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Programming Fundamentals 4

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Programming Fundamentals

This lecture introduces the following iteration control structure topics:– Do Loops– For-Next Loops– For Each-Next

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Loop Structures

Visual Basic provides 3 major loop structures.– Do-Loop (Do-While, Do-Until)– For-Next– For Each-Next

There are 4 variations of the Do-Loop. The For Each-Next loop only applies to

objects in a collection and arrays.

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Do-While Loop Structures

There are 2 “While” structures available:1) Do While condition test at top of loop

statements[Exit Do]

Loop

2) Dostatements[Exit Do]

Loop While condition test at bottom of loop

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Do-While Loop Example 1

Dim blnFlag as boolean = FalseDo While blnFlag = True Debug.Writeline “Hi Mom”Loop

How many times will this execute?

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Do-While Loop Example 2

Dim blnFlag as boolean = FalseDo Debug.Writeline “Hi Mom”Loop While blnFlag = True

How many times will this execute?

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Do-Until Loop Structures

There are 2 “Until” structures available:1) Do Until condition test at top of loop

statements[Exit Do]

Loop

2) Dostatements[Exit Do]

Loop Until condition test at bottom of loop

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Do-Until Loop Example 1

Dim shoNum as short = 7Dim shoGuess as short = 0shoGuess = InputBox(“Enter Guess”)Do Until shoGuess = shoNum

shoGuess = InputBox(Enter Guess”)Loop

What happens when this executes?

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Do-Until Loop Example 2

Dim shoNum as short = 7Dim shoGuess as short = 0shoGuess = InputBox(“Enter Guess”)Do

shoGuess = InputBox(Enter Guess”)Loop Until shoGuess = shoNum

What happens when this executes?

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For-Next Loops

For-Next loops let you execute a block of statements a fixed number of times.

For counter = start To end [Step increment]

statements

[Exit For]

Next [counter]

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For-Next Loops – Alternate Syntax

A newer syntax exists for For-Next loops that does not require you to define the counter variable prior to the loop. This utilizes new block structure scope for variables.

For counter [AS datatype] = start To end [Step increment]

statements

[Exit For]

Next [counter]

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For-Next Example 1

Dim shoCnt as ShortFor shoCnt = 0 to 9

Debug.Writeline shoCntNext shoCnt

OR

For shoCnt as short = 0 to 9Debug.Writeline shoCnt

Next shoCnt

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Nested Loops

Dim Check As Boolean = True Dim Counter As Integer = 0 Do ' Outer loop

Do While Counter < 20 ' Inner loop Counter Counter += 1 ' Increment Counter If Counter = 10 Then

Check = FalseExit Do ' Exit inner loop

End If Loop ' Exit inner loop

Loop Until Check = False ' Exit outer loop

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For-Next Example 2

For shoRow as short = 0 to 9For shoCol as short = 0 to 4

Debug.Writeline shoRow; shoColNext shoCol

Next shoRow

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For-Next Loops

For-Next loops are commonly used to increment subscripts to process the elements of an array.

If you have a multi-dimension array, use nested For-Next loops.

When nested, the outer For-Next increments slowest and the inner most For-Next increments fastest.

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For-Next Example 3

Dim strName(9,4) as String

...load the array

...For shoRow as integer = 0 to 9

For shoCol as integer = 0 to 4Debug.Writeline strName (shoRow, shoCol)

Next shoColNext shoRow

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For Each-Next Structure

For Each-Next loops are similar to For-Next loops except that it repeats a block of statements for each element of a collection of objects or elements in an array.

This is useful when you do not know how many elements there are in a collection or array.

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For Each-Next Structure

For Each element In group

statements

[Exit For]

Next [element]

If you are using this on a multi-dimension array, you do not need to nest For Each statements to access all elements.

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For Each-Next Structure - Alternate Syntax

For Each element [AS datatype] In group

statements

[Exit For]

Next [element]

The ‘datatype’ must be the type of object that is present in the collection. For example, if the group is a collection of strings, then the datatype would be ‘string’.

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For Each-Next Example 1

Dim Found As Boolean = False Dim objCollection As ObjectFor Each objObject as object In objCollection

If CStr(objObject.Text)= "Hello" Then Found = True Exit For

End If Next

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For Each-Next Example 2

Dim intX (9, 19) as integerDim objCnt as object...For Each objCnt In intX initializes matrix

intX = 5Next objCnt...For Each objCnt In intX prints matrix

Print.Writeline objCntNext objCnt

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In-Class Loop Example 1

Use a Do-While loop that tests at the top to sum the integers 1 to 100. Print the sum after you have computed it.

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Answer Example 1

Dim shoSum as short = 0Dim shoCnt as short = 1Do While shoCnt < 101

shoSum += shoCntshoCnt += 1

LoopDebug.Writeline shoSum

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In-Class Loop Example 2

Use a Do-Until loop that tests at the bottom to ask the user for a password until their answer is the same as a public variable named txtPassword. Assume that txtPassword is already defined and assigned elsewhere. Print “success” when they give the correct answer.

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Answer Example 2

Dim strAnswer As String

Do strAnswer = InputBox("Enter the Password")Loop Until strAnswer = strPassword

Debug.Writeline "success"

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In-Class Loop Example 3

Modify example case #2 to give the user 3 tries to get the correct password. After the 3rd try, tell them “sorry”.

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Answer Example 3

Dim strAnswer As StringDim shoCnt As Short = 1

Do strAnswer = InputBox("Enter the Password") shoCnt = shoCnt + 1Loop Until strAnswer = strPassword Or shoCnt = 4

If strAnswer = strPassword Then Debug.Writeline "success"Else Debug.Writeline "sorry"End If