Iteration

1
Iteration

• Dr. Tim Margush
• University of Akron

2
Goals

• Know how to use the basic looping structures in
  Java (while, for, and do)
• Be able to implement nested structures when
  appropriate
• Understand typical design methods for handing
  input
• Explore random simulation techniques

3
Iteration

• Basic Control Structures
• Sequence
• Decision
• Iteration (repetition, looping)
• Java provides 3 basic iteration structures
• while
• do
• for

4
While Loop

• Java's while structure
• Allows a statement to be repeated 0 or more times
• Repetition is controlled by a Boolean expression
• Warning
• Loops may never end

But the cat came back the very next day,The cat
came back, we thought he was a gonerBut the cat
came back it just couldn't stay away.Away,
away, yea, yea, yea
5
While Syntax
//How to become a millionaire

• while (expression) statement
• while (expression)
• Statement list

netWorthlt 1e6?
netWorthdeposit
true branch (iterate)
false branch (exit)
Block Statement
6
Application Input Validation

• The task is to require a user to enter a positive
  integer
• At least one input is required
• We cannot predict in advance how many illegal
  entries will occur
• This may go on forever!

• Scanner console
• new Scanner(System.in)
• int posInt0
• while (posIntlt0)
• System.out.print("Enter a positive number ")
• posIntconsole.nextInt()

7
Error Infinite Loop

• Congratulations on your new job!
• Day 1 your wage is 1 penny
• Each additional day your wage doubles
• When can we party?
• Can you spot the error?

• //How to become a
• //millionaire in a few days
• double wage.01, netWorth0
• int days0
• while (netWorthlt1e6)
• days 1 //count days
• netWorth wage //payday
• System.out.println( "Your net worth after
  " days " days is " netWorth)
• wage 2 //double wage

Click to view animation steps
Animation Complete
8
Error Off By One

• Calculating n factorial
• n! 1 2 3 n
• Initial values?
• nfact 0 or 1?
• Condition?
• gt0, gt0?
• Pre or Post Decrement?
• --n or n
• Decrement placement?
• In the condition (--n gt 0) or in the
  multiplication?

• //Calculate n!
• static public int factorial(int n)
• int nfact1 //the answer
• while (n gt 0) nfact n--
• return nfact

9
Do Loop

• While is a pre-test loop
• Loop condition is evaluated at the 'top' of the
  loop
• Do (while) is a post-test Loop
• Loop condition is evaluated at the bottom of the
  loop
• Used when the loop body must be executed at least
  once
• Used when the condition depends on the outcome of
  the calculations in the loop body

10
Do-While Syntax

• do
• statement
• while (expression)
• do
• Statement list
• while (expression)

xdice.roll()
!x.doubles()
true branch (iterate)
false branch (exit)
Keep rolling until you get doubles
11
Application Until

• Retrieve coordinates from a map until the point
  is in quadrant 1
• until xgt0 AND ygt0
• The while condition is the negation of the until
  condition
• while (!(xgt0 ygt0))
• Apply DeMorgan's Law
• while (xlt0 ylt0))

• //First quadrant coordinates
• double x, y
• do
• xdata.getAbscissa()
• ydata.getOrdinate()
• while (xlt0 ylt0)
• System.out.println(
• "("x","y") is in Quad 1"
• )

12
For Loop

• Combines the three fundamental parts of every
  loop
• Initialization
• Iteration control
• Condition update
• Most commonly used for count-controlled loops

13
For Syntax
int total0 int count1

• for (exp1 exp2 exp3)
• statement
• for (exp1 exp2 exp3)
• Statement list

count
countlt5?
true branch (iterate)
false branch (exit)
totaldice.roll()
Roll the dice 5 times
Click to view animation steps
Animation Complete
14
Application Sequences

• Find the sum of the first 20 terms of the
  Fibonacci sequence
• 11235
• This is a common example of a counting loop
• Notice that term is declared and initialized in
  the loop structure and is only defined for the
  life of the loop

• //fib_a and fib_b are the previous
• // two terms of the sequence
• int fib_n, fib_a1, fib_b1
• int sum2 //includes first 2 terms
• for (int term3 termlt20 term)
• fib_n fib_afib_b
• sum fib_n
• fib_a fib_b //keep only fib_b
• fib_b fib_n // and fib_n
• System.out.println("The sum is "sum)

15
Simple Bar Chart

• //data comes from data object
• for (xdata.next() //initialize
• xgt0 //stop when data lt0
• xdata.next()) //update
• //row of x asterisks
• for (int i0 iltx i)
• System.out.print("")
• //append row size
• System.out.println("("x")")

• (6)
• (12)
• (2)
• (8)
• These are nested loops
• The outer loop iterates through the data values
• The inner loop iterates to display the correct
  number of 's

