Looping
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looping. LOOPING. Used to perform repetitive tasks Tasks use the same set of statements that are executed again and again until a specific condition is met Statements will loop until the condition is false. Do…loop WHILE. A looping structure that evaluates the loop at least one time

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Looping

looping


Looping1

LOOPING

  • Used to perform repetitive tasks

  • Tasks use the same set of statements that are executed again and again until a specific condition is met

  • Statements will loop until the condition is false


Do loop while

Do…loop WHILE

  • A looping structure that evaluates the loop at least one time

  • A posttest loop – evaluates the condition after the loop has run

  • Dim intNum as Integer

  • Do

  • intNum = intNum + 2

  • Loop While intNum < 10


Do while loop

Do while … loop

  • A looping structure that may or may not evaluate the loop.

  • A pretest loop - the condition is FIRST, so the loop may never run

  • Dim intNum as Integer

  • Do While intNum < 10

  • intNum = intNum + 2

  • Loop


Infinite loops

Infinite loops

  • Continues forever because the condition is never false

  • Happens when there is a logic error – will eventually cause a run time error

  • Dim intNum as Integer = 1

  • Do While intNum > 0

  • intNum = intNum + 1

  • Loop


Input boxes

Input boxes

  • The Inputbox() function displays an input box with a prompt and a text box

  • The information entered by the user is the new value for a string variable

  • Dim strName as String

  • strName = InputBox(“Enter your name to continue”, “What is your name”)

  • “Enter your name to continue” is the prompt

  • “What is your name?” is the title of the InputBox


Accumulator variables

Accumulator variables

  • Counters used within loops to update a numeric variable are called accumulator variables

  • intTotalScore = intTotalScore + intNewScore

  • Each time this statement is executed, the value of intNewScore is added to the current value of intTotalScore and then this new value is assigned to intTotalScore


Flags

flags

  • A flag, also called a sentinel, is a condition used to signify that a loop should stop executing

  • It’s an easy way to end a loop


For next statement

For…next statement

  • Executes a set of statements a fixed number of times – not Boolean

  • Forcounter = startToend

  • statements

  • Next counter

  • Counter is initialized to the start number when the loop first executes and is automatically incremented by 1


For next statement1

For…next statement

  • Dim intCount as Integer

  • ForintCount = 1 To 10

  • Messagebox.Show(intCount)

  • Next intCount


For next statement2

For…next statement

  • You create an integer counter to use after the keyword FOR.

  • The first time the For line executes, your counter is set to whatever is after the “=”. It does NOT reset each time the loop repeats.

  • Eachtime “Next” is executed, the counter is updated by 1 (default value is 1)

  • The condition is tested on the FOR line

  • The condition is still true when counter reaches the value after the “TO”. (Loop still executes)

  • When the condition is tested false, execution jumps to statement after the NEXT.

  • Remember, counter is updated to false value and holds that value when you exit the loop.


For next statement3

For…next statement

  • The loop below executes until intCountis equal to 5, by checking one last time (it is no longer true), jumps to Next and exits the loop.

  • Dim intCount as Integer

  • Dim intTotal as Integer

  • For intCount = 1 To 4 ‘accumulatorintTotal = intTotal + intCountNext intCount

  • The variable counter “intCount” holds a 5 after the loop ends, but the code inside only executed four times.


For next statement4

For…next statement

  • You may create the counter variable (intCount) in the For line by using the optionalAs Integer keywords, rather than using a Dim command before the loop.

  • The lifetime of the variable counter created this way is the lifetime of the loop. (When you exit the loop, the variable counter no longer exists).

  • For intCount As Integer = 0 To 4intTotal = intTotal + intCountNext intCount

  • intCountis only in scope in the loop


For next statement5

For…next statement

Step

Changes the way the counter is incremented.

Can be positive or negative

Syntax

Forcounter = start To end Stepstepnum

Statements

Nextcounter


For next statement6

For…next statement

  • The step integer can be a positive number to tell the compiler to increment the counter.

  • Make sure your start is the low number, the end is the high number.

    For intCount= 1To 10Step 2

    MessageBox.Show("Counting by twos: " & intCount)

    Next intCount


For next statement7

For…next statement

  • The step integer can be a negative number to tell the compiler to decrement the counter.

  • Make sure your start is the high number, the end is the low number.

    For intCount= 5 To 1 Step -1

    MessageBox.Show("Counting Down " & intCount)

    Next intCount


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