conditionals and loops

1

5

Conditionals and Loops

2

Conditionals and Loops• Now we will examine programming statements that allow us to:

– make decisions– repeat processing steps in a loop

• Chapter 5 focuses on:

– boolean expressions– conditional statements– comparing data– repetition statements– iterators– more drawing techniques– more GUI components (skip)

3

Outline

The if Statement and Conditions – Oct. 25

Other Conditional Statements – Oct. 25

Comparing Data – Oct. 30

The while Statement – Nov. 8

Iterators – Nov. 8

Other Repetition Statements – Nov. 8, 13

Decisions and Graphics – Nov. 13

More Components (skip)

4

Flow of Control• Unless specified otherwise, the order of statement

execution through a method is linear: one statement after another in sequence

• 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 controlled by boolean expressions (or conditions) that evaluate to true or false

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

5

Conditional Statements• A conditional statement lets us choose which statement

will be executed next

• Therefore they are sometimes called selection statements

• Conditional statements give us the power to make basic decisions

• The Java conditional statements are the:

– if statement– if-else statement– switch statement

6

Examples of if statementsDouble hoursWorked;

// Scan in a value of hoursWorked.

if (hoursWorked > 40.0 )

grossPay = 40 * payRate + 1.5 * payRate * (hoursWorked

– 40);

int heightInInches;

// Scan in a value of heightInInches

if (heightInInches < 48 )

system.out.println(“Customer too small,” +

“ do not let on ride.”);

7

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.

8

Logic of an if statement

conditionevaluated

statement

truefalse

9

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

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

10

The if Statement

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

• See Age.java (page 216)

• An example of an if statement:

11

Age.java (page 216) import java.util.Scanner;

public class Age{ // Reads the user's age and prints comments accordingly. public static void main (String[] args) { final int MINOR = 21;

Scanner scan = new Scanner (System.in);

System.out.print ("Enter your age: "); int age = scan.nextInt();

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."); } }

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Age.java (page 216) output

• Enter your age: 19You entered: 19Youth is a wonderful thing. Enjoy.Age is a state of mind.

• Enter your age: 57You entered: 57Age is a state of mind.

13

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 to understanding the flow of the program.

"Always code as if the person who ends up maintaining your code will be a violent psychopath who knows where you live."

-- Martin Golding

14

The if Statement• What do the following statements do?

if (top >= MAXIMUM) top = 0;

Sets top to zero if the current value of top is greater than or equal to the value of MAXIMUM

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

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

15

Logical Operators• Boolean expressions can also use the following logical

operators:

! Logical NOT

&& Logical AND

|| Logical OR

• They all take boolean operands and produce boolean results

• Logical NOT is a unary operator (it operates on one operand)

• Logical AND and logical OR are binary operators (each operates on two operands)

16

Logical NOT• The logical NOT operation is also called logical

negation or logical complement

• If some boolean condition a is true, then !a is false; if a is false, then !a is true

• Logical expressions can be shown using a truth table

a !a

true false

false true

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Logical AND and Logical OR• The logical AND expression

a && b

is true if both a and b are true, and false otherwise

• The logical OR expression

a || b

is true if either a or b or both are true, and false otherwise

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Logical Operators

• Expressions that use logical operators can form complex conditions

if (total < MAX+5 && !found) System.out.println ("Processing…");

• All logical operators have lower precedence than the relational operators

• Logical NOT has higher precedence than logical AND and logical OR

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Logical Operators• A truth table shows all possible true-false

combinations of the terms

• Since && and || each have two operands, there are four possible combinations of conditions a and b

a b a && b a || b

true true true true

true false false true

false true false true

false false false false

20

Boolean Expressions

• Specific expressions can be evaluated using truth tables

total < MAX

found !foundtotal < MAX && !

found

false false true false

false true false false

true false true true

true true false false

21

Short-Circuited Operators• The processing of logical AND and logical OR is

“short-circuited”

• If the left operand is sufficient to determine the result, the right operand is not evaluated

• This type of processing must be used carefully

• Usually short circuiting has no effect on your code and can be ignored.

if (count != 0 && total/count > MAX) System.out.println ("Testing…");

