Loop - CIS 1068 Program Design and Abstraction

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Loop - CIS 1068 Program Design and Abstraction
Zhen Jiang CIS Dept. Temple University SERC 347,
Main Campus Email zhen.jiang_at_temple.edu
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Table of Contents

• Taste of Loop
• While Loop
• Do While Loop
• For Loop
• Variations
• Controlling Number of Loop Iterations
• Loop Development
• Mapping Iterations to Counter Values
• Controlling Event of Loop Iterations
• Random number generator
• Fencepost problem (an interesting scenario)
• Summary of Learning Materials

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Taste of Loop

• Price is right.
• Sample execution (click on this link to try)
• Before you get the price right, the program will
  REPEAT

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while loop

• while loop A control structure that repeatedly
  performs a test and executes a group of
  statements if the test evaluates to true.
• while loop, general syntax
• ltinitializationgt
• while (lttestgt)
• ltbody, consisting of statement(s)gt
• Example
• int number 1
• while (number lt 200)
• System.out.print(number " ")
• number 2
• Output
• 1 2 4 8 16 32 64 128

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• The ltinitializationgt prepares the variable
  declarations and their values that are used in
  the test, update, and body of the loop.
• The lttestgt checks whether the repetition of the
  loop body can stop.
• The statement or group of statements to be
  repeated is called the ltbodygt of the loop.
• Each repetition of the loop body is called an
  ltiterationgt of the loop.

Not clear?
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• ltinitializationgt
• while (lttestgt)
• ltbodygt

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• Finds and prints a number's first factor other
  than 1
• Scanner console new Scanner(System.in)
• System.out.print("Type a number ")
• int number console.nextInt()
• int factor 2
• while (number factor ! 0)
• factor
• System.out.println("First factor " factor)
• Sample run
• Type a number 91
• First factor 7

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• Example, WhileDemo.java, P202

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Variant 1 do/while

• do/while loop A control structure that executes
  statements repeatedly while a condition is true,
  testing the condition at the end of each
  repetition.
• do/while loop, general syntax
• ltinitializationgt
• do
• ltstatement(s)gt
• while (lttestgt)
• Example
• // roll until we get a number other than 3
• Random rand new Random()
• int die
• do
• die rand.nextInt()
• while (die 3)

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• How does this differ from the while loop?
• The controlled ltstatement(s)gt will always execute
  the first time, regardless of whether the lttestgt
  is true or false.

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• Example, DoWhileDemo.java, P206

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Variant 2 for

• for loop A block of Java code that executes a
  group of statements repeatedly until a given test
  fails.
• General syntax
• for (ltinitializationgt lttestgt ltupdategt)
• ltstatementgt
• ltstatementgt
• ...
• ltstatementgt
• Example
• for (int i 1 i lt 30 i)
• System.out.println("I will not throw...")

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for (ltinitgt lttestgt ltupdategt) ltbodygt
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• Example, ForDemo.java, P219

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• Summary

Body first, and then event change/update
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For loop?
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• Initialization, test, and body, and execution
  results of loop

• Code
• for (int i 1 i lt 4 i)
• System.out.println(i " squared is " (i
  i))
• Output
• 1 squared is 1
• 2 squared is 4
• 3 squared is 9
• 4 squared is 16

1st iteration? 2nd iteration? 3rd iteration?
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Variations

• The initial and final values for the loop
  counter/event variable can be arbitrary
  expressions
• Example
• for (int i -3 i lt 2 i)
• System.out.println(i)
• Output
• -3
• -2
• -1
• 0
• 1
• 2
• Example
• for (int i 1 3 4 i lt 5248 100 i)
• System.out.println(i " squared is " (i
  i))

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• The update can be a -- (or any other operator).
• Caution This requires changing the test from lt
  to gt .
• System.out.println("T-minus")
• for (int i 3 i gt 1 i--)
• System.out.println(i)
• System.out.println("Blastoff!")
• Output
• T-minus
• 3
• 2
• 1
• Blastoff!

