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Iteration. Chapter 6 Fall 2005 CS 101 Aaron Bloomfield. Java looping. Options while do-while for Allow programs to control how many times a statement list is executed. Averaging values. Averaging. Problem

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Iteration

Iteration

Chapter 6

Fall 2005

CS 101

Aaron Bloomfield


Java looping
Java looping

  • Options

    • while

    • do-while

    • for

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



Averaging
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


Averaging1
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


Averaging2
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.

    4.5

    0.5

    1.3

    -1

    Average 2.1


Iteration

public class NumberAverage {

// main(): application entry point

public static void main(String[] args) {

// set up the input

// prompt user for values

// get first value

// process values one-by-one

while (value >= 0) {

// add value to running total

// processed another value

// prepare next iteration - get next value

}

// display result

if (valuesProcessed > 0)

// compute and display average

else

// indicate no average to display

}

}


Iteration

int valuesProcessed = 0;

double valueSum = 0;

// set up the input

Scanner stdin = new Scanner (System.in);

// prompt user for values

System.out.println("Enterpositivenumbers1perline.\n"

+ "Indicate end of the list with a negative number.");

// get first value

double value = stdin.nextDouble();

// process values one-by-one

while (value >= 0) {

valueSum += value;

++valuesProcessed;

value = stdin.nextDouble();

}

// display result

if (valuesProcessed > 0) {

double average = valueSum / valuesProcessed;

System.out.println("Average: " + average);

} else {

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

}


Program demo
Program Demo

  • NumberAverage.java



While syntax and semantics

Logical expression that

Action is either a single

determines whether Action

statement or a statement

is to be executed

list within braces

While syntax and semantics

Expression

Action

while

(

)


While semantics for averaging problem

Test expression is evaluated at the

start of each iteration of the loop.

If test expression is true, these statements

are executed. Afterward, the test expression

is reevaluated and the process repeats

While semantics for averaging problem

// process values one-by-one

while ( value >= 0 ) {

// add value to running total

valueSum += value;

// we processed another value

++valueProcessed;

// prepare to iterate – get the next input

value = stdin.nextDouble();

}


While semantics

Expression is

evaluated at the

start of each

iteration of the

loop

If Expression is

true, Action is

executed

If Expression is

false, program

execution

continues with

next statement

While Semantics

Expression

false

true

Action


Execution trace
Execution Trace

Suppose input contains: 4.50.51.3-1

Suppose input contains: 4.50.51.3 -1

Suppose input contains: 4.5 0.5 1.3 -1

Suppose input contains: 4.5 0.5 1.3 -1

Suppose input contains: 4.50.5 1.3 -1

valuesProcessed

1

0

3

2

4.5

valueSum

0

6.3

5.0

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);

value

1.3

4.5

-1

0.5

average

2.1



Converting text to strictly lowercase
Converting text to strictly lowercase

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);

}


Sample run

An empty line

was entered

A Ctrl+z was

entered. I

t is the

Windows escape

sequence for

indicating

end-of-file

Sample run


Program demo1
Program Demo

  • LowerCaseDisplay.java


Program trace
Program trace

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);

}

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);

}


Program trace1

The append assignment operator updates the representation

of converted to include the current input line

Representation of lower case

Newline character is needed

conversion of current input line

because method nextLine()

"strips" them from the input

Program trace

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



Loop design
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?


Reading a file

Same Scanner class!

filename is a String

The File class allows access to files

It’s in the java.io package

Reading a file

  • Background

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


Reading a file1
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


Reading a file2
Reading a file

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();

}

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 stream

Determine file name

Set up file stream

Process lines one by one

Get first line

Make sure got a line to process

Display current line

Get next line

Make sure got a line to process

If not, loop is done

Close the file stream


All your base are belong to us
All your base are belong to us

  • All your base history: http://en.wikipedia.org/wiki/All_your_base

  • Flash animation: http://www.planettribes.com/allyourbase/AYB2.swf



The for statement1

The body of the loop iterates

while the test expression is

Initialization step

true

is performed only

After each iteration of the

once -- just prior

body of the loop, the update

to the first

expression is reevaluated

evaluation of the

test expression

The body of the loop displays the

current term in the number series.