22

Outline









23

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

to make an if-else statementif ( 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

• See Wages.java (page 219)

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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;

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)); } }

25

Wages.java (page 219) output

• Enter the number of hours worked: 40

Gross earnings: $330.00

• Enter the number of hours worked: 50

Gross earnings: $453.75

26

Logic of an if-else statement

conditionevaluated

statement1

true false

statement2

27

The Coin Class

• Let's examine a class that represents a coin that can be flipped

• Instance data is used to indicate which face (heads or tails) is currently showing

• See CoinFlip.java (page 220)

• See Coin.java (page 221)

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

public class CoinFlip{ // Creates a Coin object, flips it, and prints the results. public static void main (String[] args) { Coin myCoin = new Coin();

myCoin.flip();

System.out.println (myCoin);

if (myCoin.isHeads()) System.out.println ("You win."); else System.out.println ("Better luck next time."); }}

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CoinFlip.java (page 220) output

• TailsBetter luck next time.

• HeadsYou win.

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Coin API Constructor public Coin () Sets up the coin by flipping it initially

Methods public void flip () - Flips the coin by randomly choosing a face value. . public boolean isHeads () - Returns true if the current face of the coin is heads. public String toString() - Returns the current face of the coin as a string, “Heads” or

“Tails”

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public class Coin { private final int HEADS = 0; private final int TAILS = 1; private int face;

// Constructor Sets up the coin by flipping it initially. public Coin () { flip(); }

// Flips the coin by randomly choosing a face value. public void flip () { face = (int) (Math.random() * 2); }

// Returns true if the current face of the coin is heads. public boolean isHeads () { return (face == HEADS); } // Check this out

// Returns the current face of the coin as a string. public String toString() // Can you reduce this by 1 stment? { String faceName; if (face == HEADS) faceName = "Heads"; else faceName = "Tails"; return faceName; }}

32

Indentation Revisited

• 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

33

Block Statements• Several statements can be grouped

together into a block statement delimited by braces

• A block statement can be used wherever a statement is called for in the Java syntax rules

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

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Block Statements• In an if-else statement, the if portion, or the else

portion, or both, could be block statementsif (total > MAX){ System.out.println ("Error!!"); errorCount++;}else{ System.out.println ("Total: “ + total); current = total*2;}

• See Guessing.java (page 223)

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public class Guessing{ // Plays a simple guessing game with the user. public static void main (String[] args) { final int MAX = 10; int answer, guess;

Scanner scan = new Scanner (System.in); Random generator = new Random();

answer = generator.nextInt(MAX) + 1;

System.out.print ("I'm thinking of a number between 1 and " + MAX + ". Guess what it is: ");

guess = scan.nextInt();

if (guess == answer) System.out.println ("You got it! Good guessing!"); else { System.out.println ("That is not correct, sorry."); System.out.println ("The number was " + answer); } }}

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Guessing.java (page 223) output

• I'm thinking of a number between 1 and 10. Guess what it is: 7That is not correct, sorry.The number was 2

• I'm thinking of a number between 1 and 10. Guess what it is: 1You got it! Good guessing!

37

The Conditional Operator• Java has a conditional operator that uses a boolean

condition to determine which of two expressions is evaluated

• Its syntax is:

condition ? expression1 : expression2

• If the condition is true, expression1 is evaluated; if it is false, expression2 is evaluated

• The value of the entire conditional operator is the value of the selected expression

38

The Conditional Operator

• The conditional operator is similar to an if-else statement, except that it is an expression that returns a value

• For example:

larger = ((num1 > num2) ? num1 : num2);

• If num1 is greater than num2, then num1 is assigned to larger; otherwise, num2 is assigned to larger

• The conditional operator is ternary because it requires three operands

39

Answer: 5

What is the value of larger when this is done?

