1

Iteration

Chapter 6Fall 2005CS 101Aaron Bloomfield

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Java looping Options

while do-while for

Allow programs to control how many times a statement list is executed

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Averaging valuesAveraging values

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Averaging Problem

Extract a list of positive numbers from standard input and produce their average Numbers are one per line A negative number acts as a sentinel to indicate that

there are no more numbers to process Observations

Cannot supply sufficient code using just assignments and conditional constructs to solve the problem Don’t how big of a list to process

Need ability to repeat code as needed

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Averaging Algorithm

Prepare for processing Get first input While there is an input to process do {

Process current input Get the next input

} Perform final processing

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Averaging Problem

Extract a list of positive numbers from standard input and produce their average Numbers are one per line A negative number acts as a sentinel to indicate that

there are no more numbers to process Sample run

Enter positive numbers one per line.Indicate end of list with a negative number.4.50.51.3-1Average 2.1

public class NumberAverage {// main(): application entry pointpublic static void main(String[] args) {

// set up the input// prompt user for values// get first value

// process values one-by-onewhile (value >= 0) {

// add value to running total// processed another value// prepare next iteration - get next value

}// display resultif (valuesProcessed > 0)

// compute and display averageelse

// indicate no average to display}

}

int valuesProcessed = 0;double valueSum = 0;// set up the inputScanner stdin = new Scanner (System.in);// prompt user for valuesSystem.out.println("Enter positive numbers 1 per line.\n" + "Indicate end of the list with a negative number.");// get first valuedouble value = stdin.nextDouble();// process values one-by-onewhile (value >= 0) {

valueSum += value;++valuesProcessed;value = stdin.nextDouble();

}// display resultif (valuesProcessed > 0) {

double average = valueSum / valuesProcessed;System.out.println("Average: " + average);

} else {System.out.println("No list to average");

}

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Program DemoProgram Demo NumberAverage.javaNumberAverage.java

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Today’s dose of Today’s dose of demotivatorsdemotivators

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While syntax and semantics

Logical expression thatdetermines whether Action

is to be executed

while ( Expression ) Action

Action is either a singlestatement or a statement

list within braces

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While semantics for averaging problem

// process values one-by-onewhile ( value >= 0 ) {

// add value to running totalvalueSum += value;

// we processed another value++valueProcessed;

// prepare to iterate – get the next inputvalue = stdin.nextDouble();

}

Test expression is evaluated at thestart of each iteration of the loop.

If test expression is true, these statementsare executed. Afterward, the test expression

is reevaluated and the process repeats

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While Semantics

Expression

Action

true false

Expression isevaluated at the

start of eachiteration of the

loop

If Expression istrue, Action is

executed If Expression isfalse, program

executioncontinues with

next statement

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int valuesProcessed = 0;double valueSum = 0;

double value = stdin.nextDouble();

while (value >= 0) {valueSum += value;++valuesProcessed;value = stdin.nextDouble();

}

if (valuesProcessed > 0) {double average = valueSum / valuesProcessed;System.out.println("Average: " + average);

}else {

System.out.println("No list to average");}

int valuesProcessed = 0;double valueSum = 0;

double value = stdin.nextDouble();

while (value >= 0) {valueSum += value;++valuesProcessed;value = stdin.nextDouble();

if (valuesProcessed > 0) {double average = valueSum / valuesProcessed;System.out.println("Average: " + average);

Execution TraceSuppose input contains: 4.5 0.5 1.3

-10valuesProcessed

valueSum 0value 4.5

Suppose input contains: 4.5 0.5 1.3 -1

4.51

Suppose input contains: 4.5 0.5 1.3 -1

0.55.02

1.36.3

Suppose input contains: 4.5 0.5 1.3 -1

3

-1

Suppose input contains: 4.5 0.5 1.3 -1

average 2.1

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Converting text to lower caseConverting text to lower case

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Converting text to strictly lowercasepublic static void main(String[] args) {

Scanner stdin = new Scanner (System.in);System.out.println("Enter input to be converted:");

String converted = "";while (stdin.hasNext()) { String currentLine = stdin.nextLine(); String currentConversion =

currentLine.toLowerCase(); converted += (currentConversion + "\n");}

System.out.println("\nConversion is:\n" + converted);

}

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Sample run

A Ctrl+z wasentered. It is theWindows escape

sequence forindicatingend-of-file

An empty linewas entered

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Program DemoProgram Demo LowerCaseDisplay.javaLowerCaseDisplay.java