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• What if we wanted the output to be the following?
• T-minus 3 2 1 Blastoff!
• System.out.print prints the given output without
  moving to the next line.
• System.out.print("T-minus ")
• for (int i 3 i gt 1 i--)
• System.out.print(i " ")
• System.out.println("Blastoff!")

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• When controlling a single statement, the
  braces are optional.
• for (int i 1 i lt 6 i)?
• System.out.println(i " squared is " (i
  i))
• This can lead to errors if a line is not properly
  indented.
• for (int i 1 i lt 3 i)?
• System.out.println("This is printed 3
  times")
• System.out.println("So is this... or is
  it?")
• Output
• This is printed 3 times
• This is printed 3 times
• This is printed 3 times
• So is this... or is it?
• Moral Always use curly braces and always use
  proper indentation.

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• Extra semicolon in a loop (P218).
• int i
• for (i 1 i lt 6 i)?
• System.out.println(i " squared is " (i
  i))
• Output
• 7 squared is 49
• Comman in a loop (P220).
• int i, sum
• for (i 1, sum 0 i lt 10 i)?
• sum sum i i
• System.out.println("Result is " sum)
• Output
• 385

int sum for (int i0, sum
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• Invalidation Loops that never execute.
• for (int i 10 i lt 5 i)
• System.out.println("How many times do I
  print?")
• ERROR Loop tests that never fail.
• A loop that never terminates is called an
  infinite loop.
• for (int i 10 i gt 1 i)
• System.out.println("Runaway Java
  program!!!")

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• Loops that go on forever
• while (true)
• ltstatement(s)gt
• If it goes on forever, how do you stop?

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• break statement Immediately exits a loop (for,
  while, do/while).
• Example
• while (true)
• ltstatement(s)gt
• if (lttestgt)
• break
• ltstatement(s)gt
• Why is the break statement in an if statement?

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• Sentinel loop using break
• Scanner console new Scanner(System.in)
• int sum 0
• while (true)
• System.out.print("Enter a number (-1 to
  quit) ")
• int inputNumber console.nextInt()
• if (inputNumber -1) // don't add -1 to
  sum
• break
• sum inputNumber // inputNumber ! -1
  here
• System.out.println("The total was " sum)

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• Special case If a variable is declared in the
  ltinitializationgt part of a for loop, its scope is
  the for loop.
• public static void main(String args)
• int x 3
• int i
• for (i 1 i lt 10 i)
• System.out.println(x)
• // i no longer exists here
• // x ceases to exist here

x's scope
is scope
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• ERROR Using a variable outside of its scope.
• public static void main(String args)
• for (int i 1 i lt 10 i)
• int y 5
• System.out.println(y)
• System.out.println(i) // illegal
• System.out.println(y) // illegal

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• COMMON ERROR Using the wrong loop counter
  variable.
• But barely possible when you develop code with
  our process.
• What is the output of the following piece of
  code?
• for (int i 1 i lt 10 i)
• for (int j 1 i lt 5 j)
• System.out.print(j)
• System.out.println()
• What is the output of the following piece of
  code?
• for (int i 1 i lt 10 i)
• for (int j 1 j lt 5 i)
• System.out.print(j)
• System.out.println()

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• Exercises
• http//www.cis.temple.edu/jiang/1068LoopExecution
  .pdf

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Loop Development
 population   TV purchase  1248... 1234...
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• http//www.cis.temple.edu/jiang/LoopDevelopment.h
  tm

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Controlling Number of Loop Iterations

• If the number of iterations is known before the
  loop starts, the loop is called a
  count-controlled loop.
• Counter 0, counter, counter ltnumber
• Counter 1, counter, counter ltnumber
• Use for loop for an easy development.

