Additional loop presentation

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Additional loop presentation
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Additional problems requiring loops

• Fibonacci sequence
• Euclids GCD algorithm
• Reading data from a file
• Changing base of numbers base b to base 10.
• Changing base 10 to base b

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Fibonacci numbers

• These numbers used in Biology (and in Computer
  Science!) are the sequence
• 1, 1, 2, 3, 5 , 8, 13, 21, ?, ?, .
• Using any two adjacent numbers in the sequence,
  you can generate the rest of the sequence. How?

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Fibonacci numbers

• Fib(i2)Fib(i)Fib(i1) is the recurrence
  relation.
• Set old1 and current1. Build a loop for as
  many fibonacci values as youd like
• Display them in a listbox

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Fibonacci numbers
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Fibonacci numbers

• Should display first two
• Remember to declare a counter for your for loop
• Remember to subtract two from how many loop
  iterations

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Fibonacci numbers Button click code

• Dim i, old, cur As Integer
• old 1
• cur 1
• Dim n As Integer
• n Integer.Parse(TextBox1.Text)
• 'print first two fibs
• ListBox1.Items.Add(cur)
• ListBox1.Items.Add(cur)
• For i 1 To n - 2 'need to count the
  first two already shown
• cur cur old
• old cur - old
• ListBox1.Items.Add(cur)
• Next

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GCDgreatest common divisor
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GCD discussion

• This application gets the greatest common divisor
  two ways and counts how many times it has to loop
  for each technique.
• The crude method set your guess to be the
  smaller number. As long as your guess doesnt
  evenly divide both numbers, keep subtracting 1
  from it.

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Euclids algorithm

• Get the remainder
• Iterate the following
• Rembig mod small note rem is reserved word
• If rem is zero quit, small is the gcd
• If not, set big to small and small to remainder,
  and do it again.

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Button click

• Dim a, b, gcd, big, small As Integer
• Try
• a Integer.Parse(TextBox1.Text)
• b Integer.Parse(TextBox2.Text)
• If a gt b Then
• big a
• small b
• Else
• big b
• small a
• End If
• gcd getGCD1(big, small)
• Label3.Text "gcd using crude
  method" gcd.ToString()
• gcd getGCD2(big, small)
• Label4.Text "gcd using Euclid"
  gcd.ToString()
• Label5.Text "method looped"
  loopct1.ToString()
• Label6.Text "method looped"
  loopct2.ToString
• Catch ex As Exception
• MessageBox.Show("must enter
  integers")

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Crude method code

• Function getGCD1(ByVal big, ByVal small) As
  Integer
• Dim diff As Integer small
• Loopct10
• Do While big Mod diff ltgt 0 Or small Mod
  diff ltgt 0
• diff diff - 1
• loopct1 1 loop1ct is a global
  integer
• Loop
• Return diff 'should be gcd
• End Function

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Euclids method

• Function getGCD2(ByVal big, ByVal small) As
  Integer
• Dim remainder As Integer
• loopct20
• remainder big Mod small
• Do While remainder ltgt 0
• loopct2 1 loop2ct is a global
  integer
• big small
• small remainder
• remainder big Mod small
• Loop
• Return small
• End Function

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Reading from a data file

• Nothing here especially requires a loop except
  processing a lot of data.
• Typically, the application needs to read numbers
  or names or whatever until it reaches end of file.

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Notes on text files

• Create a data file by typing numbers (or
  something) into a file.
• I used Textpad. Wordpad and Notepad editors will
  also work.
• If you use MS Word, be sure to select save as
  option ascii text

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Create a data file save as txt
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Save data file to project/thisproj/bin/debug
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I built an ap with just a button and listbox
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Button click code opens file and reads (strings)
to end of file

• Private Sub btnread_Click(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles btnread.Click
• Dim data As String
• Lstdisplay.Items.Clear()
• Dim sr As IO.StreamReader
  IO.File.OpenText("data.txt")
• Do While sr.Peek ltgt -1
• data sr.ReadLine()
• Lstdisplay.Items.Add(data)
• Loop
• sr.Close()
• End Sub

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Running form just displays the data
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Improvements

• Sum the values in the data file
• Search for a name or number in the data file
• Allow user to enter a file name

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Base changer

• In base b, only digits 0b-1 may appear
• Remarks on Radix positional notation
• A number in any base consists of legal digits,
  each represents how many there are of the base to
  some power

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Base changer

• So
• 12345 base 10 represents 110000210003100 and
  so on
• 101111 in base 2 represents 111214180161
  3247

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Base changer

• Convert to base 10
• 33221 in base 4
• 10101 in base 2
• 4210 in base 5

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Base changer

• Putting together an ap that converts numbers from
  base b to base 10
• Get input from user, a String (number in base b)
  and an integer (the original base).
• Set a sum value to 0
• LOOP
• Use the substring function to peel off digits
  from the string and multiply by the appropriate
  power of the base and add to the running total

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Numbers in base 10 are unchanged
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But if base was 5 22535469
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Handles any base up to including 10
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Buttonclick code

• Dim i, j, ans, base As Integer
• Dim x, y As String
• ans 0 ' will hold answer
• base Integer.Parse(Txtbase.Text)
• x Txtnum.Text
• For i 0 To x.Length - 1
• y x.Substring(i, 1) ' this short
  string 1 character long
• j Integer.Parse(y)
• ans ans base y
• Next
• Lblans.Text ans.ToString

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To handle larger bases

• Need a function to return proper character value

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Going the other way base 10 to base b
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Use mod and div (\) to build a string
representation of the answer

• Dim num, base As Integer
• Try
• num Integer.Parse(txtnum.Text)
• base Integer.Parse(txtbase.Text)
• Dim ans As String ""
• Do While num ltgt 0
• ans ans num Mod base
• num num \ base
• Loop
• lblans.Text "answer is" ans
• Catch ex As Exception
• End Try

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Still to do

• Handle bases bigger than 10 I havent done
  that.
• If the remainder is larger than 9, you need to
  use alpha representation for the digits A is
  10, and so on.