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Herbert G. Mayer, PSU CS Status 7/29/2013 Initial content copied verbatim from CS 106 material developed by CS professors: Cynthia Brown & Robert Martin. CS 106 Computing Fundamentals II Chapter 61 “ Loops ”. Syllabus. Repetition Real-Life Examples Multiplication Table Example - PowerPoint PPT Presentation
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CS 106Computing Fundamentals II
Chapter 61“Loops”
Herbert G. Mayer, PSU CSHerbert G. Mayer, PSU CSStatus 7/29/2013Status 7/29/2013
Initial content copied verbatim fromInitial content copied verbatim fromCS 106 material developed byCS 106 material developed by
CS professors: Cynthia Brown & Robert MartinCS professors: Cynthia Brown & Robert Martin
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Syllabus RepetitionRepetition
Real-Life ExamplesReal-Life Examples
Multiplication Table ExampleMultiplication Table Example
For Next LoopFor Next Loop
Do WhileDo While
Do UntilDo Until
Nested LoopsNested Loops
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Repetition
• Repetition of the same or similar actions is often Repetition of the same or similar actions is often needed in process designneeded in process design
• Three styles of repetition:Three styles of repetition: Repeat for a fixed number of times Repeat as long as or while a certain condition is true Repeat until a condition becomes true Repeat for every element of a group or set
• We’ll look at the first two types in this sessionWe’ll look at the first two types in this session
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Real-Life Examples
• Keep ringing up items while the customer has moreKeep ringing up items while the customer has more
• Keep adding numbers until you get to the known end Keep adding numbers until you get to the known end of the listof the list
• Cut a paycheck for each employee in the rosterCut a paycheck for each employee in the roster
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Multiplication Table Example
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Printing a Multiplication Table
• Our job: input a number between 1 and 12 in a text Our job: input a number between 1 and 12 in a text box (txtM)box (txtM)
• Print a multiplication table for this number in a list Print a multiplication table for this number in a list box (lstAnswer)box (lstAnswer)
• This is possible but painful with our current set of This is possible but painful with our current set of toolstools
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Possible code…
strM = txtM.Text strM = txtM.Text 'text form of M'text form of M
numM = numM = CIntCInt(strM) (strM) 'numeric form of M'numeric form of M
lstAnswer.ClearlstAnswer.Clear ‘nothing in the list box‘nothing in the list box
lstAnswer.AddItem( strM & " X 1 = " & lstAnswer.AddItem( strM & " X 1 = " & CStrCStr(numM * 1 ) )(numM * 1 ) )
lstAnswer.AddItem( strM & " X 2 = " & lstAnswer.AddItem( strM & " X 2 = " & CStrCStr(numM * 2 ) )(numM * 2 ) )
lstAnswer.AddItem( strM & " X 3 = " & lstAnswer.AddItem( strM & " X 3 = " & CStrCStr(numM * 3 ) )…(numM * 3 ) )…
lstAnswer.AddItem( strM & " x 12 = " & lstAnswer.AddItem( strM & " x 12 = " & CStrCStr(numM * 12 ) )(numM * 12 ) )
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Ugh!
• This is clumsy and unbearably repetitiveThis is clumsy and unbearably repetitive
• If we wanted to change the upper limit in some way If we wanted to change the upper limit in some way (say do up to 8 * 8, or 10 * 10, instead of going to 12 (say do up to 8 * 8, or 10 * 10, instead of going to 12 each time), we would need even uglier codeeach time), we would need even uglier code
• Doing a large number of these would be awfulDoing a large number of these would be awful
• Luckily, VBA has some nice repetition constructsLuckily, VBA has some nice repetition constructs
• Now for some samples with upper bounds, 4 or 10 . . .Now for some samples with upper bounds, 4 or 10 . . .
