INF110 Visual Basic Programming AUBG Spring semester 2011

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INF110Visual Basic Programming

AUBG Spring semester 2011

Reference books:Schneider D., An Introduction to Programming Using Visual Basic, Prentice Hall, Pearson Education Inc., 7th Ed. 2008, 6th Ed. 2006 Liberty J., Learning Visual Basic .NET, O’Reilly, 2002 Any Visual Basic book available in AUBG library

Course lecturer: Assoc. Prof. Svetla Boytcheva, PhD

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INF110 Visual Basic Programming AUBG Spring semester 2011

Lecture 7Title:

Visual Basic(Repetition/Iteration

statements 1)

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Lecture Contents:

Repetition/Iteration statements:Pre Test LoopsPost Test LoopsLogically Controlled LoopsCounter Controlled LoopsNested Loops

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Quiz 2on

Decision Operators and Expressions

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Control Structures IIControl Structures II

RepetitionRepetition

(aka Iteration)(aka Iteration)

INF110

Visual Basic Programming

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An iterative statement is one which executes one or more other statements a certain number of times

i.e. the execution of one or more statements is repeated

The iterative statement comes in a variety of “flavours”.

They are used for a number of purposes, such as:• searching a collection of data items for one with a specific value, or• performing a calculation involving a collection of data items (e.g. adding them all together to get a total) or• performing some action repeatedly until an event occurs which tells it to stop (imagine a robot placing boxes on a conveyor belt ...)

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• Repetition statements (also iterative statements) may classify in two sub groups:

Do…Loop statements

For…Next statements

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Do Loops

• A loop is one of the most important structures in programming.

• Used to repeat a sequence of statements a number of times.

• The Do loop repeats a sequence of statements either:• as long as, a certain condition is True.• until, a certain condition becomes True.

• It has a few variants …

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Comments

• Be careful to avoid infinite loops – loops that never end.

• VB.NET allows for the use of either the While keyword or the Until keyword at the top or the bottom of a loop. We will mostly use While at the top of a loop (pretest loop) and Until at the bottom of a loop (posttest loop).

• Semantics:

“Do these actions while condition is true”

or

“Do these actions until a condition becomes true”

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•Variations of the Do…Loop control structure:

Do … Loop1/ Pre test loops

with While and Until

2/ Post test loops

with While and Until

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Do LoopsDo While boolean expression

statementsLoop------------------------------------------Do Until boolean expression

statementsLoop------------------------------------------Do

statementsLoop While boolean expression------------------------------------------Do

statementsLoop Until boolean expression------------------------------------------Do

statementsLoop

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Do Loops in VBasic

Pretest loops

----------------------------------------- Do While boolean expressionstatements

Loop-----------------------------------------Do Until boolean expressionstatements

Loop-----------------------------------------

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Do Loops in VBasic

Posttest loops

-----------------------------------------Do statements

Loop While boolean expression-----------------------------------------Do statements

Loop Until boolean expression-----------------------------------------

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Do Loops in VBasic

Infinite /endless/ loops

-----------------------------------------Do statements

Loop -----------------------------------------

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Do While <condition> … Loop

Statement

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Do While … Loop Syntax

Do While condition

statement(s)

Loop

Condition is tested.If it is True,

the loop is run.If it is False,

the statements following the Loop statementare executed.

These statements are inside the body of the loop and are run if the condition

above is True.

condition is a Boolean expression

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Pseudocode and Flow Chart for a Do Loop

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Dim Counter As Integer = 20Do While Counter <= 100

Console.Writeline(Counter)

Counter = Counter + 10

Loop

The above example will keep on adding until Counter’s value becomes > 100

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Example

Dim PassWord As String

PassWord = “”

Do While PassWord <> “SHAZAM”

Console.Writeline(“Enter password:")

PassWord = Console.Readline()

Loop

PassWord is the loop control variable because the value stored in PassWord

is what is tested to determine if the loop should continue or stop.

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‘Accumulate sum of the numbers from 1 to 7

Dim Num As Integer = 1 Dim Sum As Integer = 0

Do While Num <= 7 Sum = Sum + Num Num = Num + 1 'Add 1 to the value of Num Loop

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Do … Loop Until <condition>

Statement

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Do … Loop Until

Do

statement(s)

Loop Until condition

Loop is executed once and then the conditionis tested. If it is False, the loop is run again.

If it is True, the statements following the Loop statement are executed.