The Conditional Operatorint num1 = 5, num2 = 3;

larger = ((num1 > num2) ? num1 : num2);

Evaluatethis

expression

Return this value if expression is

true

Return this value

if expression is

false

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The Conditional Operator

• Another example:

System.out.println ("Your change is " + count +((count == 1) ? "Dime" : "Dimes"));

• If count equals 1, then "Dime" is printed

• If count is anything other than 1, then "Dimes" is printed

41

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

• See MinOfThree.java (page 227)

• 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

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MinOfThree.java (page 227) bad example int num1, num2, num3, min = 0;


System.out.println ("Enter three integers: "); num1 = scan.nextInt(); num2 = scan.nextInt(); num3 = scan.nextInt();

if (num1 < num2) // more confusing than it needs to be if (num1 < num3) min = num1; else min = num3; else if (num2 < num3) min = num2; else min = num3;

System.out.println ("Minimum value: " + min);

43

MinOfThree.java (page 227) output• Enter three integers:

5 1 3Minimum value: 1

• Enter three integers: 1 3 5Minimum value: 1

• Enter three integers: 3 5 1 Minimum value: 1

44

MinOfThree.java (page 227) improved(but no longer demonstrates nested if else)

int num1, num2, num3, min = 0;


System.out.println ("Enter three integers: "); num1 = scan.nextInt(); num2 = scan.nextInt(); num3 = scan.nextInt();

min = num1;

if (num2 < min) min = num2; if (num3 < min) min = num3; System.out.println ("Minimum value: " + min);

45

Tax Rate Problem

• Let’s say that there are 4 income tax rates in some country.

• The rates are:0 <= income < 10,000: tax rate = 0%

10,000 <= income < 30,000: tax rate = 5%

30,000 <= income < 60,000: tax rate = 10%

income >= 60,000: tax rate = 15%• Write a program that inputs a person’s income

and prints out their maximum tax rate.

46

Tax Program designWhat do we need to do? Let’s set up some steps.

1. Ask the user to input their amount of income, can be dollars and cents.

2. Read the income

3. Based on income, figure out tax rate:

A person making less than $10,000 has 0% tax

A person making $10,000 but less than $30,000

has 5% tax

A person making $30,000 but less than $60,000

has 10% tax

A person making more than $60,000 has 15% tax.

4. Print out the maximum tax rate.

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Tax rate program, first try, no if else double income, maxTax = 0.0;


System.out.print ("Enter the annual income: ");income = scan.nextDouble(); if (income < 10000.0) // This test can be eliminated, why? maxTax = 0.0; if (income >= 10000.0 && income < 30000.0 ) maxTax = 0.05;

if (income >= 30000.0 && income <60000.0 ) maxTax = 0.10;

if (income >= 60000.0) maxTax = 0.15; System.out.println ("Maximum tax rate: " + NumberFormat.getPercentInstance().format(maxTax));

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Tax rate program, cleaner version double income, maxTax = 0.0;


System.out.print ("Enter the annual income: ");income = scan.nextDouble(); if (income < 10000) maxTax = 0.0; else if (income < 30000 ) maxTax = 0.05;else if (income < 60000 ) maxTax = 0.10;else maxTax = 0.15; System.out.println ("Maximum tax rate: " + NumberFormat.getPercentInstance().format(maxTax));

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Logic of an if-else statement

conditionevaluated

statement1

true false

statement2

Statement2 consists of several if else’s but the entire thing is skipped if the condition is true.

50

The switch Statement• The switch statement provides another way to

decide which statement to execute next

• The switch statement evaluates an expression, then attempts to match the result to one of several possible cases

• Each case contains a value and a list of statements

• The flow of control transfers to statement associated with the first case value that matches

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The switch Statement

// char option has a value at this pointswitch (option){ case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break;}

• An example of a switch statement:

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• The general syntax of a switch statement is:

switch ( expression ){ case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case ...

}

switchandcaseare

reservedwords

If expressionmatches value2,control jumpsto here

If the expression matches value2, statement-list2 is executed and all the statement lists following this one are executed. Put a break statement in so only statement-list2 is executed.

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The switch Statement, usual form

switch ( expression ){ case value1 : statement-list1 break; case value2 : statement-list2 break; case value3 : statement-list3 break; case ...

}next-statement

If expressionmatches value2,control jumpsto here

• Break placed after each statement-list

Break causes control to transfer to the next statement, the one directly following the switch

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• The actions in the switch are usually mutually exclusive. Only one of the options is executed.

