5-1

Chapter 5: Conditionals and Loops

• Topics:

– Boolean expressions– Conditional statements– Increment and Decrement Operators (Chapter

2.4)– Repetition statements– the Random class (Chapter 3.4)– Comparing Strings

5-2

Flow of Control

• The order of statement execution is called the flow of control

• Unless specified otherwise, the order of statement execution through a method is linear: one statement after another in sequence

5-3

Flow of Control

• Some programming statements allow us to:

– decide whether or not to execute a particular statement

– execute a statement over and over, repetitively

• These decisions are based on boolean expressions (or conditions) that evaluate to true or false

– For example, num > 5

5-4

Conditional Statements

• A conditional statement lets us choose which statement will be executed next

• The Java conditional statements are the:

– if statement– if-else statement– switch statement

5-5

The if Statement

• The if statement has the following syntax:

if ( condition ) statement;

if is a Javareserved word

The condition must be aboolean expression. It mustevaluate to either true or false.

If the condition is true, the statement is executed.If it is false, the statement is skipped.

5-6

continue

System.out.println ("You entered: " + age);

if (age < MINOR) System.out.println ("Youth is a wonderful thing. Enjoy.");

System.out.println ("Age is a state of mind."); }}

5-7

Boolean Expressions

• A condition often uses one of Java's equality operators or relational operators, which all return boolean results:

== equal to

!= not equal to

< less than

> greater than

<= less than or equal to

>= greater than or equal to

5-8

Equality Operator and Assignment Operator

• Note the difference between the equality operator (==) and the assignment operator (=)

• num == 4 is a comparison: the boolean expression has the value true or false

• num = 4 is an assignment: the expression has the value 4 because that’s the value of the left side

5-9

A Mistake You’ll Probably Make

• What if you accidentally drop an equals sign in the boolean expression?

int num = 5;if (num = 4) System.out.println("Hello");

• You will get a “incompatible types” error:found : intrequired: boolean

5-10

The if Statement

• An if statement lets a program decide whether a particular statement is executed.

if (sum > MAX) delta = sum - MAX;System.out.println ("The sum is " + sum);

• First the condition is evaluated -- the value of sum is either greater than the value of MAX, or it is not

• If the condition is true, the assignment statement is executed -- if it isn’t, it is skipped.

• Either way, the call to println is executed next

5-11

Indentation

• The statement controlled by the if statement is indented to indicate that relationship

• The use of a consistent indentation style makes a program easier to read and understand

• Although it makes no difference to the compiler, proper indentation is crucial

5-12

Indentation

• Remember that indentation is for the human reader, and is ignored by the computer

if (total > MAX) System.out.println ("Error!!"); errorCount++;

Despite what is implied by the indentation, the increment will occur whether the condition is true or not

5-13

The if Statement

• The precedence of the arithmetic operators is higher than the precedence of the equality and relational operators

if (total != stock + warehouse) inventoryError = true;

Sets a flag to true if the value of total is not equal to the sum of stock and warehouse

5-14

The if-else Statement

• An else clause can be added to an if statement to make an if-else statement

if ( condition ) statement1;else statement2;

• If the condition is true, statement1 is executed; if the condition is false, statement2 is executed

• One or the other will be executed, but not both

5-15

//********************************************************************// Wages.java Author: Lewis/Loftus//// Demonstrates the use of an if-else statement.//********************************************************************

import java.text.NumberFormat;import java.util.Scanner;

public class Wages{ //----------------------------------------------------------------- // Reads the number of hours worked and calculates wages. //----------------------------------------------------------------- public static void main (String[] args) { final double RATE = 8.25; // regular pay rate final int STANDARD = 40; // standard hours in a work week

Scanner scan = new Scanner (System.in);

double pay = 0.0;

continue

5-16

continue

System.out.print ("Enter the number of hours worked: "); int hours = scan.nextInt();

System.out.println ();

// Pay overtime at "time and a half" if (hours > STANDARD) pay = STANDARD * RATE + (hours-STANDARD) * (RATE * 1.5); else pay = hours * RATE;

NumberFormat fmt = NumberFormat.getCurrencyInstance(); System.out.println ("Gross earnings: " + fmt.format(pay)); }}

5-17

Logic of an if-else statement

conditionevaluated

statement1

true false

statement2

5-18

Exercise

• Write an if/else statement that compares the variable age with 65 , adds 1 to the variable seniorCitizens if age is greater than or equal to 65 , and adds 1 to the variable nonSeniors otherwise

5-19

Block Statements

• Keep in mind that the two alternatives in an if else statement must be single statements.

