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looping

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looping

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  1. looping

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. For…next statement • Dim intCount as Integer • ForintCount = 1 To 10 • Messagebox.Show(intCount) • Next intCount

  11. 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.

  12. 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.

  13. 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

  14. For…next statement Step Changes the way the counter is incremented. Can be positive or negative Syntax Forcounter = start To end Stepstepnum Statements Nextcounter

  15. 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

  16. 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