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


	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


	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 solves the problem of testing a long sequence of alternatives


	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


	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”


	ElseIf is not a nested test as seen before, though it is similar


	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


	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


	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 easy way to get the idea of iteration is to print out the iteration variable ...


	Try the same computation with a different termination condition


	What does this code print? Dim i As Integer i = 2 Do While i <= 4 Print i i = i + 1 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.


	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.