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Program tracepublic static void main(String[] args) {

Scanner stdin = new Scanner (System.in);

System.out.println("Enter input to be converted:");

String converted = "";

while (stdin.hasNext()) { String currentLine = stdin.nextLine(); String currentConversion = currentLine.toLowerCase(); converted += (currentConversion + "\n");}

System.out.println("\nConversion is:\n" + converted);

}

public static void main(String[] args) {

Scanner stdin = new Scanner (System.in);

System.out.println("Enter input to be converted:");

String converted = "";

while (stdin.hasNext()) { String currentLine = stdin.nextLine(); String currentConversion = currentLine.toLowerCase(); converted += (currentConversion + "\n");}

System.out.println("\nConversion is:\n" + converted);

}

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Program trace

Representation of lower caseconversion of current input line

converted += (currentConversion + "\n");

The append assignment operator updates the representationof converted to include the current input line

Newline character is neededbecause method nextLine()

"strips" them from the input

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Loop Design & Reading From Loop Design & Reading From a Filea File

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Loop design Questions to consider in loop design and analysis

What initialization is necessary for the loop’s test expression?

What initialization is necessary for the loop’s processing? What causes the loop to terminate? What actions should the loop perform? What actions are necessary to prepare for the next

iteration of the loop? What conditions are true and what conditions are false

when the loop is terminated? When the loop completes what actions are need to

prepare for subsequent program processing?

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Reading a file Background

Same Scanner class!

Scanner fileIn = new Scanner (new File (filename) );

The File class allows access to filesIt’s in the java.io package

filename is a String

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Reading a file Class File

Allows access to files (etc.) on a hard drive

Constructor File (String s) Opens the file with name s so that values can be

extracted Name can be either an absolute pathname or a pathname

relative to the current working folder

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Reading a fileScanner stdin = new Scanner (System.in);

System.out.print("Filename: ");String filename = stdin.nextLine();

Scanner fileIn = new Scanner (new File (filename));

String currentLine = fileIn.nextLine();

while (currentLine != null) {System.out.println(currentLine);

currentLine = fileIn.nextLine();}

Scanner stdin = new Scanner (System.in);

System.out.print("Filename: ");String filename = stdin.nextLine();

Scanner fileIn = new Scanner (new File (filename));

String currentLine = fileIn.nextLine();

while (currentLine != null) {System.out.println(currentLine);

currentLine = fileIn.nextLine();}

Set up standard input streamDetermine file nameSet up file streamProcess lines one by oneGet first lineMake sure got a line to processDisplay current lineGet next lineMake sure got a line to processIf not, loop is doneClose the file stream

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The For statementThe For statement

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The For Statement

currentTerm = 1;

for ( int i = 0; i < 5; ++i ) {System.out.println(currentTerm);currentTerm *= 2;}

After each iteration of thebody of the loop, the updateexpression is reevaluated

The body of the loop iterateswhile the test expression is

trueint

Initialization stepis performed onlyonce -- just prior

to the firstevaluation of thetest expression

The body of the loop displays thecurrent term in the number series.It then determines what is to be thenew current number in the series

ForExpr

Action

true false

ForInit

PostExpr

Evaluated onceat the beginning

of the forstatements's

execution The ForExpr isevaluated at the

start of eachiteration of the

loopIf ForExpr is true,

Action isexecuted

After the Actionhas completed,

thePostExpression

is evaluated

If ForExpr isfalse, program

executioncontinues with

next statement

After evaluating thePostExpression, the next

iteration of the loop starts

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for statement syntax

Logical test expression that determines whether the action and update step areexecuted

for ( ForInit ; ForExpression ; ForUpdate ) Action

Update step is performed afterthe execution of the loop body

Initialization step prepares for thefirst evaluation of the test

expression

The body of the loop iterates wheneverthe test expression evaluates to true

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for vs. while A for statement is almost like a while statement

for ( ForInit; ForExpression; ForUpdate ) Action

is ALMOST the same as:

ForInit;while ( ForExpression ) {

Action;ForUpdate;

}

This is not an absolute equivalence! We’ll see when they are different below

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Variable declaration You can declare a variable in any block:

while ( true ) {int n = 0;n++;System.out.println (n);

}System.out.println (n);

Variable n gets created (and initialized) each time

Thus, println() always prints out 1

Variable n is not defined once while

loop ends

As n is not defined here, this causes

an error

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Variable declaration You can declare a variable in any block:

if ( true ) {int n = 0;n++;System.out.println (n);