It then determines what is to be the

new current number in the series

The For Statement

int

currentTerm = 1;

for ( int i = 0; i < 5; ++i ) {

System.out.println(currentTerm);

currentTerm *= 2;

}


Iteration

Evaluated once

at the beginning

of the for

statements's

The ForExpr is

execution

evaluated at the

start of each

iteration of the

loop

If ForExpr is true,

Action is

executed

After the Action

If ForExpr is

has completed,

false, program

the

execution

PostExpression

continues with

is evaluated

next statement

After evaluating the

PostExpression, the next

iteration of the loop starts

ForInit

ForExpr

true

false

Action

PostExpr


For statement syntax

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

executed

Initialization step prepares for the

first evaluation of the test

Update step is performed after

expression

the execution of the loop body

The body of the loop iterates whenever

the test expression evaluates to true

for statement syntax

ForInit

ForExpression

ForUpdate

Action

for

(

;

;

)


For vs while
for vs. while and update step are

  • 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


Variable declaration
Variable declaration and update step are

  • 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


Variable declaration1
Variable declaration and update step are

  • 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


Execution trace1
Execution Trace and update step are

i

0

2

1

3

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

}

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

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

}

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

i is 0

i is 1

i is 2

all done

for (

int i = 0;

int i = 0;

i < 3;

i < 3;

++i

++i

) {

Variable i has gone out of scope – it

is local to the loop


For vs while1
for vs. while and update step are

  • 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++;

    }


For vs while2
for vs. while and update step are

  • 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++;

    }

only difference

count is NOT defined here

count IS defined here


For loop indexing
for loop indexing and update step are

  • 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++ ) { ... }


Nested loops

i is 0 and update step are

j is 0

j is 1

i is 1

j is 0

j is 1

i is 2

j is 0

j is 1

Nested loops

int 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);

}

}


Nested loops1

i is 0 and update step are

i is 1

j is 0

i is 2

j is 0

j is 1

i is 3

j is 0

j is 1

j is 2

Nested loops

int 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);

}

}


The 2005 ig nobel prizes
The 2005 Ig Nobel Prizes and update step are

  • Agricultural history

  • Physics

  • Medicine

  • Literature

  • Peace

  • Economics

  • Chemistry

  • Biology

  • Nutrition

  • Fluid dynamics

“The Significance of Mr. Richard Buckley’s Exploding Trousers”

The pitch drop experiment, started in 1927

Neuticles – artificial replacement testicles for dogs

The 409 scams of Nigeria for a “cast of rich characters”

Locust brain scans while they were watching Star Wars

For an alarm clock that runs away, thus making people more productive

“Will Humans Swim Faster or Slower in Syrup?”

For cataloging the odors of 131 different stressed frogs

To Dr. Yoshiro Nakamats who catalogued and analyzed every meal he ate for the last 34 years (and counting)

“Pressures Produced When Penguins Pooh – Calculations on Avian Defaecation”


Do while loops

do-while loops and update step are


The do while statement

Action and update step are

true

Expression

false

The do-while statement

  • Syntax

    doAction

    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


Picking off digits
Picking off digits and update step are

  • 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 behavior

    Enter a positive number: 1129

    9

    2

    1

    1


Guessing a number
Guessing a number and update step are

  • 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 );


Program demo2
Program Demo and update step are

  • GuessMyNumber.java


While vs do while
while vs. do-while and update step are

  • 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


While vs do while1
while vs. do-while and update step are

  • 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;

    }


Hand paintings
Hand Paintings and update step are


Loop controls

Loop controls and update step are


The continue keyword
The continue keyword and update step are

  • 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 odd

    3 is odd

    5 is odd

    7 is odd

    9 is odd


The break keyword
The break keyword and update step are

  • 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 five

    1 is less than five

    2 is less than five

    3 is less than five

    4 is less than five


Four hobos

Four Hobos and update step are


Four hobos1
Four Hobos and update step are

  • 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


Problem
Problem and update step are

  • 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?


Analysis
Analysis and update step are

  • 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


Implementation
Implementation and update step are

public 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 + ")");

}

}

}

}

}

}

}

}


Program demo3
Program Demo and update step are

  • FourHobos.java


Results
Results and update step are

  • The output:

    (2, 4, 6, 12)

    (6, 6, 8, 8)

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


Alternate implementation
Alternate implementation and update step are

  • 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) )


Alternate implementation1
Alternate implementation and update step are

  • 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 + ")");


Today s demotivators
Today’s demotivators and update step are


3 card poker

3 card poker and update step are


3 card poker1
3 Card Poker and update step are

  • 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


The card class
The Card class and update step are

  • 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.


Card class methods
Card class methods and update step are

  • 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.


The hand class
The Hand class and update step are

  • 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


Provided hand methods
Provided Hand methods and update step are

  • 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


Hand methods to implement
Hand Methods to Implement and update step are

  • 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()


Class handevaluation
Class HandEvaluation and update step are

  • 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


Program demo4
Program Demo and update step are

  • HandEvaluation.java


Today s demotivators1
Today’s demotivators and update step are


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