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Mapping iterations to counter values

• Suppose that we have the following loop
• for (int count 0 count lt 49 count)
• ...
• What statement could we write in the body of the
  loop that would make the loop print the following
  output?
• 0 2 4 6 8
• Answer
• for (int count 0 count lt 49 count)
• System.out.print(2 count " ")

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• Now consider another loop of the same style
• for (int count 0 count lt 49 count)
• ...
• What statement could we write in the body of the
  loop that would make the loop print the following
  output?
• 3 5 7 9 11
• Answer
• for (int count 0 count lt 49 count)
• System.out.print(2 count 3 " ")

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• What statement could we write in the body of the
  loop that would make the loop print the following
  output?
• 2 7 12 17 22
• To find the pattern, it can help to make a table.
• Each time count goes up by 1, the number should
  go up by 5.
• But count 5 is too big by 3, so we must
  subtract 3.

number to print
count 5
count 5 - 3
count
2
5
2
1
7
10
7
2
12
15
12
3
17
20
17
4
22
25
22
5
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• Caution This is algebra, not assignment!
• Recall slope-intercept form (y mx b)?
• Slope is defined as rise over run (i.e. rise /
  run). Since the run is always 1 (we increment
  along x by 1), we just need to look at the
  rise. The rise is the difference between the y
  values. Thus, the slope (m) is the difference
  between y values in this case, it is 5.
• To compute the y-intercept (b), plug in the value
  of y at x 1 and solve for b. In this case, y
  2.
• y m x b
• 2 5 1 b
• Then b -3
• So the equation is
• y m x b
• y 5 x 3
• y 5 count - 3

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• Algebraically, if we always take the value of y
  at
• x 1, then we can solve for b as follows
• y m x b
• y1 m 1 b
• y1 m b
• b y1 m
• In other words, to get the y-intercept, just
  subtract the slope from the first y value (b 2
  5 -3)?
• This gets us the equation
• y m x b
• y 5 x 3
• y 5 count 3
• (which is exactly the equation from the previous
  slides)?

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• What statement could we write in the body of the
  loop that would make the loop print the following
  output?
• 17 13 9 5 1
• Let's create the loop table together.
• Each time count goes up 1, the number should ...
• But this multiple is off by a margin of ...

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• Coding (different from execution check)
• nkeyboard.nextInt() // try 6!
• for (int i 1 i lt n i)
• System.out.print("")
• System.out.println()
• Output

What is the body? Counter-controlled loop?
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• More complicate case
• nkeyboard.nextInt() // try 6!
• for (int i 1 i lt n i)
• for (int j 1 j lt n j)
• System.out.print("")
• System.out.println()
• Output

What is the body? Counter-controlled loop?
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• Code
• nkeyboard.nextInt() // try 5!
• for (int i 1 i lt n i)
• for (int j 1 j lt 10 j)
• System.out.print(
• (i j) " ")
• System.out.println()
• Output
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20
• 3 6 9 12 15 18 21 24 27 30
• 4 8 12 16 20 24 28 32 36 40
• 5 10 15 20 25 30 35 40 45 50

What is the body? Counter-controlled
loop? Initialization and body detail?
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• How to confirm the initialization correct?
• On preparing the 1st iteration
• How to ensure the detail of the body?
• A consistent view of 1st, 2nd, 3rd iterations
• Map of the counter value to the iteration
  expression

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• Code
• nkeyboard.nextInt() // try 6!
• for (i 1 iltn i) System.out.print()
• System.out.println()
• for (i 1 i lt n-2 i)
• System.out.print()
• for (int j 1 j lt n-2 j)
• System.out.print( )
• System.out.println()
• for (i 1 iltn i) System.out.print()
• System.out.println()
• Output

What is the body? Counter controlled loop
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• Code
• nkeyboard.nextInt() // try 6!
• for (int i 1 i lt n i)
• for (int j 1 j lt i j)
• System.out.print("")
• System.out.println()
• Output

i each line!
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• Code
• nkeyboard.nextInt() // try 6!
• for (int i 1 i lt n i)
• for (int j 1 j lt i j)
• System.out.print(i)
• System.out.println()
• Output
• 1
• 22
• 333
• 4444
• 55555
• 666666

How many numbers each line? What are they?
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• Code
• nkeyboard.nextInt() // try 5!
• for (int i 1 i lt n i)
• for (int j 1 j lt (n - i) j)
• System.out.print(" ")
• for (int k 1 k lt i k)
• System.out.print(i)
• System.out.println()
• Output
• 1
• 22
• 333
• 4444
• 55555

