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Two dimensional arrays

Two dimensional arrays. 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

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Two dimensional arrays

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  1. Two dimensional arrays

  2. 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 can’t store (for example) both floats and Strings

  3. 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

  4. Manipulating 2-d arrays • For a one dimensional array we usually use 1 loop to traverse contents. • For 2-D array we’ll 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

  5. 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}}

  6. 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

  7. 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)

  8. Row by row traversal

  9. 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

  10. Display of functionality

  11. The UPS rates application using comboboxes

  12. Contents of weight combobox

  13. Contents of zone combobox

  14. 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"}

  15. Same,but using textboxes

  16. 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 <= 3 If wtin <= 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 <= 3 If Me.txtzone.Text.ToUpper() = zone(zoneindex) Then zonefound = True Else zoneindex += 1 End If End While If zoneindex <= 3 Then ' txtdisplay.Text = weightindex & " " & zoneindex txtdisplay.Text = rates(weightindex, zoneindex).ToString("N") Else MessageBox.Show("weight or zone incorrect", "data error", MessageBoxButtons.OK, MessageBoxIcon.Exclamation) End If

  17. A Rainfall simulator

  18. 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.

  19. 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}

  20. 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.Millisecond) 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

  21. 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) > 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

  22. 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 < 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

  23. Other buttons and functionality • I didn’t code the other buttons. • You could add a button: Redo simulation to refill the rain array again randomly

  24. An exam recorder

  25. Recording grades

  26. Add grades for students

  27. Compute averages for each student

  28. Fields (globals) used Const classsize = 4 Public students As String() = {"Bob", "Marcy", "Jane", "Pete"} Dim exams(classsize, 3) As Integer

  29. 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 (row=student#,col=exam#) of the exams array.

  30. 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 >= 0 And row < 4 And col >= 0 And col < 3 Then exams(row, col) = Integer.Parse(txtexam.Text) Else MessageBox.Show("row=" & row & " col=" & col, "error", MessageBoxButtons.OK) End If txtexam.Text = "" End Sub

  31. 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.

  32. 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

  33. 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 sum=0. • Add across the row (inner loop) to the sum. • Display the name and average in the listbox.

  34. 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.

  35. Generic array manipulation

  36. 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}}

  37. 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

  38. 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

  39. Sum by row

  40. Sum a column

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