Zhen Jiang

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CSC141 Computer Science I

• Zhen Jiang
• Dept. of Computer Science
• West Chester University
• West Chester, PA 19383
• zjiang_at_wcupa.edu

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Loop

• Smart if-decision making, smart program work,
  talent programmer
• Research experience (REU) - click on this link
• Temperature/humidity detection every second
• A repetition process for the 7x24 hours seamless
  surveillance
• Needs a computer support to REPEAT

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• While loop
• Format Logic, page 197, Figure 4-1.
• Sample, code 4-3, page 198.

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• ltinitializationgt
• while (lttestgt)
• ltbodygt

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• Do-while loop
• Format, page 208
• Logic, page 209, Figure 4-6.
• Sample, code 4-6, page 209.

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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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• For loop
• Format, page 212, Figure 4-7.
• Logic, page 212, Figure 4-8.
• Sample, code 4-7, page 213.

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for (ltinitgt lttestgt ltupdategt) ltbodygt
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• Summary

Body first, and then event change/update
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• Ex10
• http//www.cs.wcupa.edu/zjiang/141_ex10.pdf
• Ex11
• http//www.cs.wcupa.edu/zjiang/141_ex11.pdf

11

• Trial
• population 
• http//www.cs.wcupa.edu/zjiang/6billion.exe
• TV purchase
• http//www.cs.wcupa.edu/zjiang/tv563.exe
• 1248...
• http//www.cs.wcupa.edu/zjiang/1_2_4.exe
• 1234...99
• http//www.cs.wcupa.edu/zjiang/1to99.exe

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• Development process
• 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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• 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?
http//www.cs.wcupa.edu/zjiang/141_ex11.pdf
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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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• Exercise
• population 
• http//www.cis.temple.edu/jiang/6billion.exe
• TV purchase
• http//www.cis.temple.edu/jiang/tv563.exe
• 1248...
• http//www.cis.temple.edu/jiang/1_2_4.exe
• 1234...99
• http//www.cis.temple.edu/jiang/1to99.exe

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Solution
45

• Ex 12
• http//www.cs.wcupa.edu/zjiang/141_ex12.pdf
• Ex 13
• http//www.cs.wcupa.edu/zjiang/141_ex13.pdf

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• File writing, page 237-240
• Filename
• PringWriter
• Println
• Close
• Sample, code 4-17, page 237

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• Appending data to a (existing) file
• FileWriter (, true), page 240

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• File Reading, page 241-245
• File
• Scanner
• nextXXXX( )
• close
• Sample, code 4-18, page 242.

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• Detecting the end of a file
• hasNext
• Code 4-19, page 245.
• Detecting the existence of a file
• exists
• Code 4-21, page 249.

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• Random number generator
• randomNumbers.nextXXX( )
• Sample, code 4-23, page 253.

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• 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