CSCI 3131.01

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CSCI 3131.01 Chapter 8 Arrays Instructor
Bindra Shrestha University of Houston Clear
Lake
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Acknowledgement Dr. Xinhua Chen And
Starting Out with Visual Basic 2010 by Tony
Gaddis and Kip Irvine
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• Chapter 8 Arrays, Timers, and More
• Topics
• Declare and initialize a one-dimensional array
• Search a one-dimensional array
• Compute the sum and average of a one-dimensional
  array
• Sort a one-dimensional array
• Copy between arrays
• Using For Each Next with array
• Create and manipulate parallel one-dimensional
  arrays
• Create and initialize a two-dimensional array
• Enabled property
• Timer
• Random number generation

4
Introduction

• Arrays are like groups of variables that allow
  you to store sets of similar data
• A single dimension array is useful for storing
  and working with a single set of data, like a
  column or a row of data.
• A multidimensional array can be used to store and
  work with multiple sets of data. A
  two-dimensional array deals with data in a table.
• Array programming techniques covered
• Summing and averaging all the elements in an
  array
• Summing all the columns in a two-dimensional
  array
• Searching an array for a specific value
• Using parallel arrays

5
Array Characteristics

• An array stores multiple values of same type
• Like a group of variables with a single name
• For example, the days of the week might be
• a set of 7 string variables
• with a maximum length of 9 characters
• All variables within an array are called elements
  and must be of the same data type
• You access the elements in an array through a
  subscript

6
Subscript Characteristics

• A subscript, also called an index, is a number
  that identifies a specific element within an
  array
• Subscript numbering works like a list box index
• Subscript numbering begins at 0
• 1st element in an array is always subscript 0
• Last element is total number of elements 1
• An array with 7 elements refers to the 1st
  element as subscript 0 and the last element as
  subscript 6

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Declaring a One-Dimensional Array
Dim ArrayName(UpperSubscript) As DataType

• ArrayName is the variable name of the array
• UpperSubscript is the value of the array's
  highest subscript
• Must be a positive whole number
• DataType may be any Visual Basic data type
• VB knows an array is declared when name is
  followed by parentheses with number of elements

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Array Declaration Example
Dim intHours(5) As Integer
intHours(0) intHours(1) intHours(2) intHours(3) i
ntHours(4) intHours(5)

• Note that 6 elements are available when an array
  with an upper subscript of 5 is declared
• Each element is an integer type

9
Default Initialization

• All elements of an Integer array are initialized
  to zero
• Same initialization as an integer variable
• Each array element is initialized exactly the
  same as a simple variable of that data type
• A Single is initialized to zero (0.0)
• A String is initialized to Nothing

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Implicit Array Sizing Initialization

• An array can be initialized when declared
• Example
• This array is implicitly sized
• Upper subscript value is left blank
• Number of elements implied from initialization
• Upper subscript of 5 implied by this example
• This results in a 6 element array
• Elements are assigned the values shown

Dim intNumbers() As Integer 2, 4, 5, 7, 9, 12
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Named Constant as Upper Subscript

• A named constant may be used as an array's
  highest subscript instead of a number
• This is a common use for named constants
• Highest subscript is often used multiple times
• If highest subscript changes, use of a named
  constant allows it to be changed in one place

Const UPPER_SUB As Integer 100 Dim
array(UPPER_SUB) As Integer
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Working With Array Elements

• Simply include the subscript to use an array
  element like an ordinary variable
• intNumbers(2) refers to 3rd element of array

intNumbers(0) 100 intNumbers(1)
200 intNumbers(2) 300 intNumbers(3)
400 intNumbers(4) 500 intNumbers(5) 600
decPay intHours(3) decRate intNumbers(0)
1 MessageBox.Show(decPay(5).ToString())
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Arrays and Loops

• Loops are frequently used to process arrays
• Use a loop to prompt for 10 values
• Use a loop to reset values in the strNames array
  to an empty string

Dim intNbrs(9) as Integers Dim intCount As
Integer For intCount 0 To 9 intNbrs(intCount)
CInt(InputBox("Enter number")) Next intCount
Dim strNames(999) as String Dim intCount As
Integer For intCount 0 To 999 strNames(intCount
) "" Next intCount
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Array Bounds Checking

