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Iteration

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Title: Iteration

1
Iteration
2
Java looping

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

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
Dont 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

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)

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()

notice the format!
8
While syntax and semantics
Expression

Action
while

(

)

9
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()

10
While Semantics
Expression
false
true
Action
11
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
12
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 ""
String currentLine stdin.nextLine()
while (currentLine ! null)
String currentConversion
currentLine.toLowerCase()
converted (currentConversion "\n")
currentLine stdin.nextLine()
System.out.println("\nConversion is\n"
converted)

13
Sample run
14
Program trace
public static void main(String args)
Scanner stdin new Scanner (System.in) Syst
em.out.println("Enter input to be
converted") String converted "" String
currentLine stdin.nextLine() while
(currentLine ! null) String
currentConversion currentLine.toLowerCase()
converted (currentConversion
"\n") currentLine stdin.nextLine() Syst
em.out.println("\nConversion is\n"
converted)
public static void main(String args)
Scanner stdin new Scanner (System.in) Syst
em.out.println("Enter input to be
converted") String converted "" String
currentLine stdin.nextLine() while
(currentLine ! null) String
currentConversion currentLine.toLowerCase()
converted (currentConversion
"\n") currentLine stdin.nextLine() Syst
em.out.println("\nConversion is\n"
converted)
15
Program trace
converted (currentConversion "\n")
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 ""
String currentLine stdin.nextLine()
while (currentLine ! null)
String currentConversion
currentLine.toLowerCase()
converted (currentConversion "\n")
currentLine stdin.nextLine()
System.out.println("\nConversion is\n"
converted)

17
All your base are belong to us

Flash animation
Reference http//en.wikipedia.org/wiki/All_your_b
ase_are_belong_to_us

18
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?

19
Reading a file

Background

Scanner fileIn new Scanner (new File (filename)
)
20
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

21
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
22
The For Statement
currentTerm 1
int
for ( int i 0 i lt 5 i )
System.out.println(currentTerm)
currentTerm 2

23
ForInit
ForExpr
true
false
Action
PostExpr
24
for statement syntax
for
ForInit

ForExpression

ForUpdate

Action

(

)
25
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

26
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
27
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
28
End of lecture on 1 November 2004
29
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
30
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

31
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 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
32
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 ) ...

33
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

34
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

35
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

36
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

37
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
38
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 ( flag WhileExpression )
flag false
Action

39
End of lecture on 3 November 2004

We watched the Kerry concession speech and the
Bush acceptance speech in class today, so after
the announcements, we only got to about 20-25
minutes of actual lecture time

40
A digression Perl again

Consider the statement
if ( !flag )
...
else
...
Perl has a command unless
unless ( flag )
...
else
...
An unless command is a if statement with a
negated condition
It can get a bit confusing, though

41
A digression Perl again

Consider the statement
while ( !flag )
...
Perl has a command until
until ( flag )
...
An until command is a while loop with a negated
condition
As most people are quite used to if-else and
while, unless and until are rarely used

42
Problem solving
43
Data set manipulation

Often five values of particular interest
Minimum
Maximum
Mean
Standard deviation
Size of data set
Lets design a data set representation
The data set represents a series of numbers
Note that the numbers themselves are not
remembered by the DataSet
Only properties of the set (average, minimum,
etc.)

44
Implication on facilitators

public double getMinimum()
Returns the minimum value in the data set. If the
data set is empty, then Double.NaN is returned,
where Double.NaN is the Java double value
representing the status not-a-number
public double getMaximum()
Returns the maximum value in the data set. If the
data set is empty, then Double.NaN is returned

45
Implication on facilitators

public double getAverage()
Returns the average value in the data set. If the
data set is empty, then Double.NaN is returned
public double getStandardDeviation()
Returns the standard deviation value of the data
set. If the data set is empty, then Double.NaN is
returned
Left to the interested student
public int getSize()
Returns the number of values in the data set
being represented

46
Constructors

public DataSet()
Initializes a representation of an empty data set
public DataSet(String s)
Initializes the data set using the values from
the file with name s
public DataSet(File filep)
Initializes the data set using the values from
the file represented by filep
Left to interested student

47
Other methods

public void addValue(double x)
Adds the value x to the data set being
represented
public void clear()
Sets the representation to that of an empty data
set
public void load(String s)
Adds the vales from the file with name s to the
data set being represented
public void load(File filep)
Adds the vales from the file represented by filep
to the data set being represented
Left to interested student

48
Instance variables