Two%20dimensional%20arrays

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Two dimensional arrays
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Two dimensional arrays

• A two dim array is doubly subscripted
• It has rows and columns
• Tables of data are examples of two-dim arrays.
• As with one-dim arrays, row and column length is
  fixed once the array is allocated
• Arrays are homogeneous, so an array cant store
  (for example) both floats and Strings

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Applications in this ppt

• UPS lookup using comboboxes
• UPS lookup using textboxes
• Yearly Rainfall simulator
• Roster/exam grades
• Processing rows (or columns) of a 2-d array

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Manipulating 2-d arrays

• For a one dimensional array we usually use 1 loop
  to traverse contents.
• For 2-D array well usually use 2 loops, a row
  loop and a column loop.
• The outer loop is the slow loop and determines
  the order of the traversal.
• Each row (or column) or a 2-d array is a 1-D array

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Declaring data

• If the data is known in advance it can be
  declared and initialized as a field value
• Dim data(,) As Integer 12, 34, 56, 75, 9,
  2, 100, 6, 500, 400, 300, 200

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To display the data (row by row) in a listbox

• For row 0 To 2
• strdata ""
• For col 0 To 3
• strdata strdata data(row,
  col) ControlChars.Tab
• Next
• Listdata.Items.Add(strdata)
• Next

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exercises

• For the previous example, write code for summing
  all the contents of the array.
• For the previous example, write code for finding
  the largest (smallest) element in the array.
• What code would access the diagonal elements of
  a square array?
• What code would transpose the contents of a
  square array? (swap rows to columns)

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Row by row traversal
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We might be interested in row (or column)
computations say sum contents of a row

• Dim row, col, sum As Integer
• sum 0
• 'one loop needed to traverse a one
  dimensional subarray
• row Integer.Parse(txtrow.text)
• For col 0 To 3
• sum sum data(row, col)
• Next

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Display of functionality
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The UPS rates application using comboboxes
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(No Transcript)
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Contents of weight combobox
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Contents of zone combobox
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Global (field) array declarations note doubly
subscripted array

• Dim rates(,) As Decimal 1D, 1.5D, 1.65D,
  1.85D, _
• 1.58D, 2D, 2.4D, 3.05D, _
• 1.71D, 2.52D, 3.1D, 4D, _
• 2.04D, 3.12D, 4D, 5.01D, _
• 2.52D, 3.75D, 5.1D, 7.25D
• Dim weight() As Integer 1, 3, 5, 10
• Dim zone() As String "A", "B", "C", "D"

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Same,but using textboxes
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The button code

• Dim weightindex, zoneindex As Integer
• Dim wtin As Integer
• wtin Integer.Parse(txtwt.Text)
• Dim wtfound, zonefound As Boolean
• wtfound False
• zonefound False
• weightindex 0
• zoneindex 0
• While (Not wtfound) And weightindex lt 3
• If wtin lt weight(weightindex) Then
• wtfound True
• Else
• weightindex 1
• End If
• End While
• If wtfound False Then
• weightindex 4
• End If
• While zonefound False And zoneindex lt
  3

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A Rainfall simulator
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remarks

• This is not very realistic, as I did not use real
  rainfall data to fill my rainfall array.
• Real data could easily be incorporated get
  (historic) max and min rainfall for each day, and
  use these numbers in your random generator.
• But it shows how a simulator could be built to
  model weather (or something else). For example,
  a temperature simulator could be built similarly.

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Global (field) values

• Dim rain(12, 31) As Double
• Dim nummonths As Integer 12
• Dim months As String() "January",
  "February", "March", "April", "May", "June",
  "July", "August", "September", "October",
  "November", "December"
• Dim days As Integer() 31, 28, 31, 30, 31,
  30, 31, 31, 30, 31, 30, 31

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Form load fills rainfall array randomly

• Private Sub RainFall_Load(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles MyBase.Load
• Dim r As New Random(DateTime.Now.Milliseco
  nd)
• Dim i, j As Integer
• For i 0 To nummonths - 1
• For j 0 To days(i) - 1
• rain(i, j) r.NextDouble()
  r.Next(0, 5)
• not a very realistic simulation
• Next
• Next
• End Sub

