Notes
Outline
Iteration -- Once Is Not Enough
Though people don’t like to repeat themselves, repetition is a valuable facility that a computer can provide.  If program instructions are to be performed more than once, as in Alphabetize CDs, repetition is needed
Two Additional Control Statements
The conditional statement (If-Then-Else) is the only way (so far) to control which statements are executed
We will introduce two more:
ElseIf -- a variation on the If-Then-Else for long sequences of tests
Do While -- a control facility allowing statements to be repeated as long as some condition is true
ElseIf
ElseIf solves the problem of testing a long sequence of alternatives
Example
If   txtNum.Text = 1   Then
     MsgBox(“John”)
ElseIf txtNum.Text = 2 Then
     MsgBox(“Paul”)
ElseIf txtNum.Text = 3 Then
     MsgBox(“George”)
ElseIf txtNum.Text = 4 Then
     MsgBox(“Ringo”)
Else
     MsgBox(“Who?”)
EndIf
Mini-Exercise #1
What does this print?
Dim x As Integer
x = 10
If   x = 1   Then
     Print “octopus”
ElseIf x = 2 Then
     Print “squid”
Else
     Print “clam”
EndIf
Print “mollusc”
Mini-Exercise #1 -- Answer
What does this print?
Dim x As Integer
x = 10
If   x = 1   Then
     Print “octopus”
ElseIf x = 2 Then
     Print “squid”
Else
     Print “clam”
EndIf
Print “mollusc”
Contrast  Elseif With Nested If
ElseIf is not a nested test as seen before, though it is similar
Caution With Else If
An If statement that uses Else If passes through all of the previous cases before reaching a given test … think about the consequences
If   someVar < 20   Then
     MsgBox(“Less than 20”)
ElseIf someVar < 10 Then
     MsgBox(“Less than 10”)
Else
     ...
EndIf
Repeating Terms
Iteration is the repeated execution of a series of statements in programming
To perform iteration, programming languages include special statements often called iteration statements
There are two crucial components of all iterations:
The statements that will be repeated -- called the loop body
A test specifying when to repetition stops -- termination test
Additionally, loops typically have at least one variable that is explicitly changed “inside” the loop -- this is called the iteration variable
General Form Of VB6 Iteration
VB6, like most languages, has several iteration statements.  We’ll just use one in CSE/IMT 100:
The semantics are as follows:
The termination condition is tested and if it is false the statements are all skipped; execution continues after Loop
If it is true, the statements are performed once
The termination condition is tested again, and if it is false the loop is over and the statements are skipped; continue after Loop
If it is true, the statements are performed a second time
…
An Example
An easy way to get the idea of iteration is to print out the iteration variable ...
Execution of Example
Try the same computation with a different termination condition
Mini-Exercise #2
What does this code print?

Dim i As Integer
i = 2
Do While i <= 4
   Print i
   i = i + 1
loop
Mini-Exercise #2  -- Answer
What does this code print?

Dim i As Integer
i = 2
Do While i <= 4
   Print i
   i = i + 1
loop
Mini-Exercise #3
What does this code print?

Dim i As Integer
i = 10
Do While i <= 4
   Print i
   i = i + 1
loop
Mini-Exercise #3 -- Answer
What does this code print?

Dim i As Integer
i = 10
Do While i <= 4
   Print i
   i = i + 1
loop
Mini-Exercise #4
What does this code print?

Dim i As Integer
i = 1
Do While i >= 1
   Print i
   i = i + 1
loop
Mini-Exercise #4 -- Answer
What does this code print?

Dim i As Integer
i = 1
Do While i >= 1
   Print i
   i = i + 1
loop
To get out of an infinite loop:

In Visual Basic, type
     control-break

Then you are in the debugger, and can see where you are stuck.  Select “continue” to proceed
If a program (any program) gets stuck in Windows, you can also type control-alt-delete and get a task manager.  Select the offending task and end it.  Caution: you’ll lose any changes for that program since the last save.
Mini-Exercise #5
Suppose we have a procedure “squid” that takes a single integer argument.  The argument is only read by squid, not changed.  Write a loop that calls a “squid” with 2, 4, 6, 8, 10.
Mini-Exercise #5 -- Answer
Suppose we have a procedure “squid” that takes a single integer argument.  The argument is only read by squid, not changed.  Write a loop that calls a “squid” with 2, 4, 6, 8, 10.

Dim i As Integer
i = 2
Do While i <= 10
   Call squid(i)
   i = i + 2
loop