16
Iterating For Input

• Looping to process a list of input values is a
  common task
• How can the looping process be controlled?
• Count control
• Know in advance how many inputs
• Query control
• Ask before each input if there are more values
• Sentinel control
• Continue reading data until a special value is
  encountered

17
Count Controlled Input

• First data value is the number of items in the
  set
• Common when data comes from a file
• Not friendly when data is entered by a human
• This example uses a Scanner named console

• System.out.print("How many items? ")
• int num console.nextInt()
• for (int ct1 ctltnum ct)
• System.out.print( "Enter next
  item ")
• String data console.nextLine()//process
  input data

18
Query Controlled Input

• Only used for interactive input
• Requires the user to respond to a query before or
  after every input action
• The example uses a Scanner named console and asks
  after each input

• final String LOOP_RESP"y"
• do
• System.out.print("Input ")
• String data console.nextLine()//process
  input data
• System.out.print( "More data?
  (y/n) ")
• while ( LOOP_RESP.equals(
• console.nextLine())
• //Loop terminates when input is
• // NOT equal to
• // LOOP_RESPONSE

19
Sentinel Controlled Input

• Each input item can be either a data value to be
  processed or a special value that halts input
  (sentinel)
• In this example, the value "" is used as the
  sentinel value

• final String SENTINEL""
• boolean isSentinelfalse
• while(!isSentinel)
• System.out.print("Input ")
• String data console.nextLine()
• if (data.equals(SENTINEL))
• isSentinel true
• else
• //process input data
• //Loop terminates when sentinel value is detected

20
Loop and a Half

• boolean isSentinelfalse
• while(!isSentinel)
• System.out.print("Input ")
• String data console.nextLine()
• if (data.equals(SENTINEL))
• isSentinel true
• else
• //process input data

isSentinelfalse
!isSentinel
dataconsole.nextLine()
data.equals(SENTINEL)?
//process data
isSentineltrue
Problem Iteration test occurs in the MIDDLE of
the loop
21
Priming Input
data is primed before loop
dataconsole.nextLine()

• System.out.print("Input ")
• String data console.nextLine()
• while(!data.equals(SENTINEL))
• //process input data
• System.out.print("Input ")
• data console.nextLine()

!data.equals( SENTINEL)
//process data
dataconsole.nextLine()
data is updated before condition is tested
22
Middle Of Loop Exit

• while(true)
• System.out.print("Input ")
• String data console.nextLine()
• if (data.equals(SENTINEL))
• break //terminate loop
• //process input data
• //Loop exits from middle when sentinel is detected

//Branch never taken
true
true
dataconsole.nextLine()
true
data.equals(SENTINEL)?
break
//process data
Actual loop exit
23
Alternate Sentinel Control

• Sentinel controlled input may be accomplished in
  the control expression
• The value is assigned and tested against the
  sentinel in this single expression
• This loop terminates naturally at the top
• The loop body is all about processing data
• The private method combines the prompt and input
  extraction actions

• while(
• !( data getInput(console) )
• .equals(SENTINEL) )
• //process input data
• private String getInput(
• Scanner con)
• System.out.print("Input ")
• return con.nextLine()

24
Character Access

• Access to individual characters of a String is
  possible using the charAt method
• aString.charAt(pos)
• Iterating is commonly done with a for loop
• Legal positions range from 0 through
  aString.length()-1

• String s "3a5ru831T"
• final String DGT"0123456789"
• int dv, sum 0
• for (int p0 plts.length() p)
• //sum the digits in string
• char x s.charAt(p)
• if ((dvDGT.indexOf(x))gt0)
• sum dv

25
Monte Carlo Simulations

• Using random numbers to simulate events and make
  statistical inferences
• Require the ability to access a sequence of
  random numbers
• import java.util.Random
• Random gennew Random()
• The object gen is capable of producing random
  numbers on demand

This should execute only once in the life of your
program!
26
Robin Hood

• Random coordnew Random()
• int totalscores0
• final int TRYS10000, ARROWS6, SIZE10
• for (int trial0 trialltTRYS trial)
• int score0
• for (int shot0 shotltARROWS shot)
• double x,y
• xSIZE-coord.nextDouble()2SIZE
• ySIZE-coord.nextDouble()2SIZE
• double distMath.sqrt(xxyy)
• if (distltSIZE) scoreSIZE-Math.floor(dist)
• totalscoresscore
• System.out.println( "Avg " (double)totalscores/
  TRYS )

• Problem What is the expected score for 6 arrows
  shot at a target given that the archer always
  hits the target square (randomly)?
• MC Solution
• Generate 6 random 'hit' locations, total score
• Do this many times and determine the average score

27
Summary

• Java provides 3 basic looping structures
• while, do-while, and for
• Input loops can be controlled several ways
• count, query, and sentinel control
• The break statement can cause a middle-of-loop
  exit
• The Random class can generate numbers that can be
  used in Monte Carlo simulations