Arrays

1
Chapter 7

• Arrays

2
Outline and Objective

• Arrays in Visual Basic
• One-dimensional arrays
• Control arrays
• Two-dimensional arrays
• Searching
• Sorting

3
Array verses Simple Variable

• Simple variable is used to store a single value.
• Array variable is used to represent many values
  of the same type with one variable name.

4
Elements of an Array

• Array Name A valid variable name for the
  structure.
• Subscript or Index A value that refers to a
  particular array element.
• Element or Subscripted variable An individual
  data item within an array.

5
Array Declaration

• SyntaxDim arrayName(m To n) As varTypewhere m
  and n are integers
• Examples
• Dim month(1 To 12) As String
• Dim population(1987 To 2003) As Single
• Dim students(1 To 30) As String

6
The Dim Statement

• Used to declare an array
• A Dim statement must occur before the first
  reference to the array elements.

7
Initializing an Array

• Private Sub cmdWhoWon_Click()
• Dim teamName(1 To 4) As String
• Dim n As Integer
• teamName(1) "Red Sox"
• teamName(2) "Giants"
• teamName(3) "White Sox"
• teamName(4) "Cubs"
• n Val(txtNumber.Text)
• picWinner.Print "The " teamName(n) " won
  World Series " n
• End Sub

8
Array teamName()
Array Name

• Dim teamName( 1 To 4) As String

Red Sox Giants White Sox Cubs
teamName(1)
Index
9
Initializing an Array by Reading from a File

• Dim student (1 To 30) As String
• Dim count As Integer
• Open STUDENTS.TXT For Input As 1
• For count 1 To 30
• Input 1, student(count)
• Next count

10
Adding Up Elements in an Array

• Dim score(1 To 30) As Single, student(1 To 30) As
  String
• Dim count As Integer, average as Integer, sum As
  Integer
• Open STUDENT.TXT For Input As 1
• For count 1 To 30
• Input 1, student(count), score(count)
• Next count
• sum 0
• For count 1 To 30
• sum sum score(count)
• Next count
• average sum/30

11
Parallel Arrays

• Two arrays are said to be parallel if subscripted
  variables having the same subscript are related.

12
Example of Parallel Arrays

• Dim nom(1 To 8) As String, score(1 To 8) As
  Integer
• Dim student As Integer
• Open SCORE.TXT For Input As 1
• For student 1 To 8
• Input 1, nom(student), score(student)
• Next student
• Close 1

13
Form_Load()

• Use it to Initialize Form Level variables
• Use it to initialize Form Level Arrays
• Use it to Dim Dynamic Arrays

14
Dynamic Arrays

• ReDim can only be used inside a procedure
• Defines or dimensions a Dynamic array
• ReDim arrayName (m To n) as varType
• m, n can be a variables or expressions
• Use Dim in General Declaration as
• Dim arrayName() as varType
• Cannot be used till you ReDim it in a procedure

15
Ordered Array

• An array is ordered if its values are in either
  ascending or descending order.
• For string arrays, the ANSI table is used to
  evaluate the less than or equal to condition.

16
Merging two ordered arrays

• 1.Compare the two first elements of the first and
  second arrays.
• A. If one name alphabetically precedes the other,
  copy it onto the third list and cross it off the
  original array.
• B. If the names are the same, copy the name onto
  the third list and cross out the name from both
  arrays.
• 2. Repeat Step 1 with the new name in the array
  until you reach the end of either array.
• 3. Copy the names from the remaining array onto
  the third array.

17
Passing an Array

• An array can be passed to another procedure by
  reference.
• Example
• Private Sub cmddisplay_Click()
• 'Pass array to subprogram and function
• Dim score(1 To 5) As Integer
• Call FillArray(score())
• picAverage.Print Average is" Sum(score()) /
  5
• End Sub

18
Array Score is passed to a Sub procedure

• Private Sub FillArray(s() As Integer)
• ' Fill array with scores
• s(1) 85
• s(2) 92
• s(3) 75
• s(4) 68
• s(5) 84
• End Sub

This array is pointing to the same location as
array score
19
Array score is passed to a Function

• Private Function Sum(s() As Integer) As Integer
• Dim total As Integer, index As Integer
• 'Add up scores
• total 0
• For index 1 To 5
• total total s(index)
• Next index
• Sum total
• End Function

20
Sorting

• A common practice involving arrays is to sort the
  elements of the array in either ascending or
  descending order.
• Sorting techniques
• Bubble Sort
• Shell Sort

21
Bubble Sort

• The bubble sort involves comparing adjacent
  elements and swapping the values of those
  elements when they are out of order.
• One complete time through an array is called a
  pass.