• Visual Basic checks each subscript value of each
  array reference used at run time
• A subscript is invalid if it is
• Negative
• Greater than the UpperSubscript declared
• An invalid subscript causes VB to throw a
  run-time exception
• Bounds checking is not done at design time

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For Each Loop Array Processing

• A ForEach loop is similar to a ForNext
• Iterates for each element in the array
• strName is assigned the value of the next array
  element at each iteration
• Tutorial 8-1 demonstrates array processing

Dim strEmployees() As String "Jim", "Sally",
_ "Henry", "Jean", "Renee" Dim strName As
String For Each strName In strEmployees MessageBo
x.Show(strName) Next strName
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Use For Each To Sum an Array
Private intArray() as Integer 10, 20, 90,
-20) Dim intSum as Integer 0 For Each intVal as
Integer in intArray intSum intSum
intVal Next

• Start of iteration 1 intVal 10 intSum 0
• End of iteration 1 intVal 10 intSum 10
• Start of iteration 2 intVal 20 intSum 10
• End of iteration 2 intVal 20 intSum 30
• Start of iteration 3 intVal 90 intSum 30
• End of iteration 3 intVal 90 intSum 120
• End of iteration 4 intVal -20 intSum 120
• Start of iteration 4 intVal -20 intSum 100

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Use For Each To Find Largest Value
Dim intArray() as Integer 10, 20, 90, -20) Dim
intLargest as Integer intArray(0) For Each
intVal as Integer in intArray If intVal gt
intLargest Then intLargest intVal Next

• Start of iteration 1 intVal 10 intLargest 10
• End of iteration 1 intVal 10 intLargest 10
• Start of iteration 2 intVal 20 intLargest 10
• End of iteration 2 intVal 20 intLargest 20
• Start of iteration 3 intVal 90 intLargest 30
• End of iteration 3 intVal 90 intLargest 90
• End of iteration 4 intVal -20 intLargest 90
• Start of iteration 4 intVal -20 intLargest 90

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Determining Number of Elements

• Arrays have a Length property that holds the
  number of elements in the array
• Note that length is number of array elements, not
  the upper subscript of the array
• Length property always 1 greater than the upper
  subscript

Dim intValues(25) As Integer For intCount 0 to
(intValues.Length 1) MessageBox.Show(intValues(i
ntCount).ToString) Next intCount
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Total the Values in an Array

• Variable to hold sum (intTotal)is initialized to
  zero prior to loop
• Iterate over all elements, adding each element to
  intTotal

Dim intUnits(24) as Integer 'Declare array Dim
intTotal As Integer 0 'Initialize
accumulator Dim intCount as Integer 'Declare loop
counter 'Find total of all values held in
array For intCount 0 To (intUnits.Length
1) intTotal intUnits(intCount) Next intCount
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Average the Values in an Array

• Similar to previous example but then divides
  total by number of elements to get average

Dim intUnits(24) as Integer 'Declare array Dim
intTotal As Integer 0 'Initialize
accumulator Dim dblAverage as Double 'Declare
average var Dim intCount as Integer 'Declare loop
counter 'Find total of all values held in
array For intCount 0 To (intUnits.Length
1) intTotal intUnits(intCount) Next
intCount 'Use floating-point division to compute
average dblAverage intTotal / intUnits.Length
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Find Highest Lowest Array Values

• Pick first element as the highest
• Search rest of array for higher values
• If a higher value is located, save the value
• Use similar logic to find lowest value

Dim intNumbers(24) as Integer Dim intCount as
Integer Dim intHighest as Integer
intNumbers(0) For intCount 1 To
(intNumbers.Length - 1) If intNumbers(intCount)
gt intHighest Then intHighest
intNumbers(intCount) End If Next intCount
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Array Copy Using Loop

• Copy one element at a time
• Note that array copy cannot be done by a simple
  assignment intNewValues intOldValues
• Causes intNewValues array to reference the same
  memory location as intOldValues
• Thus any change to intOldValues will affect
  intNewValues and vice versa
• This is because arrays are object variables

