Iteration

1
Iteration
2
While loops

• Often we want to continue to do the same thing
  many times, until some condition has been met
• While the user continues to enter input, keep
  processing it
• while(!input.equals(QUIT))
• System.out.println(You entered input)
• input console.readLine()

3
While flowchart

• while(condition)
• statements

!input.equals(QUIT)
False
True
System.out.println(You entered input)
input console.readLine()
4
Infinite loops

• while(!input.equals(QUIT))
• System.out.println(You entered input)
• This loop never ends!
• We never advance the variable that controls the
  loop (input)

5
Common forms

• int start 0, total 0
• while(start lt 10)
• total total start
• start
• Iterate through the loop some number of times
• increment some counting variable at each iteration

6
For loops

• Easy shorthand for the most common kind of loop
• for(initialization condition update)
• statements

7
While vs For

• int start 0, total 0
• while(start lt 10)
• total total start
• start

for(int start 0 start lt 10 start) total
total start
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Other for examples

• int start
• for(start 0 start lt 10 start)
• for(int years n years gt 0 years years 1)
• for(int rate -10 rate lt 10 rate rate
  .05)
• for(rate 5 years-- 0 System.out.println(bala
  nce))
• //bad style

9
Scope of variables

• for(int start 0 start lt 10 start)
• total total start
• The scope of start extends to the end of the loop
• for(int start 0 start lt 10 start)
• total total start
• for(int start 5 start lt 100 start 5)
• total total - start

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Stylistic issues

• Always use
• for(int i 0 i lt 10 i)
• total total (totali)
• Dont use !
• for(int i 0 i ! n i)
• total total (totali)
• Dont update your control variable in the loop
• for(int i 0 i ! n i)
• i n i

11
break and continue

• break lets you get out of a loop early
• while(true)
• if(input.equals(QUIT))
• break
• System.out.println(You entered input)

12
break and continue

• continue lets you skip the rest of the loop and
  start again at the top
• while(a lt b)
• if(input null)
• continue
• System.out.println(You entered input)

13
Nested loops

• Suppose we want to print out

14
Goal

• User enters the number of rows and the number of
  columns
• Print a rectangle (of characters) that matches
  the users specification

15
Examples

• User enters 1, 3
• User enters 4, 5

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How do we do it?

• find out how many rows the user wants to print
  (numrows)
• print out numrows rows

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PrintRect.java

• public class PrintRect
• public static void main(String args)
• for(int i 0 i lt numrows i) //print a row

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How do we print a row?

• Find out how many columns each row should have
  (numcols), and print a star for each column
• for(int j 0 j lt numcols j)
  System.out.print()

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PrintRect.java

• public class PrintRect
• public static void main(String args)
• for(int i 0 i lt numrows i) //print a row

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PrintRect.java

• public class PrintRect
• public static void main(String args)
• for(int i 0 i lt numrows i)
• for(int j 0 j lt numcols j)
  System.out.print()