Iteration
Chapter 6
Fall 2005
CS 101
Aaron Bloomfield
1
Java looping
 Options
 while
 do-while
 for
 Allow programs to control how many times a statement list is
executed
2
Averaging values
3
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
4
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
5
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
6
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
}
}
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 >= 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

NumberAverage.java
9
Today’s dose of demotivators
10
While syntax and semantics
while
(
Expression
)
Action
Logical expression that
determines whether Action
is to be executed
Action is either a single
statement or a statement
list within braces
11
While semantics for averaging problem
Test expression is evaluated at the
start of each iteration of the loop.
// 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();
}
If test expression is true, these statements
are executed. Afterward, the test expression
is reevaluated and the process repeats
12
While Semantics
Expression is
evaluated at the
start of each
iteration of the
loop
Expression
If Expression is
true, Action is
executed
true
Action
false
If Expression is
false, program
execution
continues with
next statement
13
Suppose input contains: 4.5 0.5 1.3 -1
Execution Trace
valuesProcessed
int valuesProcessed = 0;
double valueSum = 0;
0
1
2
3
valueSum
4.5
6.3
5.0
0
value
0.5
1.3
4.5
-1
average
2.1
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");
}
14
Converting text to lower case
15
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);
}
16
Sample run
An empty line
was entered
A Ctrl+z was
entered. tI is the
Windows escape
sequence for
indicating
end-of-file
17
Program Demo

LowerCaseDisplay.java
18
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);
19
Program trace
The append assignment operator updates the representation
of converted to include the current input line
converted += (currentConversion + "\n");
Representation of lower case
conversion of current input line
Newline character is needed
because method nextLine()
"strips" them from the input
20
Loop Design & Reading From
a File
21
Loop design
 Questions to consider in loop design and analysis
 What initialization
expression?
is
necessary
for
the
loop’s
test
 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?
22
Reading a file
 Background
Same Scanner class!
filename is a String
Scanner fileIn = new Scanner (new File (filename) );
The File class allows access to files
It’s in the java.io package
23
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
24
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();
}
Close
Determine
Set
Process
Display
Get
Make
up
first
next
sure
the
standard
file
current
lines
line
line
file
stream
got
fileone
stream
name
aline
input
line
by one
tostream
process
If not, loop is done
25
All your base are belong to us
26
The For statement
28
The For Statement
The body of the loop iterates
while the test expression is
true
Initialization step
is performed only
After each iteration of the
once -- just prior int currentTerm = 1;
body of the loop, the update
to the first
expression is reevaluated
evaluation of the for ( int i = 0; i < 5; ++i ) {
test expression
System.out.println(currentTerm);
currentTerm *= 2;
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
29
Evaluated once
at the beginning
of the for
statements's
execution
If ForExpr is true,
Action is
executed
After the Action
has completed,
the
PostExpression
is evaluated
ForInit
ForExpr
true
Action
PostExpr
After evaluating the
PostExpression, the next
iteration of the loop starts
The ForExpr is
evaluated at the
start of each
iteration of the
loop
false
If ForExpr is
false, program
execution
continues with
next statement
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
expression
for
( ForInit
; ForExpression
Update step is performed after
the execution of the loop body
; ForUpdate
) Action
The body of the loop iterates whenever
the test expression evaluates to true
31
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
32
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
33
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
34
Execution Trace
i
for ( int i = 0; i < 3; ++i ) {
0
3
2
1
System.out.println("i is " + i);
}
System.out.println("all done");
i is 0
i is 1
i is 2
all done
Variable i has gone
out of scope – it
is local to the loop
35
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++;
}
36
for vs. while
 An example when a for loop CANNOT be directly translated
into a while loop:
only difference
for ( int count = 0; count < 10; count++ ) {
System.out.println (count);
}
count is NOT defined here
 Would (mostly) translate as:
int count = 0;
while (count < 10) {
System.out.println (count);
count++;
}
count IS defined here
37
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++ ) { ... }
38
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);
}
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
39
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);
}
}
i is 0
i is 1
j is
i is 2
j is
j is
i is 3
j is
j is
j is
40
0
0
1
0
1
2
The 2005 Ig Nobel Prizes






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”
41
do-while loops
42
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
Expression
false
43
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
44
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 );
45
Program Demo

GuessMyNumber.java
46
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
47
while vs. do-while
 A do-while statement
statement as follows:
can
be
translated
into
a
while
do {
Action;
} while ( WhileExpression );
 can be translated into:
boolean flag = true;
while ( WhileExpression || flag ) {
flag = false;
Action;
}
48
Hand Paintings
49
Loop controls
50
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
3
5
7
9
is
is
is
is
is
odd
odd
odd
odd
odd
51
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
1
2
3
4
is
is
is
is
is
less
less
less
less
less
than
than
than
than
than
five
five
five
five
five
52
Four Hobos
53
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
54
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?
55
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
56
Implementation
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 + ")");
}
}
}
}
}
}
}
}
57
Program Demo

FourHobos.java
58
Results
 The output:
(2, 4, 6, 12)
(6, 6, 8, 8)
 Not surprisingly, the smart hobo picks the short straw of the
first combination
59
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) )
60
Alternate implementation
 This is the new code for the inner-most for loop:
if ( (a > b) || (b > c)
continue;
}
if ( a*a+b*b+c*c+d*d !=
continue;
}
System.out.println ("("
+ c
|| (c > d) ) {
200 ) {
+ a + ", " + b + ", "
+ ", " + d + ")");
61
Today’s demotivators
62
3 card poker
64
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
65
 Straight flush: both a flush and a straight
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.
66
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.
67
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
68
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
69
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()
70
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
71
Program Demo

HandEvaluation.java
72
Today’s demotivators
73