• If a break statement is not used, the flow of control will continue into the next case. Sometimes this may be appropriate, but usually we want to execute only the statements associated with one case

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The switch Statement with default// char option has a value at this pointswitch (option){ case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break; case ‘D’: case ‘F’: dfCount++; break; default: System.out.println(“Input must “ + “be A,B,C, D, F.\n<” + option + “> was entered.”);

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default clause

• A switch statement can have an optional default case

• Control will transfer to it if no other case value matches

• If there is no default case, and no other value matches, control falls through to the statement after the switch

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The switch Statement• The expression of a switch statement must result in

an integral type, meaning an integer (byte, short, int, long) or a char

• It cannot be a boolean value (use an if instead) or a floating point value (float or double)

• The implicit boolean condition in a switch statement is equality, you cannot perform relational checks with a switch statement

• See GradeReport.java (page 233)

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int grade, category;


System.out.print ("Enter a numeric grade (0 to 100): "); grade = scan.nextInt(); category = grade / 10;

System.out.print ("That grade is ");

switch (category) { case 10: System.out.println ("a perfect score. Well done."); break; case 9: System.out.println ("well above average. Excellent."); break; case 8: System.out.println ("above average. Nice job."); break; case 7: System.out.println ("average."); break; case 6: System.out.println ("below average. You should see the"); System.out.println ("instructor to clarify the material " + "presented in class."); break; default: System.out.println ("not passing."); }

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Question

• What if a student got 110 as the numeric grade?

60

Outline









61

Comparing Data• When comparing data using boolean

expressions, it's important to understand the nuances of certain data types

• Let's examine some key situations:

– Comparing floating point values for equality– Comparing characters– Comparing strings (alphabetical order)– Comparing object vs. comparing object references

62

Comparing Float Values• You should rarely use the equality operator (==) when

comparing two floating point values (float or double)

• Two floating point values are equal only if their underlying binary representations match exactly

• Floating point computations often result in slight differences that may be irrelevant to you but not to the computer

• In many situations, you might consider two floating point numbers to be "close enough" even if they aren't exactly equal

• Often use relation tests (e.g. <, <=, >, >=) with floating point values.

63

Comparing Float Values• To determine the equality of two floats, you

may want to use the following technique:

if (Math.abs(f1 - f2) < TOLERANCE) System.out.println ("Essentially equal");

• If the difference between the two floating point values is less than the tolerance, they are considered to be equal

• The tolerance could be set to any appropriate level, such as 0.000001

64

Comparing Characters• As we've discussed, Java character data is based on the

Unicode character set

• Unicode establishes a particular numeric value for each character, and therefore an ordering

• We can use relational operators on character data based on this ordering

• For example, the character '+' is less than the character 'J' because it comes before it in the Unicode character set

• Appendix C provides an overview of Unicode

65

Comparing Characters• In Unicode, the digit characters (0-9) are

contiguous and in order

• Likewise, the uppercase letters (A-Z) and lowercase letters (a-z) are contiguous and in order

Characters Unicode Values

0 – 9 48 through 57

A – Z 65 through 90

a – z 97 through 122

66

Comparing Strings• Remember that in Java a character string is an object

• Using the test if (name1 == name2) will return true if name1 and name2 have the same address in memory, i.e. if they are the same String 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");

67

Comparing Strings• We cannot use the relational operators to compare strings

• The String class contains a method called compareTo to determine if one string comes before another

• A call to name1.compareTo(name2)

– returns zero if name1 and name2 are equal (contain the same characters)

– returns a negative value if name1 is less than name2

– returns a positive value if name1 is greater than name2

68

Comparing Strings

name1.compareTo(name2)

name1 < name2: returns a negative value

name1 = name2: returns zero (name1 and name2 contain the same characters but are not necessarily the same object)

name1 > name2: returns a positive value

69

Comparing Strings

if (name1.compareTo(name2) < 0) System.out.println (name1 + "comes first");else if (name1.compareTo(name2) == 0) System.out.println ("Same name"); else System.out.println (name2 + "comes first");

