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Chapter 7 Loops

Chapter 7 Loops. Programming In Visual Basic.NET Prepared by Johnny Tsui, CIM@VTC. Loops. Repeating a series of instructions Types of Loops Do Use when the number of iterations is unknown For Next Use when the number of iterations known. Do Loops.

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Chapter 7 Loops

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  1. Chapter 7Loops Programming In Visual Basic.NET Prepared by Johnny Tsui, CIM@VTC

  2. Loops • Repeating a series of instructions • Types of Loops • Do • Use when the number of iterations is unknown • For Next • Use when the number of iterations known

  3. Do Loops • Ends based on a condition you specify, either • Loop While a condition is True • Loop Until a condition becomes True • Condition can be located at • Top of Loop, Pretest • Bottom of Loop, Posttest

  4. Do Loop General Form Do {While |Until} condition Statements to execute Loop OR Do Statements to execute Loop {While | Until} condition Top of Loop Condition, Pretest Bottom of Loop Condition, Posttest

  5. Do's - When Evaluated • Top Evaluation, not guaranteed to run once since evaluated BEFORE running Do While … Loop Do Until … Loop

  6. Example: intA = 0 Do Until intA = 10 intA = intA + 1 Loop

  7. Example: intA = 0 Do While intA < 10 intA = intA + 1 Loop

  8. Do's - When Evaluated • Bottom Evaluation, will always run at least one time Do … Loop While Do … Loop Until

  9. Example: intA = 0 Do intA = intA + 1 Loop Until intA = 10

  10. Example: intA = 0 Do intA = intA + 1 Loop While intA < 10

  11. Is the number of repetitions known? Must the loop execute once? NO NO Top Eval Do Loop YES YES For Next Loop Bottom Eval Do Loop Is the expression initially true? Do Until Loop Do While Loop NO YES Do Loops / For Next Loops:Deciding Which To Use

  12. For Next Loops • Use when you know the number of iterations • Uses a numeric counter variable, called Loop Index, to control number of iterations • Loop Index is incremented at the bottom of the loop on each iteration • Step value can be included to specify the incrementing amount to increment Loop Index, step can be a negative number

  13. For Next Loop General Form For LoopIndex = InitialValue To TestValue [Step Increment] Statements Next [LoopIndex]

  14. Example: intA = 0 intB = 0 For intA = 1 To 10 intB = intB + 1 Next

  15. Example: intA = 0 intB = 0 For intA = 1 To 10 Step 2 intB = intB + 1 Next

  16. Manually Exiting For Next Loops • In some situations you may need to exit the loop prematurely • Use theExit For statement inside the loop structure

  17. Example: intA = 0 intB = 0 For intA = 1 To 10 intB = intB + 1 If intB = 3 Exit For End If Next

  18. Exercises Name: Class: • Idenifty the statements that are correctly formed and those that have errors. For those with errors, state what is wrong and how to correct it. (a) For decIndex = 3.5 to 6.0, Step 0.5 Next (b) For 4 = 1 to 10 Step 2 Next (c) For intIndex = 100 to 0 Step -25 Next (d) For intIndex = 0 to -10 Step -1 Next (e) For intIndex = 10 to 1 Next

  19. Exercises Name: Class: • How many times will the body of the loop be executed for each of these examples? (a) For intIndex = 1 to 3 (b) For intIndex = 2 to 11 Step 3 (c) For intIndex = 10 to -1 Step -1 (d) For decIndex = 3.0 to 6.0 Step 0.5 (e) For intIndex = 5 to 1

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