• If you need more than one statement for if or else, you must use braces to collect them into a single block statement.

5-20

Block Statements

• The following code produces a compiler error:

• The compiler sees it as a simple if statement that ends with the println ("Error!!"); statement. Then there is a errorCount++ statement. But then there is what the compiler perceives as an unattached else, and that is flagged as a syntax error.

if (total > MAX) System.out.println ("Error!!"); errorCount++;else System.out.println ("Total: " + total); current = total*2;

5-21

Block Statements

• Convert the code to what we want by adding braces

if (total > MAX){ System.out.println ("Error!!"); errorCount++;}else{ System.out.println ("Total: " + total); current = total*2;}

Exercise

• Modify the Ideal Weight (from lab 2) program so it will not compute the ideal weight if the height is less than 5 feet and print a message.

5-22

5-23

Nested if Statements

• The statement executed as a result of an if statement or else clause could be another if statement

• These are called nested if statements

if (num1 > num2) System.out.println ("greater");else{ if (num1 == num2) System.out.println ("same"); else System.out.println ("less");}

5-24

Nested if Statements

• An else clause is matched to the last unmatched if (no matter what the indentation implies)

• Braces can be used to specify the if statement to which an else clause belongs

5-25

Exercise

• Read in three integers from the user and determine the minimum.

5-26

Repetition Statements

• Repetition statements allow us to execute a statement multiple times. Often they are referred to as loops

• Java has three kinds of repetition statements:

– the while loop– the do loop– the for loop

• The programmer should choose the right kind of loop for the situation

5-27

Assignment Revisited

• The right and left hand sides of an assignment statement can contain the same variable

First, one is added to theoriginal value of count

Then the result is stored back into count(overwriting the original value)

count = count + 1;

5-28

Increment and Decrement

• The increment operator (++) adds one to its operand

• The decrement operator (--) subtracts one from its operand

• The statement

count++;

is functionally equivalent to

count = count + 1;

5-29

Increment and Decrement

• The increment and decrement operators can be applied in postfix form:

count++ count --

• or prefix form:

++count --count

• Postfix and prefix are functionally equivalent when used alone in a statement, for example, count currently contains the value 5, the two increment statements assign 6 to count.

5-30

Increment and Decrement

• When used as part of a larger expression, the two forms can have different effects, for example, count currently contains the value 5,

– Postfix:

• total = count ++; // assign 5 to total and 6 to count

– Prefix:

• total = ++count; // assign 6 to total and 6 to count

5-31

Exercises

• Given an integer variable strawsOnCamel , write a statement that uses the increment operator to increase the value of that variable by 1

• Given an integer variable timer , write a statement that uses the decrement operator to decrease the value of that variable by 1

5-32

The while Statement

• A while statement has the following syntax:while ( condition ) statement;

• If the condition is true, the statement is executed

• Then the condition is evaluated again, and if it is still true, the statement is executed again

• The statement is executed repeatedly until the condition becomes false

5-33

Logic of a while Loop

statement

true false

conditionevaluated

5-34

The while Statement

• An example of a while statement:

int count = 1;while (count <= 5){ System.out.println (count); count++;}

• If the condition of a while loop is false initially, the statement is never executed

• Therefore, the body of a while loop will execute zero or more times

5-35

Exercise

• What output is produced by the following code fragment?

int num = 0, max = 20;while (num < max){

System.out.println(num);num +=4;

}

5-36Copyright © 2012 Pearson Education, Inc.

//********************************************************************// Average.java Author: Lewis/Loftus//// Demonstrates the use of a while loop, a sentinel value, and a// running sum.//********************************************************************

import java.text.DecimalFormat;import java.util.Scanner;

public class Average{ //----------------------------------------------------------------- // Computes the average of a set of values entered by the user. // The running sum is printed as the numbers are entered. //----------------------------------------------------------------- public static void main (String[] args) { int sum = 0, value, count = 0; double average;

Scanner scan = new Scanner (System.in);

System.out.print ("Enter an integer (0 to quit): "); value = scan.nextInt();

continue

5-37

continue

while (value != 0) // sentinel value of 0 to terminate loop { count++;

sum += value; System.out.println ("The sum so far is " + sum);

System.out.print ("Enter an integer (0 to quit): "); value = scan.nextInt(); }

continue

5-38

continue

System.out.println ();

if (count == 0) System.out.println ("No values were entered."); else { average = (double)sum / count;

DecimalFormat fmt = new DecimalFormat ("0.###"); System.out.println ("The average is " + fmt.format(average)); } }}

Exercise: Input Validation

• Modify the Ideal Weight program. If the user enters a number less than 5 for feet, the program should print an error message and keep asking for another input.

