1 Iteration Chapter 6 Fall 2005 CS 101 Aaron Bloomfield
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1
Iteration
Chapter 6Fall 2005CS 101Aaron Bloomfield
2
Java looping Options
while do-while for
Allow programs to control how many times a statement list is executed
33
Averaging valuesAveraging 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 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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All your base are belong to All your base are belong to usus
All your base history: All your base history: http://en.wikipedia.org/wiki/All_your_basehttp://en.wikipedia.org/wiki/All_your_base Flash animation: Flash animation: http://www.planettribes.com/allyourbase/AYB2.swfhttp://www.planettribes.com/allyourbase/AYB2.swf
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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
35
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
36
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++;
}
37
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
40
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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Agricultural Agricultural historyhistory
PhysicsPhysics MedicineMedicine LiteratureLiterature PeacePeace EconomicsEconomics
ChemistryChemistry BiologyBiology NutritionNutrition
Fluid dynamicsFluid dynamics
The 2005 Ig Nobel PrizesThe 2005 Ig Nobel Prizes““The Significance of Mr. Richard Buckley’s Exploding The Significance of Mr. Richard Buckley’s Exploding
Trousers”Trousers”The pitch drop experiment, started in 1927The pitch drop experiment, started in 1927Neuticles – artificial replacement testicles for dogsNeuticles – artificial replacement testicles for dogsThe 409 scams of Nigeria for a “cast of rich characters”The 409 scams of Nigeria for a “cast of rich characters”Locust brain scans while they were watching Star WarsLocust brain scans while they were watching Star WarsFor an alarm clock that runs away, thus making people For an alarm clock that runs away, thus making people
more productivemore productive““Will Humans Swim Faster or Slower in Syrup?”Will Humans Swim Faster or Slower in Syrup?”For cataloging the odors of 131 different stressed frogsFor cataloging the odors of 131 different stressed frogsTo Dr. Yoshiro Nakamats who catalogued and analyzed To Dr. Yoshiro Nakamats who catalogued and analyzed
every meal he ate for the last 34 years (and every meal he ate for the last 34 years (and counting)counting)
““Pressures Produced When Penguins Pooh – Pressures Produced When Penguins Pooh – Calculations on Avian Defaecation”Calculations on Avian Defaecation”
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do-while loopsdo-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
Action
true
false
Expression
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 behaviorEnter a positive number: 11299211
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 );
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Program DemoProgram Demo GuessMyNumber.javaGuessMyNumber.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
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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
5050
Loop controlsLoop 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 <= 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
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 <= 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
5353
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
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:
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
57
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
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 <= 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
6464
3 card poker3 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 <= 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.
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()
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
70
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
7373
Today’s demotivatorsToday’s demotivators
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