• The above results in comparing strings in the same way as they are compared (and sequenced) in a dictionary. Another word for “dictionary” is “lexicon”. This sequencing is traditionally called lexicographic ordering

70

Lexicographic Ordering

• Lexicographic ordering is not strictly alphabetical when uppercase and lowercase characters are mixed

• For example, the string "Great" comes before the string "fantastic" because all of the uppercase letters come before all of the lowercase letters in Unicode

• Also, short strings come before longer strings with the same prefix (lexicographically)

• Therefore "book" comes before "bookcase"

71

Lexicographic Ordering continued

• When ordering strings it is often useful to change them all to upper or lower case letters then order them. There is no problem with mixing of cases then.

String strng1 = “Great”;

String strng2 = “fantastic”;

strng1Upper = strng1.toUpperCase();

strng2Upper = strng2.toUpperCase();

if(strng1Upper.compareTo(strng2Upper)>0) /* this will be true because

“GREAT” comes after “FANTASTIC” */

72

Comparing Objects• The == operator can be applied to objects – it returns

true if the two references are aliases of each other. This just means that they are the same object, works just like comparing strings with ==

• The equals method is defined for all objects, but unless we redefine it when we write a class, it has the same semantics as the == operator

• It has been redefined in the String class to compare the characters in the two strings

• When you write a class, you can redefine the equals method to return true under whatever conditions are appropriate

73

Adding an equal method to the circle class. private int diameter, x, y;private Color color;

public boolean equals(Circle par){ return x == par.x && y == par.y && diameter == par.diameter && color == par.color; }

Notice that we can refer to the private data of the Circle object par directly. This is a shortcut allowed for methods in a class that refer to objects in the same class. We do not have to say par.getDiameter(), though this would certainly work.

74

Outline









75

Repetition Statements• Repetition statements allow us to execute a statement

multiple times

• Often they are referred to as loops

• Like conditional statements, they are controlled by boolean expressions

• 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

76

The while Statement

• A while statement has the following syntax:

while ( condition ) statement;

• If the condition is true, the statement is executed and the condition evaluated again

• If condition is still true, the statement is executed again

• The statement is executed repeatedly until the condition becomes false

77

Logic of a while Loop

statement

true false

conditionevaluated

78

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

• The above will print the values 1 2 3 4 5 on separate lines.

79

The while Statement

• Let's look at some examples of loop processing

• A loop can be used to maintain a running sum

• A sentinel value is a special input value that represents the end of input

• See Average.java (page 237)

80

int sum = 0, value, count = 0; double average;


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

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(); }

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)); }

81

• Enter an integer (0 to quit): 89The sum so far is 89Enter an integer (0 to quit): 68The sum so far is 157Enter an integer (0 to quit): 97The sum so far is 254

Enter an integer (0 to quit): 73The sum so far is 327Enter an integer (0 to quit): 82The sum so far is 409Enter an integer (0 to quit): 77The sum so far is 486Enter an integer (0 to quit): 0

The average is 81

82

General form of input loops that use a sentinel to stop

StatementRead next value

truefalse

conditionevaluated

Read first value

The last value (sentinel) is read and tested immediately. Thus the loop is exited without processing the sentinel value. This is the desired behavior.

83

The while Statement

• A loop can also be used for input validation, making a program more robust

• See WinPercentage.java (page 239)

84

final int NUM_GAMES = 12; int won; double ratio;


System.out.print ("Enter the number of games won (0 to " + NUM_GAMES + "): "); won = scan.nextInt();

while (won < 0 || won > NUM_GAMES) { System.out.print ("Invalid input. Please reenter: "); won = scan.nextInt(); }

ratio = (double)won / NUM_GAMES; NumberFormat fmt = NumberFormat.getPercentInstance();

System.out.println (); System.out.println ("Winning percentage: " + fmt.format(ratio));

85

WinPercentage.java (page 239) output

Enter the number of games won (0 to 12):13 Invalid input. Please reenter: -2Invalid input. Please reenter: 8

Winning percentage: 67%

86

WinPercentageBetter.java• The following displays the incorrect input to the

user. while (won < 0 || won > NUM_GAMES) {

System.out.print ("Invalid input, < " + won + " >. Please reenter: "); won = scan.nextInt();}

Enter the number of games won (0 to 12): -1Invalid input, < -1 >. Please reenter: 13Invalid input, < 13 >. Please reenter: 8

Winning percentage: 67%

87

Infinite Loops

• The body of a while loop must eventually execute a statement that makes 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

88

Infinite Loops

• An example of an infinite loop:

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

• Count is never incremented so it will remain 1 which is always less than 25

• This loop will continue executing until interrupted (Control-C).