Since the condition is tested at thebottom the loop, the loop will

execute at least once.

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Pseudocode and Flowchart for a Do … Loop Until

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Dim Counter As Integer = 20

Do Console.Writeline(Counter) Counter = Counter + 10

Loop Until Counter > 100

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Example

Dim PassWord As String

Do

Console.Writeline(“Enter password:")

PassWord = Console.Readline()

Loop Until PassWord = "SHAZAM"

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Nested Loops

Statements inside a loop can contain another loop.

(Just like If statements).

Example: What is the output of this code?i = 0

Do While i < 6

j = 0

Do While j < 6

Console.Write("*")

j = j + 1

Loop

Console.WriteLine()

i = i + 1

Loop

Nested loops are quite common, especially for processing tables of data.

Assume initially that i = 0

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Nested Loops


(Just like If statements).

Example: What is the output of this code?i = 0

Do While i < 6

j = 0

Do While j < i

Console.Write("*")

j = j + 1

Loop

Console.WriteLine()

i = i + 1

Loop



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When condition is a variable of Boolean type,

the statements likeIf flagVar = True Then ...

can be replaced byIf flagVar Then ...

and

the statements likeIf flagVar = False Then ...

can be replaced byIf Not flagVar Then ...

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The statements like

Do While flagVar = True

can be replaced by

Do While flagVar

and

The statements like

Do While flagVar = False

can be replaced by

Do While Not flagVar

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The iterative statement

Do While condition

statement(s)

Loop

Has an alternative while control structure:

The While repetition structure

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While Repetition Structure

• Repetition structure

– Allows the programmer to specify that an action should be repeated, depending on the value of a condition.

• Example (pseudocode)

While there are more items on my shopping list

Purchase next item

Cross it off my list

Just like Do While ... loop

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Dim Product As Integer = 2 While Product <= 1000

Console.Writeline(Product)Product = Product * 2

End While

The decision is tested each time the loop iterates

product <= 1000

true

product = product * 2

false

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‘ Modern VB 2005/2008 versionDim Product As Integer = 2While Product <= 1000


End While

VB6 >>> VB 2005/2008The End While statement replaces the Wend statement of VB6.

‘ Obsolete VB 6 versionDim Product As Integer = 2While Product <= 1000


WEnd

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Nested Loops using While … End While control structure


Example: What is the output of this code?

i=0

While i < 6

j = 0

While j < 6

Console.Write("*")

j = j + 1

End While

Console.WriteLine()

i = i + 1

End While



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Nested Loops using While … End While control structure


Example: What is the output of this code? i=0

While i < 6

j = 0

While j < i

Console.Write("*")

j = j + 1

End While

Console.WriteLine()

i = i + 1

End While



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Do Loops• All the variations use the same basic model.

A pretest loop is executed either– While the condition is True. Uses While

reserved wordDo While conditionstatement-block

Loop

– Until the condition becomes True. Uses Until reserved wordDo Until conditionstatement-block

Loop

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Examples

I = 1

Do While (I <= 10)

Console.WriteLine(I)

I = I + 1

Loop

How many iterations are to be run? (10) Try it

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Examples

I = 1

Do Until I <= 10


I = I + 1

Loop


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Examples to achieve the same effectI = 1Do While (I <= 10)

Console.WriteLine(I)I = I + 1

Loop======================= same effectI = 1Do Until Not(I <= 10)


Loop


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Examples to achieve the same effectI = 1Do While Not(I <= 10)


Loop======================= same effectI = 1Do Until (I <= 10)


Loop


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Post test loops with While and Until

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Do Loops• All the variations use the same basic model.

A posttest loop is executed either– While the condition is True. Uses While

reserved wordDostatement-block

Loop While condition

– Until the condition becomes True. Uses Until reserved wordDostatement-block

Loop Until condition

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Examples

I = 1

Do


I = I + 1

Loop While (I <= 10)


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Examples

I = 1

Do


I = I + 1

Loop Until (I <= 10)


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Examples to achieve the same effect

I = 1Do


Loop While (I <= 10)=================== same effectI = 1Do


Loop Until Not(I <= 10)


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Examples to achieve the same effect

I = 1Do


Loop While Not(I <= 10)=================== same effectI = 1Do


Loop Until (I <= 10)


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Endless loops

with While … End While

andDo … Loop

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Endless loops - examples

While True

statement-block

End While

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Endless loops - comments

The endless loops should be avoided. They must be terminated from within the own loop body with an Exit statement

While True statement-block

if condition Then Exit Whilestatement-block

End While

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Endless loops - examples

Do

statement-block

Loop

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Endless loops - comments

The endless loops should be avoided. They must be terminated from within the own loop body with an Exit statement

Dostatement-blockIf condition Then Exit Dostatement-block

Loop

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From Logically controlled loop

ToCounter controlled loop

Introduction to

For … Next Loop statement

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You use a Do loop when you do not know how many times the statements should be run.