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The day-of-max-rain button

• Private Sub btnmaxday_Click(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles btnmaxday.Click
• Dim i, j, m, d As Integer
• m 0
• d 0
• For i 0 To nummonths - 1
• For j 0 To days(i) - 1
• If rain(i, j) gt rain(m, d) Then
• m i
• d j
• End If
• Next
• Next
• ListBox1.Items.Add("Day of max rain was "
  months(m) d "amt" rain(m,
  d).ToString("N2"))
• End Sub

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The month-of-min-rain button

• Private Sub btnminmon_Click(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles btnminmon.Click
• Dim i, j, m, d As Integer
• Dim sum, tot As Double
• tot 99999
• For i 0 To nummonths - 1
• sum 0
• For j 0 To days(i) - 1
• sum rain(i, j)
• Next
• If sum lt tot Then
• m i
• tot sum
• End If
• Next
• ListBox1.Items.Add("Month of min rain was
  " months(m) " amt " tot.ToString("N2"))
• End Sub

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Other buttons and functionality

• I didnt code the other buttons.
• You could add a button Redo simulation to refill
  the rain array again randomly

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An exam recorder
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Recording grades
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Add grades for students
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Compute averages for each student
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Fields (globals) used

• Const classsize 4
• Public students As String() "Bob",
  "Marcy", "Jane", "Pete"
• Dim exams(classsize, 3) As Integer

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Add a grade button code

• use selectedindex from the two comboboxes to see
  which student/ which exam score is being entered.
• Check to make sure a legal row/col was selected
  and insert the grade into position
  (rowstudent,colexam) of the exams array.

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Add a grade button code

• Private Sub btnadd_Click(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles btnadd.Click
• Dim row, col As Integer
• col cboexam.SelectedIndex()
• row cbostudent.SelectedIndex()
• If row gt 0 And row lt 4 And col gt 0 And
  col lt 3 Then
• exams(row, col) Integer.Parse(txtexa
  m.Text)
• Else
• MessageBox.Show("row" row "
  col" col, "error", MessageBoxButtons.OK)
• End If
• txtexam.Text ""
• End Sub

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Display button

• Clear listbox.
• Show names and which grades have been recorded.
• Exams are all initialized to -99. In the loop,
  if a legal grade has not been recorded, a message
  is displayed for this exam.
• 2 loops needed to traverse a 2-D array, only one
  loop is needed for a 1-D array. The outer loop
  adds a name from the name list onto the string to
  display, the inner loop adds an exam for this
  student, each time it goes around.

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Display button code

• Private Sub btndisplay_Click(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles btndisplay.Click
• Dim i, j As Integer
• Dim displaystring As String
• ListBox1.Items.Clear()
• For i 0 To classsize - 1
• displaystring students(i)
• For j 0 To 2
• If exams(i, j) -99 Then
• displaystring displaystring
  ControlChars.Tab "no grade"
• Else
• displaystring displaystring
  ControlChars.Tab exams(i, j)
• End If
• Next
• ListBox1.Items.Add(displaystring)
• Next
• End Sub

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Calculate averages

• Clear listbox.
• You need a sum (and, optionally, an ave variable)
  of type double
• For each set of scores (for each student) set
  sum0.
• Add across the row (inner loop) to the sum.
• Display the name and average in the listbox.

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Additions and improvements

• Add labels for checkboxes
• Add a clear-all button
• Have number of exams be a constant like number of
  students in this example.
• Provide (buttons or combobox for) other exam
  statistics, like high grade, low grade, and
  overall mean.

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Generic array manipulation
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About this example

• An array is declared as a field value and filled
  (either at compile or in form load)
• Buttons provided to add across any given row or
  column
• Array declaration at the top of class
• Dim data(,) As Integer 12, 34, 56, 75, 9,
  2, 100, 6, 500, 400, 300, 200

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About this example formload displays array
contents in listbox

• Private Sub Form1_Load(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles MyBase.Load
• Dim row, col As Integer
• Dim strdata As String
• For row 0 To 2
• strdata ""
• For col 0 To 3
• strdata strdata data(row,
  col) ControlChars.Tab
• Next
• Listdata.Items.Add(strdata)
• Next
• End Sub

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Sum by row button

• Private Sub btnrow_Click(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles btnrow.Click
• Dim row, col, sum As Integer
• sum 0
• 'one loop needed to traverse a one
  dimensional subarray
• row Integer.Parse(txtrow.text)
• For col 0 To 3
• sum sum data(row, col)
• Next
• outside loop list results
• Listdata.Items.Add("sum of row " row
  "" sum)
• End Sub

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Sum by row
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Sum a column