Iteration

1
Iteration

• Chapter 6
• Fall 2005
• CS 101
• Aaron Bloomfield

2
Java looping

• Options
• while
• do-while
• for
• Allow programs to control how many times a
  statement list is executed

3
Averaging values
4
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
• Dont how big of a list to process
• Need ability to repeat code as needed

5
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

6
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

7

• 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 gt 0)
• // add value to running total
• // processed another value
• // prepare next iteration - get next value
• // display result
• if (valuesProcessed gt 0)

8

• int valuesProcessed 0
• double valueSum 0
• // set up the input
• Scanner stdin new Scanner (System.in)
• // prompt user for values
• System.out.println("Enter positive numbers 1 per
  line.\n"
• "Indicate end of the list with a negative
  number.")
• // get first value
• double value stdin.nextDouble()
• // process values one-by-one
• while (value gt 0)
• valueSum value
• valuesProcessed
• value stdin.nextDouble()

9
Program Demo

• NumberAverage.java

10
Todays dose of demotivators
11
While syntax and semantics
Expression

Action
while

(

)

12
While semantics for averaging problem
// process values one-by-one while ( value gt 0 )
// add value to running total valueSum
value // we processed another
value valueProcessed // prepare to iterate
get the next input value stdin.nextDouble()

13
While Semantics
Expression
false
true
Action
14
Execution Trace
Suppose input contains 4.5 0.5 1.3 -1
Suppose input contains 4.5 0.5 1.3 -1
Suppose input contains 4.5 0.5 1.3 -1
Suppose input contains 4.5 0.5 1.3 -1
Suppose input contains 4.5 0.5 1.3 -1
0
valuesProcessed
1
2
3
4.5
valueSum
0
5.0
6.3
int valuesProcessed 0 double valueSum
0 double value stdin.nextDouble() while
(value gt 0) valueSum value valuesProces
sed value stdin.nextDouble() if
(valuesProcessed gt 0) double average
valueSum / valuesProcessed System.out.println("A
verage " average) else System.out.println
("No list to average")
int valuesProcessed 0 double valueSum
0 double value stdin.nextDouble() while
(value gt 0) valueSum value valuesProces
sed value stdin.nextDouble() if
(valuesProcessed gt 0) double average
valueSum / valuesProcessed System.out.println("A
verage " average)
value
4.5
0.5
1.3
-1
average
2.1
15
Converting text to lower case
16
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)

17
Sample run
18
Program Demo

• LowerCaseDisplay.java

19
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)
20
Program trace
converted (currentConversion "\n")
21
Loop Design Reading From a File
22
Loop design

• Questions to consider in loop design and analysis
• What initialization is necessary for the loops
  test expression?
• What initialization is necessary for the loops
  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?

23
Reading a file

• Background

Scanner fileIn new Scanner (new File (filename)
)
24
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

25
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.o
ut.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
26
All your base are belong to us

• All your base history http//en.wikipedia.org/wik
  i/All_your_base
• Flash animation http//www.planettribes.com/allyo
  urbase/AYB2.swf

27
End of lecture on 5 October 2005

• Also went over
• Lab 6
• HW J4
• Mid semester review results

28
The For statement
29
The For Statement
currentTerm 1
int
for ( int i 0 i lt 5 i )
System.out.println(currentTerm)
currentTerm 2

30
ForInit
ForExpr
true
false
Action
PostExpr
31
for statement syntax
for
ForInit

ForExpression

ForUpdate

Action

(





)
32
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!
• Well see when they are different below

33
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
34
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
35
Execution Trace
i
0
1
2
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

int i 0
i lt 3
i
for (
)
int i 0
i lt 3
i
Variable i has gone out of scope it is local
to the loop
36
for vs. while

• An example when a for loop can be directly
  translated into a while loop
• int count
• for ( count 0 count lt 10 count )
• System.out.println (count)
• Translates to
• int count
• count 0
• while (count lt 10)
• System.out.println (count)
• count

37
for vs. while

• An example when a for loop CANNOT be directly
  translated into a while loop
• for ( int count 0 count lt 10 count )
• System.out.println (count)
• Would (mostly) translate as
• int count 0
• while (count lt 10)
• System.out.println (count)
• count

only difference
count is NOT defined here
count IS defined here
38
for loop indexing

• Java (and C and C) indexes everything from zero
• Thus, a for loop like this
• for ( int i 0 i lt 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 lt 10 i ) ...

39
Nested loops

• int m 2
• int n 3
• for (int i 0 i lt n i)
• System.out.println("i is " i)
• for (int j 0 j lt m j)
• System.out.println(" j is " j)

• i is 0
• j is 0
• j is 1
• i is 1
• j is 0
• j is 1
• i is 2
• j is 0
• j is 1

40
Nested loops

• int m 2
• int n 4
• for (int i 0 i lt n i)
• System.out.println("i is " i)
• for (int j 0 j lt i j)
• System.out.println(" j is " j)

• i is 0
• 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

41
The 2005 Ig Nobel Prizes

• Agricultural history
• Physics
• Medicine
• Literature
• Peace
• Economics
• Chemistry
• Biology
• Nutrition
• Fluid dynamics

The Significance of Mr. Richard Buckleys
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
42
do-while loops
43
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

44
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 behavior
• Enter a positive number 1129
• 9
• 2
• 1
• 1

45
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 )

46
Program Demo

• GuessMyNumber.java

47
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
48
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

49
Hand Paintings
50
Loop controls
51
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 lt 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

52
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 lt 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

53
Four 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 NPRs Sunday Morning Edition puzzle person
• This problem is in section 6.10 of the text

55
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?

56
Analysis

• We are looking for integer solutions to the
  formula
• a2b2c2d2 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 doesnt 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

57
Implementation

• public class FourHobos
• public static void main (String args)
• for ( int a 1 a lt 14 a )
• for ( int b 1 b lt 14 b )
• for ( int c 1 c lt 14 c )
• for ( int d 1 d lt 14 d )
• if ( (a lt b) (b lt c) (c lt d) )
• if ( aabbccdd 200 )
• System.out.println ("(" a ", " b
• ", " c ", " d ")")

58
Program Demo

• FourHobos.java

59
Results

• The output
• (2, 4, 6, 12)
• (6, 6, 8, 8)
• Not surprisingly, the smart hobo picks the short
  straw of the first combination

60
Alternate implementation

• We are going to rewrite the old code in the inner
  most for loop
• if ( (a lt b) (b lt c) (c lt d) )
• if ( aabbccdd 200 )
• System.out.println ("(" a ", " b
• ", " c ", " d ")")
• First, consider the negation of
• ( (a lt b) (b lt c) (c lt d) )
• Its ( !(a lt b) !(b lt c) !(c lt d) )
• Or ( (a gt b) (b gt c) (c gt d) )

61
Alternate implementation

• This is the new code for the inner-most for loop
• if ( (a gt b) (b gt c) (c gt d) )
• continue
• if ( aabbccdd ! 200 )
• continue
• System.out.println ("(" a ", " b ", "
• c ", " d ")")

62
Todays demotivators
63
End of lecture on 10 October 2005
64
3 card poker
65
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

66
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 lt i lt 13 then the card is a club
• If 14 lt i lt 26 then the card is a diamond
• If 27 lt i lt 39 then the card is a heart
• If 40 lt i lt 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.

67
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

69
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()
• Well 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

70
Hand Methods to Implement

• The assignment required the students to implement
  the other methods of the Hand class
• We havent 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

72
Program Demo

• HandEvaluation.java

73
Todays demotivators