Space and numbers?
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Controlling Event of Loop Iterations

• Otherwise (unknown or unclear), the loop is
  called a event-controlled loop.
• Use a while loop or a do-while loop for an easy
  checkpoint development.
• Asking the user before each iteration if it is
  time to end the loop is called the
  ask-before-iterating technique.
• Appropriate status update (or event initializing)
  for a sequence of iterations

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• Finds and prints a number's first factor other
  than 1
• int n keyboard.nextInt() // try 91
• int f 2
• while (n f ! 0)
• f
• System.out.println("First factor" f)
• Sample run
• First factor7

Body exploring the factor. When/what to stop
the loop? n f 0 divisible! The range of
f? Initialization of f? The change of f?
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• Write a program that will repeatedly prompt the
  user to type a number until the user types a
  non-negative number, then square it.
• Example log
• Type a non-negative integer -5
• Invalid number, try again -1
• Invalid number, try again -235
• Invalid number, try again -87
• Invalid number, try again 11
• 11 squared is 121

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Body trying different value of n. When/what to
stop the loop? ngt 0 non-negative! The range of
n? -gt any (since nlt0) Initialization of n? The
change of n?

• System.out.print("Type a non-negative integer
  ")
• int n keyboard.nextInt()
• while (n lt 0)
• System.out.print("Invalid number, try again
  ")
• n keyboard.nextInt()
• int square n n
• System.out.println(n " squared is " square)
• Notice that the number variable had to be
  declared outside the while loop in order to
  remain in scope.

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• Write a class named DigitSum that reads an
  integer from the user and prints the sum of the
  digits of that number. You may assume that the
  number is non-negative.
• Example
• Enter a nonnegative number 29107
• prints out 19 (i.e.,29107 )
• Hint Use the operator to extract the last
  digit of a number. If we do this repeatedly,
  when should we stop?

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Body adding the last digit and extracting that
from the original number (for next
round) When/what to stop? All digits are
counted! n lt 0 no more need to count! The
change of n? -gt make the extraction
valid Initialization?

• import java.util.Scanner
• public class DigitSum
• public static void main(String args)
• Scanner keyboard new Scanner(System.in)
• int n keyboard.nextInt()
• int sum 0
• while (n gt 0)
• sum n 10 // add last digit to sum
• n n / 10 // remove last digit
• System.out.println(sum sum)

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Body try all possible numbers k, count k (sum)
only when n is divisible by k. When/what to stop?
Kgtn (trial from 1 is over) The range of k? -gt 1
to n Change? k Initialization?

• Write a program named CountFactors that reads in
  an integer and displays its number of factors.
• For example, if the user enters 60, CountFactors
  displays 12 because 1, 2, 3, 4, 5, 6, 10, 12, 15,
  20, 30, and 60 are all factors of 60.

Scanner keyboard new Scanner(System.in)
int n keyboard.nextInt() int sum 0, k
? while ( )
System.out.println(sum sum)
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• Scanner keyboard new Scanner(System.in)int n
  keyboard.nextInt()int k 1int sum
  0while (kltn) if(nk0) sum
  kSystem.out.print("sum " sum)

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• Exercises (Wednesday lab)

 population   TV purchase  1248... 1234...
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• Complete a Loop program
• http//www.cis.temple.edu/jiang/1068LoopComplete.
  pdf

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Random Number Generator

• Objects of the Random class generate
  pseudo-random numbers.
• Class Random is found in the java.util package.
• import java.util.
• The methods of a Random object

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• Random rand new Random()
• int randomNum rand.nextInt(10)
• // randomNum has a random value between 0 and 9
• What if we wanted a number from 1 to 10?
• int randomNum rand.nextInt(10) 1
• What if we wanted a number from min to max (i.e.
  an arbitrary range)?
• int randomNum rand.nextInt(ltsize of the rangegt)
  ltmingt
• where ltsize of the rangegt equals (ltmaxgt - ltmingt
  1)?