89

Nested Loops• Similar to nested if statements, loops can

be nested as well

• That is, the body of a loop can contain another loop

• For each iteration of the outer loop, the inner loop iterates completely

• See PalindromeTester.java (page 243)

90

String str, another = "y"; int left, right;


while (another.equalsIgnoreCase("y")) // allows y or Y { System.out.println ("Enter a potential palindrome:"); str = scan.nextLine();

left = 0; right = str.length() - 1;

while (str.charAt(left) == str.charAt(right) && left < right) { left++; right--; }

System.out.println();

if (left < right) System.out.println ("That string is NOT a palindrome."); else System.out.println ("That string IS a palindrome.");

System.out.println(); System.out.print ("Test another palindrome (y/n)? "); another = scan.nextLine(); }

91

PalindromeTester.java (page 243) output

Enter a potential palindrome: toot

That string IS a palindrome.

Test another palindrome (y/n)? yEnter a potential palindrome: tool

That string is NOT a palindrome.

Test another palindrome (y/n)? y Enter a potential palindrome: a toyota

That string is NOT a palindrome.

Test another palindrome (y/n)? n

92

PalindromeTester.java (page 243)• How does the program work? Use “toot” as an example.• while (str.charAt(left) == str.charAt(right) && left < right)

toot left = 0, right = 3, ‘t’ ==‘t’ && 0 < 3: true toot left = 1, right = 2, ‘o’ ==‘o’ && 1 < 2: true toot left = 2, right = 1, ‘o’ ==‘o’ && 2 < 1: false

// exit loop

• if (left < right)

2 < 1: false// execute the else part of the if else

System.out.println ("That string IS a palindrome.");

• What happens if the input string is “atoyota”?

Ans: atoyota eventually left = 3, right = 3: left < right: false exit loop

93

Nested Loops• How many times will the string "Here" be printed?

count1 = 1;while (count1 <= 10){ count2 = 1; while (count2 <= 20) { System.out.println ("Here"); count2++; } count1++;}

Ans: 10 * 20 = 200

94

Outline






Other Repetition Statements – Nov. 13


More Components (skip) – Nov. 13

95

Iterators• An iterator is an object that allows you to process a

collection of items one at a time

• It lets you step through each item in turn and process it as needed

• An iterator object has a hasNext method that returns true if there is at least one more item to process

• The next method returns the next item

• Iterator objects are defined using the Iterator interface, which is discussed further in Chapter 6

96

Iterators• Several classes in the Java standard class library are

iterators

• The Scanner class is an iterator

– the hasNext method returns true if there is more data to be scanned

– the next method returns the next scanned token as a string

• The Scanner class also has variations on the hasNext method for specific data types (such as hasNextInt)

97

Iterators• The fact that a Scanner is an iterator is particularly

helpful when reading input from a file

• Suppose we wanted to read and process a list of URLs stored in a file

• One scanner can be set up to read each line of the input until the end of the file is encountered

• Another scanner can be set up for each URL to process each part of the path

• See URLDissector.java (page 247)

98

File inputWe can input information from a file by importing java.io.*

The following program refers to a file named urls.inp. It associates this with a scanner.

The program associates a new scanner with each line in the input file. (Note: Much the same thing could be cone with the String methods but scanner works quite well.)

URL: java.sun.com/j2se/1.5 java.sun.com j2se 1.5

URL: www.linux.org/info/gnu.html www.linux.org info gnu.html

URL: duke.csc.villanova.edu/lewis/ duke.csc.villanova.edu lewis

Errors can occur in handling files and scanning. Because the program might run into an error it contains a throws clause. This says that some other module above this one should handle the error. In our case there is no other module so an execution tike error occurs.