If you know that they should be run for a specific

number of times, then use a For … Next loop.- coming on next slides!

This loop variant uses a counter which increases or decreases on each repetition of the loop and the looping ends when this counter reaches a set value.

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For … Next Loops

• Used when we know how many times we want the loop to execute.

• A counter-controlled loop

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For … Next Loop Syntax

The format is:

For counter = startNumber To endNumber Step increment

One or more VB statements

Next

starts with counter = startNumber and increments counter each time through the loop by an amount equal to increment until it reaches endNumber.

The bounds (startNumber, endNumber) are integer quantities, and Step controls the increments (which may be negative).

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One more format is:

For counter = startNumber To endNumber [Step increment]

One or more VB statements

Next [counter]

The brackets [ … ] are meta characters and indicate that the keywords in the square brackets are optional.

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For index = 0 To n Step s

...

Next

Controlvariable

Startvalue

Stopvalue

Amountto add to

index

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The most common use of the FOR loop omits Step increment – the default increment is +1, i.e. the counter is increased by 1 each time through the loop.

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Sample

For i = 1 To 5

...

Next

The loop control variable (i.e. counter), i, is – Initialized to 1– Tested against the stop value, 5– Incremented by 1 at the Next statement

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For i = 1 To 5 [Step 1] ...Next

And its corresponding Do While equivalent

i = 1

Do While i <= 5

...

i = i + 1

Loop

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(a) For counter=1 to 10

Console.Writeline(counter)

Next

(b) For counter=1 to 100 Step 10


Next

(c) For counter=100 to 5 Step -5


Next

Examples

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For Num = 1 To 10 NumSquared = Num*Num Console.Writeline(NumSquared) Next Num

The statements between the "For Num = 1 To 10" line and the "Next Num" line are executed ten times.The first time, the variable Num contains the value 1.The second time, Num contains the value 2, the third time, 3, and so on.On the tenth time around (the tenth "iteration") num contains the value 10.After that there are no further iterations - the computer moves on to the line following the "Next num" line.

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Start, Stop, and Step values

• Consider a loop beginning with

For i = m To n Step s

• The loop will be executed exactly once if m equals n no matter what value s has.

• The loop will not be executed at all if m is greater than n and s is positive.

• The loop will not be executed at all if m is less than n and s is negative.

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Altering the Control Variable

• The value of the control variable should not be altered within the body of the loop• Doing so might cause the loop to repeat

indefinitely • Or have an unpredictable number of

repetitions.

• The value of the control variable should not be referenced after the Next statement

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Altering the Control Variable

For I = 0 to 10

Console.WriteLine(i)

I = I – 1

Next I

This is a loop that never ends. Why?

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Altering the Stop Value

The stop value may be specified as literal For I = 0 to 10

The stop value may be specified as constant Const MyConst As Integer = 10

For I = 0 to MyConst

The stop value may be specified as variable Dim endValue As Integer

endValue = 10

For I = 0 to endValue

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Altering the Stop Value

Attention: keep in mind when working with For…Next loops that the stop value is set at the beginning of the loop. Changing the value of the EnDVariable in the loop’s body won’t have any effect.

For example, The following loop will execute 11 times, not 51

Dim endValue As Integer = 10

For I = 0 to endValue

endValue = 50

Console.WriteLine(endValue)

Next I

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Non-integer Step Values

• Can lead to round-off errors with the result that the loop is not executed the intended number of times.

• The loop counter should be Integer.

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Non-integer Step Example

• A loop beginning with

For i = 1 To 2 Step 0.1

will be executed only 10 times instead of the intended 11 times.

It should be replaced with

For i = 1 To 2.01 Step 0.1

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Nested For … Next Loops

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Questions?Questions?

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Thank You For

Your Attention!

Documents

INF110 Visual Basic Programming AUBG Spring semester 2011