2 == 0); // false

if (test2) {

System.out.println("under 10");

}


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Logical operators && || !

Boolean expressions can be joined together with thefollowing logical operators:

The following 'truth tables' show the effect of eachoperator on any boolean values p and q:

Operator Description Example Result

&& and (9 != 6) && (2 < 3) true

|| or (2 == 3) || (-1 < 5) true

! not !(7 > 0) false

p q p && q p || q

true true true true

true false false true

false true false true

false false false false

p !p

true false

false true


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Methods that return boolean

There are several methods in Java that return booleanvalues.

A call to one of these methods can be used as a on a for loop, while loop, or if statement.

Examples:

Scanner console = new Scanner(System.in);System.out.print("Type your age or name: ");

if (console.hasNextInt()) {

int age = console.nextInt();

System.out.println("You are " + age + " years old.");

} else {

String line = console.nextLine();if (line.startsWith("Dr.")) {

System.out.println("Will you marry me?");

}

}


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Testing for valid user input

A Scanner object has methods that can be used to test whether

the upcoming input token is of a given type:

Each of these methods waits for the user to type input tokens andpress Enter, then reports a true or false answer.

The hasNext and hasNextLine methods are not useful until we learnhow to read input from files in Chapter 6.

Method Description

hasNext() Whether or not the next token can be read as aString(always true for console input)

hasNextInt() Whether or not the next token can be read as anint

hasNextDouble() Whether or not the next token can be read as adouble

hasNextLine() Whether or not the next line of input can be readas a String(always true for console input)


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Scanner condition example

The Scanner's hasNext___ methods are very useful for testing

whether the user typed the right kind of token for our program touse, before we read it (and potentially crash!).


System.out.print("How old are you? ");

if (console.hasNextInt()) {

int age = console.nextInt();

System.out.println("Retire in " + (65 - age) + " years.");

} else {

System.out.println("You did not type an integer.");

}

System.out.print("Type 10 numbers: ");for (int i = 1; i


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Comparing objects: .equals

The relational operators such as < and == only behave correctly on

primitive values.

Objects such as String, Point, and Color should be compared forequality by calling a method named equals.

Example (incorrect):

String name = console.next();if (name == "Teacher") {

System.out.println("You must be a swell person!");

}

It turns out that this example code will compile, but it will never print themessage. The == operator on Strings usually returns false even when the

Strings have the same letters in them.

Example (correct):String name = console.next();

if (name.equals("Teacher")) {

System.out.println("You must be a swell person!");

}


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String condition methods

There are several methods of a String object that canbe used as conditions in if statements:

Method Description

equals(String) whether two Strings contain exactly the

same charactersequalsIgnoreCase(String) whether two Strings contain the same

characters, ignoring upper vs. lowercase differences

startsWith(String) whether one String contains the other'scharacters at its start

endsWith(String) whether one String contains the other'scharacters at its end


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String condition examples


System.out.print("Type your full name and title: ");String name = console.nextLine();

if (name.endsWith("Ph. D.")) {

System.out.println("Hello, doctor!");

}if (name.startsWith("Ms.")) {

System.out.println("Greetings, Ma'am.");

}

if (name.equalsIgnoreCase("boss")) {

System.out.println("Welcome, master! Command me!");}

if (name.toLowerCase().indexOf("jr.") >= 0) {

System.out.println("Your parent has the same name!");

}


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Testing for an input value

When testing user input for a certain String, the == and!= operators will not behave properly. System.out.print("Type OK to draw the badger.");String response = console.next();if (response == "OK") { // wrong

drawBadger();

}

Recall: The relational operators such as < and == onlybehave correctly on primitive values. Objects such as String, Point, and Color can be compared for

equality by calling a method named equals. Corrected code from above:System.out.print("Type OK to draw the badger.");String response = console.next();if (response.equals("OK")) { // better!

drawBadger();

}


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Boolean practice problem

Write a program that compares two words typed by the user tosee whether they "rhyme" (end with the same last two letters)and/or alliterate (begin with the same letter).

Use methods with return values to tell whether two words rhymeand/or alliterate.

Example:

Type two words: car STARThey rhyme!

(run #2)

Type two words: bare bear

They alliterate!

(run #3)

Type two words: sell shell

They alliterate!

They rhyme!


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Boolean practice problem

Write a program that reads two numbers from the userand tells whether each is prime, or if not, gives thenext prime after it; also tell whether they are relativelyprime (have no common factors).

Examples:

Type two numbers: 9 169 is not prime; the next prime after 9 is 11

16 is not prime; the next prime after 16 is 17

9 and 16 are relatively prime

(run #2)

Type two numbers: 7 21

7 is prime

21 is not prime; the next prime after 21 is 23

7 and 21 are not relatively prime


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Generating random numbers


self-checks: #20-23


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The Random class

Java has a class named Random whose objectsgenerate pseudo-random numbers.

Example:

Random rand = new Random();int randomNumber = rand.nextInt(10);

// randomNumber has a random value between 0 and 9

Class Random is found in the java.util package.

import java.util.*;

Method name Description

nextInt() returns a random integer

nextInt(max) returns a random integer in the range [0, max)in other words, from 0 up through one less than the max

nextDouble() returns a random real number in the range [0.0, 1.0)


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Random examples

Random rand = new Random();

A random number between 0 and 19 inclusive:



A random even number between 0 and 10 inclusive:

A random multiple of 10 between 100 and 200 inclus.:


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Random examples (answers)

Random rand = new Random();

A random number between 0 and 19 inclusive:int random1 = rand.nextInt(20);

A random number between 1 and 10 inclusive:int random1 = rand.nextInt(10) + 1;

A random number between 4 and 17 inclusive:int random1 = rand.nextInt(14) + 4;

A random even number between 0 and 10 inclusive:int random1 = rand.nextInt(6) * 2;

A random multiple of 10 between 100 and 200 inclus.:

int random1 = rand.nextInt(11) * 10 + 100;


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Random practice problem

Write a multiplication tutor program. Example dialogue:This program helps you learn multiplication byAsking you 5 random multiplication questionsand counts how many you get right.

#1 of 5: 10 * 25 = 250Correct!

#2 of 5: 72 * 12 = 864Correct!

#3 of 5: 87 * 21 = 1741Incorrect. The correct answer was 1827

#4 of 5: 8 * 84 = 692Incorrect. The correct answer was 672

#5 of 5: 25 * 36 = 900Correct!

You got 3 out of 5


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The do/while loop

Java has another kind of loop named the do/while loop. It is almost identical to the while loop, except that its body

statement(s) will always execute the first time, regardless ofwhether the condition is true.

The do/while loop, general syntax:do {

;} while ();

Example:// roll until we get a number other than 3Random rand = new Random();

int dice;

do {

dice = rand.nextInt();

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