99

public static void main (String[] args) throws IOException { String url; Scanner fileScan, urlScan;

fileScan = new Scanner (new File("urls.inp"));

// Read and process each line of the file while (fileScan.hasNext()) { url = fileScan.nextLine(); System.out.println ("URL: " + url);

urlScan = new Scanner (url); urlScan.useDelimiter("/");

// Print each part of the url while (urlScan.hasNext()) System.out.println (" " + urlScan.next());

System.out.println(); } }

100

No file named url.inp in the directory

• ----jGRASP exec: java URLDissector

Exception in thread "main" java.io.FileNotFoundException: urls.inp (The system cannot find the file specified)

at java.io.FileInputStream.open(Native Method)at

java.io.FileInputStream.<init>(FileInputStream.java:106)at java.util.Scanner.<init>(Scanner.java:621)at URLDissector.main(URLDissector.java:21)

----jGRASP wedge2: exit code for process is 1.

101

Outline









102

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

103

Logic of a do Loop

true

conditionevaluated

statement

false

104

The do Statement• An example of a do loop:

• The body of a do loop executes at least once

• See ReverseNumber.java (page 251)Welty used to give this on tests.

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

Output: 1 2 3 4 5 0n separate lines

105

ReverseNumber.java (page 251)• public static void main (String[] args)

{ int number, lastDigit, reverse = 0;


System.out.print ("Enter a positive integer: "); number = scan.nextInt();

do { lastDigit = number % 10; reverse = (reverse * 10) + lastDigit; number = number / 10; } while (number > 0);

System.out.println ("That number reversed is " + reverse); }

106

do { lastDigit = number % 10; reverse = (reverse * 10) + lastDigit; number = number / 10;}while (number > 0);

lastDigit = 2

reverse = 2

number = 485

lastDigit = 5

reverse = 25

number = 48

lastDigit = 8

reverse = 258

number = 4

lastDigit = 4

reverse = 2584

number = 0

Exit loop

Assume:number = 4852, reverse = 0

107

Comparing while and do

statement

true false

conditionevaluated

The while Loop (pretest)

true

conditionevaluated

statement

false

The do Loop(post test)

108

The for Statement

• A for statement has the following syntax:

for ( initialization ; condition ; increment ) statement;

The initializationis executed once

before the loop begins

The statement isexecuted until the

condition becomes false

The increment portion is executed at the end of each

iteration

109

Logic of a for loop

statement

true

conditionevaluated

false

increment

initialization

110

The for Statement

• A for loop is functionally equivalent to the following while loop structure:

initialization;while ( condition ){ statement; increment;}

111

The for Statement• An example of a for loop:

for (int count=1; count <= 5; count++) System.out.println (count);

• The initialization section can be used to declare a variable

• Like a while loop, the condition of a for loop is tested prior to executing the loop body. It is a pretest loop.

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

112

The for Statement• The increment section can perform any

calculation

• A for loop is well suited for executing statements a specific number of times that can be calculated or determined in advance

• See Multiples.java (page 255)

• See Stars.java (page 257)

for (int num=20; num > 0; num -= 5) System.out.println (num);

Q: How many times will this loop execute? Ans: 5

113

public static void main (String[] args) { final int PER_LINE = 5; int value, limit, mult, count = 0;


System.out.print ("Enter a positive value: "); value = scan.nextInt();

System.out.print ("Enter an upper limit: "); limit = scan.nextInt();

System.out.println (); System.out.println ("The multiples of " + value + " between " + value + " and " + limit + " (inclusive) are:");

for (mult = value; mult <= limit; mult += value) { System.out.print (mult + "\t");

// Print a specific number of values per line of output count++; if (count % PER_LINE == 0) System.out.println(); } }

114

• Enter a positive value: 4Enter an upper limit: 90

The multiples of 4 between 4 and 90 (inclusive) are: 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80

Multiples.java (page 255) output

115

Stars.java (page 257)

public class Stars{ // Prints a triangle shape using asterisks (stars) public static void main (String[] args) { final int MAX_ROWS = 10;

for (int row = 1; row <= MAX_ROWS; row++) { for (int star = 1; star <= row; star++) System.out.print ("*");

System.out.println(); } }}

116

Stars.java (page 257) output

*******************************************************

row = 1row = 2row = 3row = 4row = 5row = 6row = 7row = 8row = 9row = 10

117

The for Statement• The following are rarely used in beginning programs and

even in advanced programs. Can be useful but only occasionally.