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• Given the following declaration, how would you
  get
• A random number between 0 and 100 inclusive?
• A random number between 1 and 100 inclusive?
• A random number between 4 and 17 inclusive?

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• Given the following declaration, how would you
  get
• A random number between 0 and 100 inclusive?
• int random1 rand.nextInt(101)
• A random number between 1 and 100 inclusive?
• int random1 rand.nextInt(100) 1
• A random number between 4 and 17 inclusive?
• int random1 rand.nextInt(14) 4

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• Write a program that simulates the rolling of two
  six-sided dice until their combined result comes
  up as 7.
• Sample run
• Roll 2 4 6
• Roll 3 5 8
• Roll 5 6 11
• Roll 1 1 2
• Roll 4 3 7
• You won after 5 tries!

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Body try all possible numbers When/what to
stop? Sum 7 The range of sum? -gt any (since
!7) Change? Depends on roll1 and
roll2 Initialization? Any, if !7, e.g., 0

• import java.util.
• public class Roll
• public static void main(String args)
• Random rand new Random()
• int sum 0
• int tries 0
• while (sum ! 7)
• int roll1 rand.nextInt(6) 1
• int roll2 rand.nextInt(6) 1
• sum roll1 roll2
• System.out.println("Roll " roll1
  " " roll2 " " sum)
• tries
• System.out.println("You won after "
  tries " tries!")

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Fencepost Problem

• Fencepost Problem Write a class named
  PrintNumbers that reads in an integer called max
  and prints each number from 1 to max, separated
  by commas.
• Example
• java PrintNumbers
• Please enter a maximum integer 5
• should print
• 1, 2, 3, 4, 5

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• We want to print n numbers but need only n - 1
  commas.
• Similar to the task of building a fence
• If we repeatedly place a post and wire, the last
  post has an extra dangling wire.
• A flawed algorithm
• for (length of fence)
• plant a post.
• attach some wire.

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• import java.util.Scanner
• public class PrintNumbers
• public static void main(String args)
• Scanner keyboard new Scanner(System.in)
• int max keyboard.nextInt()
• for (int i 1 i lt max i)
• System.out.print(i ", ")
• System.out.println() // to end the line
• Output when user enters 5
• 1, 2, 3, 4, 5, // notice extra comma at end!

unnecessary
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• import java.util.Scanner
• public class PrintNumbers
• public static void main(String args)
• Scanner keyboard new Scanner(System.in)
• int max keyboard.nextInt()
• for (int i 1 i lt max i)
• System.out.print(", " i)
• System.out.println() // to end the line
• Output when user enters 5
• , 1, 2, 3, 4, 5 // comma at beginning

unnecessary
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• The solution is to add an extra statement outside
  the loop that places the initial "post."
• This is called a fencepost loop.
• The revised algorithm
• plant a post.
• for (length of fence - 1)
• attach some wire.
• plant a post.

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• import java.util.Scanner
• public class PrintNumbers
• public static void main(String args)
• Scanner keyboard new Scanner(System.in)
• int max keyboard.nextInt()
• System.out.print(1)
• for (int i 2 i lt max i)
• System.out.print(", " i)
• System.out.println() // to end the line
• Output when user enters 5
• 1, 2, 3, 4, 5 // no extra comma!

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Summary

• WhileDemo.java, p202
• DoWhileDemo.java, p206
• ForDemo.java, p219
• Exercises, slides 32, 45-52, 55-65, 70-71, 77

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• Test (controlling boolean expression, P201), body
  (P200), iteration (P200), and initialization
  (P228)
• While (P201-204), do-while (P204-9), and for loop
  (P217-221)
• Counter-controlled (P229) and event controlled
  loop
• Mapping iterations to counter values
• Trace and development template
• Random number generator
• The omission of and its side effect (P218)
• Extra semicolon (P222) and comma (P224)
• Invalidation and infinite loop (P213)
• The use of break and its function (P236-7)
• Scope of loop variable (P223)
• Fencepost problem and its solution