}System.out.println (n);

Only difference from last slide

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System.out.println("i is " + i);}

System.out.println("all done");

System.out.println("i is " + i);}

System.out.println("all done");

i is 0i is 1i is 2all done

Execution Tracei 0int i = 0; i < 3; ++ifor ( ) {int i = 0; i < 3; ++i 123

Variable i has gone out of scope – it

is local to the loop

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for vs. while An example when a for loop can be directly translated into a while

loop:

int count;for ( count = 0; count < 10; count++ ) {

System.out.println (count);}

Translates to:

int count;count = 0;while (count < 10) {

System.out.println (count);count++;

}

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for vs. while An example when a for loop CANNOT be directly translated

into a while loop:

for ( int count = 0; count < 10; count++ ) {System.out.println (count);

}

Would (mostly) translate as:

int count = 0;while (count < 10) {

System.out.println (count);count++;

} count IS defined here

count is NOT defined here

only difference

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for loop indexing Java (and C and C++) indexes everything from zero

Thus, a for loop like this:

for ( int i = 0; i < 10; i++ ) { ... }

Will perform the action with i being value 0 through 9, but not 10

To do a for loop from 1 to 10, it would look like this:

for ( int i = 1; i <= 10; i++ ) { ... }

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Nested loopsint m = 2;int n = 3;for (int i = 0; i < n; ++i) {

System.out.println("i is " + i);for (int j = 0; j < m; ++j) {

System.out.println(" j is " + j);}

}i is 0 j is 0 j is 1i is 1 j is 0 j is 1i is 2 j is 0 j is 1

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Nested loopsint m = 2;int n = 4;for (int i = 0; i < n; ++i) {

System.out.println("i is " + i);for (int j = 0; j < i; ++j) {

System.out.println(" j is " + j);}

}

i is 0i is 1 j is 0i is 2 j is 0 j is 1i is 3

j is 0j is 1j is 2

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do-while loopsdo-while loops

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The do-while statement Syntax

do Action while (Expression)

Semantics Execute Action If Expression is true then

execute Action again Repeat this process until

Expression evaluates to false

Action is either a single statement or a group of statements within braces

Action

true

false

Expression

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Picking off digits Consider

System.out.print("Enter a positive number: ");int number = stdin.nextInt();do { int digit = number % 10; System.out.println(digit); number = number / 10;} while (number != 0);

Sample behaviorEnter a positive number: 11299211

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Guessing a number This program will allow the user to guess the number the

computer has “thought” of

Main code block:

do {System.out.print ("Enter your guess:

");guessedNumber = stdin.nextInt();count++;

} while ( guessedNumber != theNumber );

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Program DemoProgram Demo GuessMyNumber.javaGuessMyNumber.java

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while vs. do-while If the condition is false:

while will not execute the action do-while will execute it once

while ( false ) {System.out.println (“foo”);

}

do {System.out.println (“foo”);

} while ( false );

never executed

executed once

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while vs. do-while A do-while statement can be translated into a while statement

as follows:

do {Action;

} while ( WhileExpression );

can be translated into:

boolean flag = true;while ( WhileExpression || flag ) {

flag = false;Action;

}

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Hand PaintingsHand Paintings

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Loop controlsLoop controls

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The continue keyword The continue keyword will immediately start the next iteration of the

loop The rest of the current loop is not executed

for ( int a = 0; a <= 10; a++ ) {if ( a % 2 == 0 ) {

continue;}System.out.println (a + " is odd");

}

Output: 1 is odd3 is odd5 is odd7 is odd9 is odd

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The break keyword The break keyword will immediately stop the execution of the loop

Execution resumes after the end of the loop

for ( int a = 0; a <= 10; a++ ) {if ( a == 5 ) {break;}System.out.println (a + " is less than five");}

Output: 0 is less than five1 is less than five2 is less than five3 is less than five4 is less than five

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Four HobosFour Hobos

54

Four Hobos An example of a program that uses nested for loops

Credited to Will Shortz, crossword puzzle editor of the New York Times And NPR’s Sunday Morning Edition puzzle person

This problem is in section 6.10 of the text

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Problem Four hobos want to split up 200 hours of work The smart hobo suggests that they draw straws with numbers

on it If a straw has the number 3, then they work for 3 hours on 3

days (a total of 9 hours) The smart hobo manages to draw the shortest straw How many ways are there to split up such work? Which one did the smart hobo choose?

