Iteration: Intro • Iteration/repetition/looping: Executing a sequence of instructions multi- ple times • Loop components: – Condition: determines whether loop should continue – Body: instructions being repeated – Loop control variable (LCV): its value determines when loop stops • Basic steps of loop control: Initialize Process Initialize LCV BEGIN LOOP Update Process Update LCV END LOOP • Two types of loops: 1. Pretest – Condition precedes body – Iterates 0+ times – This is the most general type of loop 1
Transcript
Iteration: Intro
• Iteration/repetition/looping: Executing a sequence of instructions multi-ple times
• Loop components:
– Condition: determines whether loop should continue
– Body: instructions being repeated
– Loop control variable (LCV): its value determines when loop stops
• Basic steps of loop control:
Initialize Process
Initialize LCV
BEGIN LOOP
Update Process
Update LCV
END LOOP
• Two types of loops:
1. Pretest
– Condition precedes body
– Iterates 0+ times
– This is the most general type of loop
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Iteration: Intro (2)
2. Posttest
– Condition follows body
– Iterates 1+ times
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Iteration: Do Loops
• VB has two versions of Do loops:
1. While Boolean expression
– Loops as long as Boolean expression is true
2. Until Boolean expression
– Loops as long as Boolean expression is false
• Both can be used to construct pretest and posttest loops
• Use whichever is easiest in given circumstances
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Iteration: Pretest Do Loops
• Do loop is pretest if the condition is at the top of the loop (after the Do)
1. Do While
– Syntax:
Do While Boolean expressionstatement(s)
Loop
– Semantics:
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Iteration: Pretest Do Loops (2)
2. Do Until
– Syntax:
Do Until Boolean expressionstatement(s)
Loop
– Semantics:
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Iteration: Pretest Do Loops (3)
• Examples:
1. While
’Print limit integers
Dim limit, counter As Integer
limit = txtBox1.Text
counter = 0
Do While counter < limit
lstBox1.Items.Add(CStr(counter))
counter++
Loop
2. Until
’Print limit integers
Dim limit, counter As Integer
limit = txtBox1.Text
counter = 0
Do Until counter = limit
lstBox1.Items.Add(CStr(counter))
counter++
Loop
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Iteration: Infinite Loops
• Infinite loop: Loop that never terminates
• Result of not updating LCV appropriately
• Example:
’Print limit integers
Dim limit, counter As Integer
limit = 10
counter = 0
Do While counter < limit
lstBox1.Items.Add(CStr(counter))
Loop
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Iteration: Loop Control
• Main issue with loops is loop control
– Determining how many times the loop body should execute and the tech-niques for controlling this
• 2 general types of loop control:
1. Definite iteration:
– Loop executes a fixed number of times
– This number is known before the loop starts
2. Indefinite iteration:
– Loop executes an indeterminate number of times
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Iteration: Definite Iteration
• Is an example of count-controlled iteration
– Body will execute a fixed number of times known before the loop starts
• LCV is an example of a variable called a counter
– It counts how many times something occurs
• Example
’count-controlled While loop: Average <limit> integers
Dim limit, counter, sum, number As Integer
limit = 10
counter = 0
sum = 0
Do While counter < limit
number = CInt(txtIn.Text)
counter++
sum += number
Loop
txtOut.Text = "The average = " & sum/limit
• LCV is counter
– Example of a counter-type variable
• Sum is an accumulator variable
– Keeps a total
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Iteration: Indefinite Iteration
• Used when don’t know ahead of time how many times loop should execute
• 3 types of control:
1. Count-controlled
– Iterations continue until a predetermined number of some occurrencehas been identified
– No knowledge of exactly when this will become true (or false) beforeiteration starts
– Example:
’Count-controlled While loop, indefinite iteration:
’ Average <limit> positive integers
Dim limit, counter, sum, number As Integer
limit = 10
counter = 0
sum = 0
Do While counter < limit
number = CInt(txtIn.Text)
If number > 0 Then
counter++
sum += number
End If
Loop
txtOut.Text = "The average = " & sum/limit
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Iteration: Indefinite Iteration (2)
2. Sentinel-controlled
– Sentinel is one or more values that signal end of iteration
– Usually outside of range of acceptable values used for processing
– Example:
’Sentinel-controlled While loop: Average positive
’integers. Terminate when a negative is input.
