Developing Software Applications Iteration in Visual Basic (Loops)

Looping Programs often have a requirement to process a set of instructions several times This sequence might be for a fixed number of times Or whether it is repeated might depend on a particular condition This means that there are a number of choices ………..

Iteration Constructs in VB For … Next Do … Loop Until Do While … Loop

Iteration constructs are of 3 types Repeat a block of code FOR a given number of times WHILE some condition is true, repeat a block of code Repeat a block of code UNTIL a condition is true

For loops The simplest loop Used when we know exactly how many times we want to repeat things e.g. Input and add together 20 exam marks

For loops - in structured English e.g. Input and add together 20 exam marks For 20 times Input a mark Add mark to total

For loops - a simple example e.g. Input and add together 20 exam marks For iCount = 1 To 20 iMark = InputBox(“Enter a mark”) iTotal = iTotal + iMark Next iCount

Expanding that code... Dim iCount, iMark, iTotal as Integer iTotal = 0 For iCount = 1 To 20 iMark = InputBox(“Enter a mark”) iTotal = iTotal + iMark Next iCount MsgBox (“Sum of marks is “ & iTotal)

A fuller syntax of the For loop.. For = To Step Next e.g. iSum = 0 For iCount = 2 To 100 Step 2 iSum = iSum + iCount Next iCount Adds the even numbers 2, 4, 6,.. 100

Step can be decreasing … For iLoopCounter = x To y [Step] Perform functions Next For a = 1 To 10 For a = 1 To 10 Step 5 For a = 10 To 1 Step -1 For a = 10 To 1 Step -5

Example: add the values of numbers 1 to 10 iTotal = 0 For iLoopCounter = 1 to 10 iTotal = iTotal + iLoopCounter Next

Nested For Loops … For iCount1 = 1 to 20 For iCount2 = 1 to 100 statements ….. Next iCount2 Next iCount1 In nested For loops, the Next statement must implicitly state which loop is progressing next

Demonstrating the For loop with a ListBox Dim iTotal as Integer Dim iCount as Integer iTotal = 0 For iCount = 1 To 10 iTotal = iTotal + iCount lstNos.AddItem iTotal Next lblTotal.Caption = iTotal

Question What about if we want the loop to stop when the total exceeds 100 ?

Stopping the For loop Dim iTotal as Integer Dim iCount as Integer iTotal = 0 For iCount = 1 To 100 iTotal = iTotal + iCount lstNos.AddItem iCount & ", " & iTotal If iTotal > 100 Then iCount = 101 End If Next lblTotal.Caption = iTotal Setting iCount to a value >100 causes this For loop to stop

trivial example works, but is not very elegant code using the IF the statement to force the exit of the FOR loop is not considered to be “good code” … it is bad practice

Do While loop Do while condition statement(s) Loop

Back to earlier example iTotal = 0 iCount = 1 Do While iTotal < 100 iTotal = iTotal + iCount lstNos.AddItem iCount & ", " & iTotal iCount = iCount + 1 Loop lblTotal.Caption = iTotal

Comparing For with Do While Dim iTotal, iCount as Integer iTotal = 0 For iCount = 1 To 100 iTotal = iTotal + iCount lstNos.AddItem iTotal If iTotal > 100 Then iCount = 101 End If Next lblTotal.Caption = iTotal Dim iTotal, iCount as Integer iTotal = 0 iCount = 1 Do While iTotal < 100 iTotal = iTotal + iCount lstNos.AddItem iTotal iCount = iCount + 1 Loop lblTotal.Caption = iTotal

Second syntax – Do …….. Loop Until …. Dim iTotal, iLoopCount as Integer iTotal = 0 iLoopCount = 1 Do iTotal = iTotal + iLoopCount lstNos.AddItem iLoopCount & ", " & iTotal iLoopCount = iLoopCount + 1 Loop Until iTotal >= 100 lblTotal.Caption = iTotal

Comparing the 2 deterministic loops.. iTotal = 0 iCount = 1 Do While iTotal < 10 iTotal = iTotal + iCount lstNos.AddItem iTotal iCount = iCount + 1 Loop lblTotal.Caption = iTotal iTotal = 0 iLoopCount = 1 Do iTotal = iTotal + iLoopCount lstNos.AddItem iTotal iLoopCount = iLoopCount + 1 Loop Until iTotal > 10 lblTotal.Caption = iTotal May look the same … but the results may be different !!

How many times is this loop executed ? Dim iTotal, iLoopCount As Integer iTotal = 0 iLoopCount = 1 Do While iTotal < 10 iTotal = iTotal + iLoopCount lstNos.AddItem iLoopCount & ", " & iTotal iLoopCount = iLoopCount + 1 Loop Label1.Caption = iTotal

How many times is this loop executed ? Dim iTotal, iLoopCount As Integer iTotal = 0 iLoopCount = 1 Do iTotal = iTotal + iLoopCount lstNos.AddItem iLoopCount & ", " & iTotal iLoopCount = iLoopCount + 1 Loop Until iTotal > 10 lblTotal.Caption = iTotal

Comparing the 2 While loops.. Do While iTotal < 10 ………… Loop Do ……………… Loop Until iTotal > 10 Will not execute at all if iTotal is already 10 or more Will always execute the code once, and then test if iTotal is over 10

Summary … For iCount = 1 To 10 ………… Next iCount Do …………. Loop Until condition Do While condition …………. Loop Loops a FIXED NUMBER OF TIMES ALWAYS EXECUTES CODE ONCE before testing whether to loop TESTS CONDITION FIRST, then executes code if appropriate

Do …. Loop Until...example Calculating a factorial e.g Factorial 6 (mathematically, 6! ) = 6 * 5 * 4 * 3 * 2 * 1 could be coded like this …..

factorial ex. using Do … Loop Until … Dim iNum, iFactorial as Integer iFactorial = 1 iNum = 6 Do iFactorial = iFactorial * iNum iNum = iNum – 1 Loop Until iNum = 1 Stop when iNum is equal to 1

Seeing how factorial ex. works Dim iNum, iFactorial as Integer iFactorial = 1 iNum = 6 Do iFactorial = iFactorial * iNum iNum = iNum – 1 Loop Until iNum = 1 iFactorial 1 iNum

Summarising again … For … Do … Loop Until … Do While ….. Loop Loops a FIXED NUMBER OF TIMES ALWAYS EXECUTES CODE ONCE before testing whether to loop or to stop TESTS CONDITION FIRST, then executes code if appropriate