[Year 12 SofDev] DLTV Exam 1

[Chris Bucknell]

Hi Damien,

I agree with your logic and working (love a bit of VB.NET code).  I think that the issue is with the algorithm given (it's wrong with two errors).  This algorithm looks like a binary search but instead of returning the quotient (from the sum of the low and high divided by 2) and using this as the middle it looks like it uses the remainder for the middle.  Hence why you have your oscillating 1 and 125 (you have assumed zero index arrays for the student datasets - who knows what would happen with non-zero ... probably a blue screen of death).  The second error is that the low and high values should hold the index position in the array (not the value in that position).

So if we resolve these the two issues (i.e. 1-use the quotient for generating the middle index value, and 2-use the index of the position of the start and end of the data set in the array) then we'd get something like:

(Note: 0 based array - continue the VB feel :))

Pass

Position
Low

Position
High

Calculation

Quotient

Remainder

Position
Middle

Student(Middle)

Found

1

0

9

(0+9)/2 =

4

1

4

500

False

2

0

4

(0+4)/2=

2

0

2

250

False

3

2

4

(2+4)/2=

3

0

3

375

True



Good catch on this on Damien.

Chris Bucknell
Head of ICT
Chris.Bucknell at haileybury.vic.edu.au<mailto:Chris.Bucknell at haileybury.vic.edu.au>
www.haileybury.vic.edu.au<http://www.haileybury.vic.edu.au>


From: sofdev-bounces at edulists.com.au [mailto:sofdev-bounces at edulists.com.au] On Behalf Of ATKINSON-BUCK, Damien
Sent: Wednesday, 8 October 2014 5:51 PM
To: Year 12 Software Development Teachers' Mailing List (sofdev at edulists.com.au)
Subject: [Year 12 SofDev] DLTV Exam 1

Hi folks,
            I'm just going through the DLTV Exam and can't quite figure the answer to the algorithm question 12 below.
[cid:image002.png at 01CFE33E.44EF3A80]
[cid:image003.png at 01CFE33E.44EF3A80]

My desk produces,
Pass

Middle

Low

High

Result

StudentNo(Middle)

1

0

1

999

False

1

2

1

0

999

False

125

3

0

1

999

False

1

4

1

0

999

False

125

AND SO ON


I've tried it as vb code below and ended in an infinite loop. Am I missing something blatantly obvious here or can anyone see the fix for this?

Module Module1
    Dim StudentNo() As Integer = {1, 125, 250, 375, 500, 625, 750, 875, 999}

    Sub Main()
        Dim low As Integer = 1
        Dim high As Integer = 999
        Dim result As Boolean = False
        Dim searchnum As Integer = 375

        Do While low <= high And result = False
            Dim middle As Integer = (low + high) Mod 2
            If StudentNo(middle) = searchnum Then
                result = True
                Dim myMessage As String = "Low " & low & ", High " & high & ", Mid " & middle
                MsgBox(myMessage, vbOK, "DONE")
            End If
            If StudentNo(middle) > searchnum Then
                high = middle
            Else
                low = middle
            End If
        Loop
    End Sub

End Module




Damien Atkinson-Buck
Head of Learning Area: Technology/Arts
p: +61 3 9490 3848

e: damien.atkinson-buck at ivanhoe.com.au<mailto:damien.atkinson-buck at ivanhoe.com.au>



f: +61 3 9490 3490

w: www.ivanhoe.com.au<http://www.ivanhoe.com.au>

w: www.ivanhoe100.com.au<http://www.ivanhoe100.com.au>

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