22
Bubble Sort

• For passNum 1 To 4
• For index 1 To 5 - passNum
• If name(index) gt name(index 1) Then
• Call SwapData(name(), index)
• End If
• Next index
• Next passNum

23
Swapping two variables

• Private Sub SwapData (A() As String, index As
  Integer)
• Dim temp As String
• temp A(index)
• A(index) A(index 1)
• A(index 1) temp
• End Sub

24
Bubble Sort

• The number of passes used to sort the elements in
  an array is equal to the number of elements in
  the array less 1.
• Bubble sort works well for sorting small arrays,
  but is often too slow for very large ones.

25
Shell Sort

• Similar to the bubble sort
• Instead of comparing and swapping adjacent
  elements A(count) and A(count1), Shell sort
  compares and swaps non-adjacent elements A(count)
  and A(count Gap), where Gap starts at roughly
  half the size of the array

26
Shell Sort

• At the end of each pass, if no elements have been
  swapped in the current pass, then Gap is halved
  for the next pass.
• Eventually, Gap becomes one, and adjacent
  elements are compared and swapped as necessary.

27
Shell Sort

• gap Int(numParts / 2)
• Do While gap gt 1
• Do
• doneFlag True
• For index 1 To numParts - gap
• If part(index) gt part(index gap)
  Then
• Call Swap(part(index),
  part(index gap)
• doneFlag False
• End If
• Next index
• Loop Until doneFlag True
• gap Int(gap / 2) 'Halve the length
  of the gap
• Loop

28
Efficiency of Bubble and Shell sort (average
number of comparisons)

• Array Elements Bubble Sort Shell
  Sort
• 5 10 15
• 15 105 115
• 25 300 302
• 30 435 364
• 100 4,950 2,638
• 500 124,750 22,517

29
Ordered Array

• An array is ordered if its values are in either
  ascending or descending order.
• For string arrays, ANSI values are used to
  evaluate the less than or equal to condition.

30
Searching Arrays

• The process of finding the position of a value in
  an array is called searching
• A sequential search examines each element,
  beginning with the first, until the specified
  value is found or the end of the array is reached

31
Example of a Sequential Search

• Dim nom(1 To 15) As String, index As Integer,
  foundFlag As Boolean
• Do While (index lt 15) And (Not foundFlag)
• If nom(index) Fred Then
• foundFlag True
• Else
• index index 1
• End If
• Loop
• picOutput.Print Fred is located at position
  index

32
Sequential Search

• Useful for small arrays.
• Very inefficient for large arrays (for example,
  names in a telephone book).
• For any size array, if the array is ordered, the
  more efficient binary search can be used.

33
Binary Search

• In a binary search, an ordered array is
  repeatedly divided in half. The half not
  containing the target value is ignored.

34
Binary Search

• Private Sub BinarySearch(corp As String, result
  As String)
• Dim foundFlag As Boolean, first as Integer,
  last as Integer
• first 1
• last numFirms
• Do While (first lt last) And (foundFlag
  False)
• middle Int((first last) / 2) Select
  Case UCase(firm(middle)) Case corp
  foundFlag True Case Is gt corp
  last middle - 1 Case Is lt corp
  first middle 1 End Select
• Loop continued on next slide

35
Binary Search (continued)

• If foundFlag True Then
• result "found
• Else
• result "not found
• End If
• End Sub

36
Two-Dimensional Arrays

• Store values as a table, grouped into rows and
  columns.
• The first subscript of a two-dimensional array
  refers to the row and the second subscript to the
  column.

37
Declaration of Two-Dimensional Array

• Syntax
• Dim arrayName(m1 To m2, n1 To n2) As varType
• Example
• Dim rm(1 To 4, 1 To 4) As Single

column
row
38
Example of a two-dimensional array rm(1 To 4, 1
To 4)
Chicago
Los Angeles
New York
Philadelphia
Chicago
Los Angeles
New York
Philadelphia
rm (2,2)
rm (3,4)
39
Example of Input File

• 0, 2054, 802, 738
• 2054, 0, 2786, 2706
• 802, 2786, 0, 100
• 738, 2706, 100, 0

40
Manipulating a Two-Dimensional Array

• Use nested For Next loops to assign or access
  elements of a two-dimensional array.
• Example
• For row 1 To 4
• For col 1 To 4
• Input 1, rm(row, col)
• Next col
• Next row

41
Examples How many elements?

• Dim rm(1 To 4) As Single
• Dim rm(4, 1 To 4) As Single
• Dim rm(1 To 4, 1 To 4) As Single
• Dim rm(4, 1 To 4) As Single
• Dim rm(0 To 3, 97 To 100)
• Dim rm (-5 To -3, 5 To 5)
• Dim (-3 To -5)