5-39

5-40

Infinite Loops

• The body of a while loop eventually must make the condition false

• If not, it is called an infinite loop, which will execute until the user interrupts the program

• This is a common logical error

• You should always double check the logic of a program to ensure that your loops will terminate normally

5-41

Infinite Loops

• An example of an infinite loop:

int count = 1;while (count <= 25){ System.out.println (count); count = count - 1;}

• This loop will continue executing until interrupted (Control-C) or until an underflow error occurs

5-42

The Random Class

5-43

The Random Class

• The Random class is part of the java.util package

• It provides methods that generate pseudorandom numbers, for example, to simulate a card shuffler.

• A Random object performs complicated calculations based on a seed value to produce a stream of seemingly random values

5-44

The Random Class

• Random ()– Creates a new pseduorandom number generator.– Random generator = new Random();

• Method:

int nextInt (int num)

– Returns a random number in the range 0 to num -1.

Random Floating Point

• float nextFloat ()

– Returns a random number between 0.0 (inclusive) and 1.0 (exclusive).

5-45

5-46Copyright © 2012 Pearson Education, Inc.

//********************************************************************// RandomNumbers.java Author: Lewis/Loftus//// Demonstrates the creation of pseudo-random numbers using the// Random class.//********************************************************************

import java.util.Random;

public class RandomNumbers{ //----------------------------------------------------------------- // Generates random numbers in various ranges. //----------------------------------------------------------------- public static void main (String[] args) { Random generator = new Random(); int num1, num2; num1 = generator.nextInt(10); System.out.println ("From 0 to 9: " + num1);

continued

5-47Copyright © 2012 Pearson Education, Inc.

continued

num1 = generator.nextInt(10) + 1; System.out.println ("From 1 to 10: " + num1);

num1 = generator.nextInt(15) + 20; System.out.println ("From 20 to 34: " + num1);

num1 = generator.nextInt(20) - 10; System.out.println ("From -10 to 9: " + num1);

num2 = generator.nextFloat(); System.out.println ("A random float (between 0-1): " + num2);

num2 = generator.nextFloat() * 6; // 0.0 to 5.999999 num1 = (int)num2 + 1; System.out.println ("From 1 to 6: " + num1);

}}

5-48

Exercise

• Write a program that simulates the rolling of a pair of dice. For each die in the pair, the program should generate a random number between 1 and 6 (inclusive).

• It should print out the result of the roll for each die and the total roll (the sum of the two dice)

5-49

Comparing Strings

• Remember that in Java a character string is an object

• The equals method can be called with strings to determine if two strings contain exactly the same characters in the same order

• The equals method returns a boolean result

if (name1.equals(name2)) System.out.println ("Same name");else System.out.println(“Not same”);

5-50

Exercise

• Modify the rolling dice program so it allows the user to keep rolling the dice as long as the user enters "yes".

5-51

Other Repetition Statements: The do Statement

5-52

The do Statement

• A do statement has the following syntax:do{ statement;}while ( condition );

• The statement is executed once initially, and then the condition is evaluated

• The statement is executed repeatedly until the condition becomes false

There is a semicolon

5-53

Comparing while and do

statement

true false

conditionevaluated

The while Loop

true

conditionevaluated

statement

false

The do Loop

5-54

The do Statement

• An example of a do loop:

• The body of a do loop executes at least once

int count = 0;do{ count++; System.out.println (count);} while (count < 5);

5-55

Exercise

• Write a program that computes the reverse number of a number entered by the user. For example, the reverse number of 8352 is 2538.

5-56

Readings and Assignments

• Reading: Chapter 5.1-5.4, 2.4, 3.4 • Lab Assignment: Java Lab 4

• Self-Assessment Exercises:– Self-Review Questions Section

• SR5.8, 5.11, 5.18, 5.21

– After Chapter Exercises• EX 5.2, 5.4, 5.8, 5.9