• Each expression in the header of a for loop is optional

• If the initialization is left out, no initialization is performed

• If the condition is left out, it is always considered to be true, and therefore creates an infinite loop

• If the increment is left out, no increment operation is performed

118

Iterators and for LoopsThis seems to be too advanced an idea at this time.

• Recall that an iterator is an object that allows you to process each item in a collection

• A variant of the for loop simplifies the repetitive processing the items

• For example, if BookList is an iterator that manages Book objects, the following loop will print each book:

for (Book myBook : BookList) System.out.println (myBook);

119

Iterators and for LoopsThis seems to be too advanced an idea at this time.

• This style of for loop can be read "for each Book in BookList, …"

• Therefore the iterator version of the for loop is sometimes referred to as the foreach loop

• It eliminates the need to call the hasNext and next methods explicitly

• It also will be helpful when processing arrays, which are discussed in Chapter 7

120

Outline









121

Drawing Techniques

• Conditionals and loops enhance our ability to generate interesting graphics

• See Bullseye.java (page 259) (I will not bother showing this. It is the usual.)

• See BullseyePanel.java (page 290)

• See Boxes.java (page 262) (I will not bother showing this. It is the usual.)

• See BoxesPanel.java (page 263)

122

BullseyePanel.java (page 290)import javax.swing.JPanel;

import java.awt.*;

public class BullseyePanel extends JPanel{ private final int MAX_WIDTH = 300, NUM_RINGS = 5, RING_WIDTH = 25;

//----------------------------------------------------------------- // Sets up the bullseye panel. //----------------------------------------------------------------- public BullseyePanel () { setBackground (Color.cyan); setPreferredSize (new Dimension(300,300)); }

123

// Paints a bullseye target. public void paintComponent (Graphics page) { super.paintComponent (page);

int x = 0, y = 0, diameter = MAX_WIDTH;

page.setColor (Color.white);

for (int count = 0; count < NUM_RINGS; count++) { if (page.getColor() == Color.black) // alternate colors page.setColor (Color.white); else page.setColor (Color.black);

page.fillOval (x, y, diameter, diameter);

diameter -= (2 * RING_WIDTH); x += RING_WIDTH; y += RING_WIDTH; }

// Draw the red bullseye in the center page.setColor (Color.red); page.fillOval (x, y, diameter, diameter); } }

124

BullseyePanel.java (page 290) output

125

BoxesPanel.java (page 263) import javax.swing.JPanel;

import java.awt.*;import java.util.Random;

public class BoxesPanel extends JPanel{ private final int NUM_BOXES = 50, THICKNESS = 5, MAX_SIDE = 50; private final int MAX_X = 350, MAX_Y = 250; private Random generator;

// Set up the drawing panel. public BoxesPanel () { generator = new Random();

setBackground (Color.black); setPreferredSize (new Dimension(400, 300)); }

126

public void paintComponent(Graphics page) { super.paintComponent (page);

int x, y, width, height;

for (int count = 0; count < NUM_BOXES; count++) { x = generator.nextInt(MAX_X) + 1; y = generator.nextInt(MAX_Y) + 1;

width = generator.nextInt(MAX_SIDE) + 1; height = generator.nextInt(MAX_SIDE) + 1;

if (width <= THICKNESS) // check for narrow box { page.setColor (Color.yellow); page.fillRect (x, y, width, height); } else if (height <= THICKNESS) // check for short box { page.setColor (Color.green); page.fillRect (x, y, width, height); } else { page.setColor (Color.white); page.drawRect (x, y, width, height); } } }}

127

Boxes.java (page 262) output

conditionals and loops

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