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For Next loop
• Repetitions are called loops in VBARepetitions are called loops in VBA
• Loops go through, what we call, Loops go through, what we call, iterationsiterations; or we say: ; or we say: loops iterateloops iterate
• A A For Next For Next loop is used when we can determine the loop is used when we can determine the number of repetitions before starting the loopnumber of repetitions before starting the loop
• Such determinations can be computable trivially, if Such determinations can be computable trivially, if the so called the so called upper bound upper bound is a constantis a constant
• Or else, if the bound is computable at the latest by Or else, if the bound is computable at the latest by the start of the first loop iterationthe start of the first loop iteration
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Simple For Next LoopstrM = txtM.TextstrM = txtM.Text ‘ strM = “5”‘ strM = “5”lstAnswer.ClearlstAnswer.Clear ‘ list box is cleared‘ list box is cleared
’ ’ print the same thing 4 times, i.e. strMprint the same thing 4 times, i.e. strMFor j = 1 To 4For j = 1 To 4 ‘ iterate 4 times, print same:‘ iterate 4 times, print same:
lstAnswer.AddItem( strM )lstAnswer.AddItem( strM )NextNext
Prints, for strM = “5”Prints, for strM = “5”55555 5 55
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Another Simple For Next Loop
lstAnswer.ClearlstAnswer.Clear‘ ‘ prints sequence 1..4prints sequence 1..4‘ ‘ with implied carriage-return after each:with implied carriage-return after each:For j = 1 To 4For j = 1 To 4
lstAnswer.AddItem( CStr( j ) ) ‘ convert intlstAnswer.AddItem( CStr( j ) ) ‘ convert intNextNext
Prints 4 lines:Prints 4 lines:11223344
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For Next Multiplication Table‘ ‘ see line continuation _ in samples:see line continuation _ in samples:‘ ‘ other line continations after , ( and before )other line continations after , ( and before )numM = CInt( txtM.Text )numM = CInt( txtM.Text )strM = txtM.TextstrM = txtM.TextlstAnswer.ClearlstAnswer.ClearFor j = 1 To 10For j = 1 To 10
lstAnswer.AddItem(strM & “ x “ & CStr(j) & “ = “ & _lstAnswer.AddItem(strM & “ x “ & CStr(j) & “ = “ & _CStr( numM * j ) )CStr( numM * j ) )
NextNext
Prints, for numM = 5Prints, for numM = 55 x 1 = 55 x 1 = 55 x 2 = 10 5 x 2 = 10 . . .. . .5 x 10 = 50 5 x 10 = 50
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Do While Concept
• Sometimes we can’t tell in advance how many times a Sometimes we can’t tell in advance how many times a loop iteratesloop iterates
• Or, it might just be clearer to use a logic that checks Or, it might just be clearer to use a logic that checks as we go alongas we go along
• In that case we can use a In that case we can use a Do While Do While loop instead of a loop instead of a For Next For Next looploop
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Do While ExampleUse Use For Next For Next loop program, but this time with a loop program, but this time with a Do While Do While looploop
numM = CInt(txtM.Text)numM = CInt(txtM.Text)strM = txtM.TextstrM = txtM.TextlstAnswer.ClearlstAnswer.Clear
j = 1 ‘ set j to initial value 1 at loop startj = 1 ‘ set j to initial value 1 at loop startDo While j <= 12Do While j <= 12 ‘ upper bund is constant‘ upper bund is constant
lstAnswer.AddItem( strM & “ x “ & CStr(j) & “ = “ & _lstAnswer.AddItem( strM & “ x “ & CStr(j) & “ = “ & _CStr( numM * j ) )CStr( numM * j ) )
j = j + 1j = j + 1 ‘ this line makes the loop stop‘ this line makes the loop stopLoop Loop ‘ end with Loop instead of Next‘ end with Loop instead of Next
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Do While Example
Here’s the numM by numM versionHere’s the numM by numM version
numM = CInt(txtM.Text)numM = CInt(txtM.Text) ‘ numM is an integer type‘ numM is an integer typestrM = txtM.TextstrM = txtM.Text ‘ strM ix a string type‘ strM ix a string typelstAnswer.ClearlstAnswer.Clearj = 1j = 1Do While j <= numMDo While j <= numM ‘ upper bound is variable‘ upper bound is variable
lstAnswer.AddItem( strM & “ x “ & CStr( j ) & “ = “ _lstAnswer.AddItem( strM & “ x “ & CStr( j ) & “ = “ _& CStr( numM * j ) )& CStr( numM * j ) )
j = j + 1j = j + 1LoopLoop
What happens if numM = 0?What happens if numM = 0?
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Do While Loop Flowchart
Do While Do While conditioncondition
statementsstatements
including nested Do including nested Do WhileWhile
LoopLoop
Loop statements can Loop statements can execute foreverexecute forever
Condition true?
Execute statements within the loop.