Dim counter, sum, number As Integer
counter = 0
sum = 0
number = CINt(txtIn.Text)
Do While number >= 0
counter++
sum += number
number = CInt(txtIn.Text)
Loop
If counter = 0 Then
txtOut.Text = "no integers were averaged"
else
txtOut.Text = "The average = " & sum/counter
End If
– Requires priming read
∗ Must get initial value for LCV prior to entering loop
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Iteration: Indefinite Iteration (3)
3. Flag-controlled
– Flag is LCV that keeps track of whether a situation has occurred
– Usually Boolean
– Can be thought of as a specialized sentinel
– Often used when multiple situations are used to terminate loop
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Iteration: Indefinite Iteration (4)
– Example:
’Flag-controlled Do loop: Average <counter> integers.
’Terminate on a negative or when average > predetermined amount.
’Print the final average and how many integers were involved.
Dim counter, sum, number As Integer
Dim done As Boolean
Dim avg, maxAvg As Single
counter = 0
sum = 0
maxAvg = 50.0
done = False
Do While Not done
number = CInt(txtIn.Text)
If number < 0
done = True
else
counter++
sum += number
avg = sum/counter
If avg >= maxAvg Then
counter--
sum -= number
avg = sum/counter
done = True
End If
End If
Loop
If counter = 0 Then
lstOut.Items.Add("no integers were averaged")
else
lstOut.Items.Add("The average = " & sum/limit)
lstOut.Items.Add("The count = " & counter)
End If
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Iteration: Post Test Loops
• Body must execute at least once
• Structures
1. Loop Until
– Syntax:
Dostatement(s)
Loop Until Boolean expression
– Semantics:
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Iteration: Post Test Loops (2)
2. Loop While
– Syntax:
Dostatement(s)
Loop While Boolean expression
– Semantics:
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Iteration: Post Test Loops (3)
• Examples:
1. Until
’Loop Until for input verification
Dim option As Integer
Do
option = InputBox("Enter a number between 5 and 10 ")
If ((option < 5) Or (option > 10)) Then
MsgBox("***Not a valid number***")
End If
Loop Until (option > 4 And option < 11)
2. While
’Loop While for input verification
Dim option As Integer
Do
option = InputBox("Enter a number between 5 and 10 ")
If ((option < 5) Or (option > 10)) Then
MsgBox("***Not a valid number***")
End If
Loop While (option < 5 And option > 10)
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Iteration: Exit Do
• The Exit Do statement immediately exits a Do statement when executed
– It halts iteration
– It can make some coding less awkward
• Consider an earlier example:
Dim counter, sum, number As Integer
Dim done As Boolean
Dim avg, maxAvg As Single
counter = 0
sum = 0
maxAvg = 50.0
done = False
Do While Not done
number = CInt(txtIn.Text)
If number < 0
done = True
else
counter++
sum += number
avg = sum/counter
If avg >= maxAvg Then
counter--
sum -= number
avg = sum/counter
done = True
End If
End If
Loop
If counter = 0 Then
lstOut.Items.Add("no integers were averaged")
else
lstOut.Items.Add("The average = " & sum/limit)
lstOut.Items.Add("The count = " & counter)
End If
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Iteration: Exit Do (2)
• Consider the same example using Exit Do
Dim counter, sum, number As Integer
Dim avg, maxAvg As Single
counter = 0
sum = 0
maxAvg = 50.0
Do While True
number = CInt(txtIn.Text)
If number < 0
Exit Do
else
counter++
sum += number
avg = sum/counter
If avg >= maxAvg Then
counter--
sum -= number
avg = sum/counter
Exit Do
End If
End If
Loop
If counter = 0 Then
lstOut.Items.Add("no integers were averaged")
else
lstOut.Items.Add("The average = " & sum/limit)
lstOut.Items.Add("The count = " & counter)
End If
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Iteration: Nested Loops
• Can place one loop inside of another
• Must make sure that initializations and updates occur for each loop and in theappropriate places
• Example:
’Nested Do loops: Print times tables.
Dim n, counter1, counter2, product As Integer
n = 5
counter1 = 1
Do While counter1 <= n
lstOut.Items.Add("" & counter1 & "’s times table: "