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Analysis We are looking for integer solutions to the formula:

a2+b2+c2+d2 = 200 Where a is the number of hours & days the first hobo

worked, b for the second hobo, etc.

We know the following: Each number must be at least 1 No number can be greater than 200 = 14 That order doesn’t matter

The combination (1,2,1,2) is the same as (2,1,2,1) Both combinations have two short and two long

straws

We will implement this with nested for loops

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Implementationpublic class FourHobos {

public static void main (String[] args) {for ( int a = 1; a <= 14; a++ ) { for ( int b = 1; b <= 14; b++ ) {for ( int c = 1; c <= 14; c++ ) { for ( int d = 1; d <= 14; d++ ) {if ( (a <= b) && (b <= c) && (c <= d) ) { if ( a*a+b*b+c*c+d*d == 200 ) {System.out.println ("(" + a + ", " + b+ ", " + c + ", " + d + ")"); }} }} }}}

}

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Program DemoProgram Demo FourHobos.javaFourHobos.java

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Results The output:

(2, 4, 6, 12)(6, 6, 8, 8)

Not surprisingly, the smart hobo picks the short straw of the first combination

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Alternate implementation We are going to rewrite the old code in the inner most for

loop:

if ( (a <= b) && (b <= c) && (c <= d) ) { if ( a*a+b*b+c*c+d*d == 200 ) {System.out.println ("(" + a + ", " + b+ ", " + c + ", " + d + ")"); }}

First, consider the negation of ( (a <= b) && (b <= c) && (c <= d) )

It’s ( !(a <= b) || !(b <= c) || !(c <= d) ) Or ( (a > b) || (b > c) || (c > d) )

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Alternate implementation This is the new code for the inner-most for loop:

if ( (a > b) || (b > c) || (c > d) ) {continue;

}if ( a*a+b*b+c*c+d*d != 200 ) {

continue;}System.out.println ("(" + a + ", " + b + ", "

+ c + ", " + d + ")");

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Today’s demotivatorsToday’s demotivators

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3 card poker3 card poker

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3 Card Poker This is the looping HW from last fall The problem: count how many of each type of hand in a 3 card

poker game

Standard deck of 52 cards (no jokers) Four suits: spades, clubs, diamonds, hearts 13 Faces: Ace, 2 through 10, Jack, Queen, King

Possible poker hands Pair: two of the cards have the same face value Flush: all the cards have the same suit Straight: the face values of the cards are in succession Three of a kind: all three cards have the same face value Straight flush: both a flush and a straight

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The Card class A Card class was provided

Represents a single card in the deck

Constructor: Card(int i) If i is in the inclusive interval 1 ... 52 then a card is

configured in the following manner If 1 <= i <= 13 then the card is a club If 14 <= i <= 26 then the card is a diamond If 27 <= i <= 39 then the card is a heart If 40 <= i <= 52 then the card is a spade If i % 13 is 1 then the card is an Ace; If i % 13 is 2, then the card is a 2, and so on.

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Card class methods String getFace()

Returns the face of the card as a String String getSuit()

Returns the suit of the card as a String int getValue()

Returns the value of the card boolean equals(Object c)

Returns whether c is a card that has the same face and suit as the invoking card

String toString() Returns a text representation of the card. You may find

this method useful during debugging.

68

The Hand class A Hand class was (partially) provided

Represents the three cards the player is holding

Constuctor: Hand(Card c1, Card c2, Card c3) Takes those cards and puts them in sorted order

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Provided Hand methods public Card getLow()

Gets the low card in the hand public Card getMiddle()

Gets the middle card in the hand public Card getHigh()

Gets the high card in the hand public String toString()

We’ll see the use of the toString() method later public boolean isValid()

Returns if the hand is a valid hand (no two cards that are the same)

public boolean isNothing() Returns if the hand is not one of the “winning” hands

described before

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Hand Methods to Implement The assignment required the students to implement the other

methods of the Hand class We haven’t seen this yet

The methods returned true if the Hand contained a “winning” combination of cards public boolean isPair() public boolean isThree() public boolean isStraight() public boolean isFlush() public boolean isStraightFlush()

71

Class HandEvaluation Required nested for loops to count the total number of each

hand

Note that the code for this part may not appear on the website

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Program DemoProgram Demo HandEvaluation.javaHandEvaluation.java

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Today’s demotivatorsToday’s demotivators