Execute statementsthat follow the loop.
No
Yes
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The Do Until Variation
Instead of repeating Instead of repeating WhileWhile a condition is true, we can a condition is true, we can repeat repeat UntilUntil the condition becomes true the condition becomes true
For example:For example:
Do While Do While varA <= 5varA <= 5
Is almost equivalent to:Is almost equivalent to:
Do Until Do Until varA > 5varA > 5
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Test at the End Variation
• Instead of testing at the beginning of the loop, we can Instead of testing at the beginning of the loop, we can test at the endtest at the end
• This is useful because, many times, we want to do the This is useful because, many times, we want to do the loop code at least once, no matter what!loop code at least once, no matter what!
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Do Until ExampleMultiplication example using Multiplication example using Do Until Do Until loop with test at the endloop with test at the end
numM = CInt( txtM.Text )numM = CInt( txtM.Text )
strM = txtM.TextstrM = txtM.Text
lstAnswer.ClearlstAnswer.Clear
j = 1j = 1
Do Do
lstAnswer.AddItem(strM & “ x “ & CStr(j) & “ = “ & CStr(numM * j))lstAnswer.AddItem(strM & “ x “ & CStr(j) & “ = “ & CStr(numM * j))
j = j + 1 j = j + 1
Loop Until j > 12Loop Until j > 12
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Do Loop Flowchart (Until, test at end)
DoDo
statement(s)statement(s)
Loop Until Loop Until conditioncondition
Do Loop statements Do Loop statements always run at least oncealways run at least once
Condition true?
Execute statements within the loop
Execute statementsthat follow the loop
No
Yes
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Nested Loops
• Remember how Remember how IfIf statements can be “nested”: you statements can be “nested”: you can have an can have an IfIf inside another inside another IfIf
• We can also put whatever we want inside a loop, We can also put whatever we want inside a loop, including another loopincluding another loop
• If we do this, the inner loop is executed completely, If we do this, the inner loop is executed completely, for all of its repetitions, each time the outer loop is for all of its repetitions, each time the outer loop is executed onceexecuted once
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Nested LoopsDim j, k As IntegerDim j, k As Integer
lstAnswer.ClearlstAnswer.Clear
For j = 1 To 12For j = 1 To 12
For k = 1 To 4For k = 1 To 4
lstAnswer.AddItem(“j = “ & CStr( j ) & “ k= “ & lstAnswer.AddItem(“j = “ & CStr( j ) & “ k= “ & CStr( k ) )CStr( k ) )
Next ‘ kNext ‘ k
Next ‘ jNext ‘ j
‘ ‘ how many lines of output??how many lines of output??
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Output of Nested Loop
j = 1 k = 1j = 1 k = 1
j = 1 k = 2j = 1 k = 2
j = 1 k = 3j = 1 k = 3
j = 1 k = 4j = 1 k = 4
j = 2 k = 1j = 2 k = 1
j = 2 k = 2j = 2 k = 2
j = 2 k = 3j = 2 k = 3
j = 2 k = 4j = 2 k = 4
j = 3 k = 1j = 3 k = 1
Etc.Etc.
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Another Nested Loop
Dim j, k As IntegerDim j, k As Integer
lstAnswer.ClearlstAnswer.Clear
For j = 1 To 12For j = 1 To 12
For k = 1 To 4For k = 1 To 4
lstAnswer.AddItem(“j = “ & CStr( j ) & “ k= “ & CStr( k lstAnswer.AddItem(“j = “ & CStr( j ) & “ k= “ & CStr( k ) )) )
Next ‘kNext ‘k
lstAnswer.AddItem(“Finished j = “ & CStr( j ) )lstAnswer.AddItem(“Finished j = “ & CStr( j ) )
Next ‘jNext ‘j
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Results of Another Nested Loopj = 1 k = 1j = 1 k = 1
j = 1 k = 2j = 1 k = 2
j = 1 k = 3j = 1 k = 3
j = 1 k = 4j = 1 k = 4
Finished j = 1Finished j = 1
j = 2 k = 1j = 2 k = 1
j = 2 k = 2j = 2 k = 2
j = 2 k = 3j = 2 k = 3
j = 2 k = 4j = 2 k = 4
Finished j = 2Finished j = 2
j = 3 k = 1j = 3 k = 1
Etc.Etc.
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