Looping

1
Looping

• Helping Unruly School Children Around the World

2
Overview

• Iteration
• 4 kinds of loops
• for
• while
• do while
• foreach
• Infinite Loops

3
Iteration

• One thing that computers do well is repeat
  commands
• Programmers use loops to accomplish this
• 4 kinds of loops in C
• for loop
• while loop
• do while loop
• foreach loop

4
Criteria for loops

• Usually have some initial condition
• Starting a counter
• Beginning in a certain state
• Must have a test to continue
• Must make progress towards finishing

5
Loops in Everyday Life

• Bad children are told to write sentences on the
  board
• I will not pour Clorox in the fish tank
• Have to write this sentence either
• A certain number of times
• Until the teacher is happy
• As many as you can during break

6
The for loop

• Good when you know exactly how many times you
  need to execute something
• Has format
• for (ltinitializationgt lttest to continuegt
  ltincrementgt)
• // everything in here is what is repeated
• // over and over again
• Initialization is where the counter is given a
  starting value
• The test determines whether or not to continue
• The increment can be any amount, including
  negative, and occurs after the loop statements
  execute

7
Satisfying the Teacher

• Example 1000 sentences? No problem
• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (I will not pour Clorox)
• // Remember, counter is the same as
• // counter counter 1

8
But I want them numbered!

• No problem
• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

9
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

1
10
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

1
true
11
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)
• Output 1 I will not pour Clorox in the Fish Tank

1
12
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

2
13
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

2
true
14
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)
• Output 2 I will not pour Clorox in the Fish Tank

2
15
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

3
16
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

3
true
17
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)
• Output 3 I will not pour Clorox in the Fish Tank

3
18
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

4
19
When will it end?

• We see that this will go on for a while
• Its a little more interesting later around 1000

20
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

999
true
21
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)
• Output 999 I will not pour Clorox in the Fish
  Tank

999
22
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

1000
23
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

true for last time
1000
24
Why this works(are we finished?)
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)
• Output 1000 I will not pour Clorox in the Fish
  Tank

1000
25
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)

1001
26
Why this works
counter

• int counter
• for (counter 1 counter lt 1000 counter)
• Console.WriteLine (counter I will not
  pour)
• // Jump down here and continue

false
1001
27
Final Output

• 1 I will not pour Clorox in the fish tank.
• 2 I will not pour Clorox in the fish tank.
• 3 I will not pour Clorox in the fish tank.
• 4 I will not pour Clorox in the fish tank.
• .
• .
• .
• 999 I will not pour Clorox in the fish tank.
• 1000 I will not pour Clorox in the fish tank.

28
The while loop

• Good for when you dont know how many times to
  repeat
• Teacher says Write until Im happy
• Has format
• while (ltboolean valuegt)
• // stuff to repeat over and over

29
Example

• bool teacherHappy false
• int lineNumber 1
• while (!teacherHappy)
• Console.WriteLine (lineNumber I will not)
• lineNumber
• teacherHappy attitudeFunction ( )
• // assume attitudeFunction can change
• // teacherHappy

30
Example Re-Writing 1-1000(using a while loop)
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)

31
Example Re-Writing 1-1000(using a while loop)
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)

32
Example Re-Writing 1-1000(using a while loop)
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• Output 1 I will not pour Clorox in the fish tank

33
Example Re-Writing 1-1000(using a while loop)
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)

34
Example Re-Writing 1-1000(using a while loop)
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)

35
Example Re-Writing 1-1000(using a while loop)
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• Output 1 I will not pour Clorox in the fish tank

36
Example Re-Writing 1-1000(using a while loop)
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)

37
Example Re-Writing 1-1000(using a while loop)
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)

38
Example Re-Writing 1-1000(using a while loop)
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• Output 1 I will not pour Clorox in the fish tank

39
Infinite Loops

• This loop isnt making a lot of progress!
• Loops that repeat forever are called infinite
  loops
• Apparently lock up
• Output
• 1 I will not pour Clorox in the fish tank
• 1 I will not pour Clorox in the fish tank
• 1 I will not pour Clorox in the fish tank
• 1 I will not pour Clorox in the fish tank
• .
• .
• .

Continue forever
40
Problem Solved
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

41
Problem Solved
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

42
Problem Solved
counter
1

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter
• Output 1 I will not pour Clorox in the fish tank

43
Problem Solved
counter
2

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

// Remember, counter is the same as// counter
counter 1
44
Example Re-Writing 1-1000(using a while loop)
counter
2

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

45
Problem Solved
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

2
46
Problem Solved
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter
• Output 2 I will not pour Clorox in the fish tank

2
47
Problem Solved
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

3
48
How does it end?
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

999
49
How does it end?
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

999
50
How does it end?
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

999
51
Problem Solved
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter
• Output 999 I will not pour Clorox in the fish
  tank

999
52
How does it end?
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

1000
53
How does it end?
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

1000
54
How does it end?
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter
• // So we never print out
• // 1000 I will not pour Clorox in the fishtank

now false
1000
55
Another Problem Solved
counter

• int counter 1
• while (counter lt 1000)
• Console.WriteLine (counter I will not)
• counter

now true
1000
56
The do-while loop

• Similar to while loop
• Must execute at least one time (test is at
  bottom)
• Has format
• do
• while (ltboolean valuegt)

57
Example(count from 1 to 3)
counter
0

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

58
Example(count from 1 to 3)
counter
0

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

59
Example(count from 1 to 3)
counter
1

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

60
Example(count from 1 to 3)
counter
1

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

Output 1
61
Example(count from 1 to 3)
counter
1

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

62
Example(count from 1 to 3)
counter
2

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

63
Example(count from 1 to 3)
counter
2

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

Output 2
64
Example(count from 1 to 3)
counter
2

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

65
Example(count from 1 to 3)
counter
3

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)
• // Note counter is now 3, but we still have
• // to finish out the loop it doesnt skip

66
Example(count from 1 to 3)
counter
3

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

Output 3
67
Example(count from 1 to 3)
counter
3

• int counter 0
• do
• counter
• Console.WriteLine (counter)
• while (counter lt 3)

now false, so loop is finished
68
The foreach loop

• Often we have a collection of data
• Wed like to do something to each one of the
  items in the collection
• Certainly we can do this with a for loop
• But C provides a new loop foreach
• References an item in the collection
• Provides access to it
• Loops through all elements in the collection

69
foreach template

• foreach (ltTYPEgt ltNAMEgt in ltCOLLECTIONgt)
• WORK TO BE DONE ON EACH ITEM

70
A sample application
71
foreach example

• Assuming we had a listbox control called
  listBoxNames with strings in it
• foreach (string currentName in listBoxNames.Items)
  
• MessageBox.Show(currentName.ToUpper())
• Displays all of the strings in capitalization to
  the user in MessageBoxes.

72
foreach vs. for loop

• The previous foreach loop on the collection using
  a for loop
• for(int i0 i lt listBoxNames.Items.Count i)
• string currentName (string)(listBoxNames.Items
  i)
• MessageBox.Show(currentName.ToUpper())

73
Summary

• for loops good for when you know how many times
  you want to repeat
• while and do-while good for when you dont
• foreach loop useful for processing collections
• All loops must finish, or they become infinite
  loops
• All loops must have a test to continue, or they
  become infinite loops