For intCount 0 To intOldValues.Length -1
intNewValues(intCount) intOldValues(intCount) Ne
xt intCount
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Array Copy Using Copy Method of Array
Syntax Array.Copy(SourceArray, DestArray,
length) Dim a() As Integer 1, 2, 3, 4, 5, 6,
7, 8 Dim b() As Integer 2, 3, 4,
5 Array.Copy(b, a, 4) '4 is the number of
elements in b ' This makes array a contain '
2, 3, 4, 5, 5, 6, 7, 8
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Parallel Arrays

• Sometimes its useful to store related data in
  two or more arrays called parallel arrays
• Causes the nth element of one array to be related
  to the nth element of another
• Allows related data to be accessed using the same
  subscript on both arrays

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Parallel Arrays Example

• Assume the following array declarations
• Element 1 refers to the same person with name in
  one array and address in the other

Dim strNames(4) As String Dim strAddresses(4) As
String
strNames(0) strNames(1) strNames(2)
strNames(3) strNames(4)
Person 1 Person 2 Person 3 Person 4 Person 5
strAddresses(0) strAddresses(1) strAddresses(2)
strAddresses(3) strAddresses(4)
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Parallel Arrays Processing

• Use parallel array processing to add name and
  address of each person to a list box
• Tutorial 8-2 has an example of parallel arrays

Dim strNames(4) As String Dim strAddresses(4) As
String For intCount 0 To 4 lstPeople.Items.Add
("Name " strNames(intCount) _ " Address "
strAddresses(intCount)) Next intCount
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Array List Box Parallel Processing

• A list box selection used as an array index

' Initialize a List Box with names lstPeople.Items
.Add("Jean James") ' Subscript 0 lstPeople.Items.A
dd("Kevin Smith") ' Subscript 1 lstPeople.Items.Ad
d("Joe Harrison") ' Subscript 2 ' Initialize an
Array with corresponding phone numbers phoneNumber
s(0) "555-2987" phoneNumbers(1)
"555-5656" phoneNumbers(2) "555-8897" '
Process a selection If lstPeople.SelectedIndex gt
-1 And _ lstPeople.SelectedIndex lt
phoneNumbers.Length Then MessageBox.Show(phoneNum
bers(lstPeople.SelectedIndex)) Else MessageBox.Sh
ow("That is not a valid selection.") End If
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Searching Arrays, The Search

• Find 1st instance of value 100 in array intScores
• intPosition gives position of this value

Dim blnFound as Boolean False Dim intCount as
Integer 0 Dim intPosition as Integer -1 '
Search for a 100 in the array Do While Not
blnFound And intCount lt scores.Length If
intScores(intCount) 100 Then blnFound
True intPosition intCount End If
intCount 1 Loop
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Searching Arrays, The Result

• Indicates whether the value was found
• If found, position is given as a test number

' Was 100 found in the array? If blnFound
Then MessageBox.Show( _ "Congratulations! You
made a 100 on test " _ (intPosition
1).ToString, "Test Results") Else MessageBox.Show
( _ "You didnt score a 100, but keep trying!",
_ "Test Results") End If
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Sorting an Array, Ascending

• Arrays have a Sort method
• Arranges elements in ascending order (lowest to
  highest)
• Sort intNumbers in this order 1, 3, 6, 7, 12
• Sort strNames in this order Alan, Bill, Kim,
  Sue

Dim intNumbers() As Integer 7, 12, 1, 6, 3
Array.Sort(intNumbers) Dim strNames() As
String "Sue", "Kim", _ "Alan", "Bill"
Array.Sort(strNames)
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Sorting an Array, Descending

• Use the Sort method in conjunction with the
  Reverse method
• Arranges elements in descending order (highest to
  lowest)
• Sort intNumbers in this order 12, 7, 6, 3, 1
• Dim intNumbers() As Integer 7, 12, 1, 6,
  3
• Array.Sort(intNumbers)
• Array.Reverse()
• Sort strNames in this order Sue, Kim, Bill,
  Alan
• Dim strNames() As String "Sue", "Kim", _
• "Alan", "Bill"
• Array.Sort(strNames)
• Array.Reverse()

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Dynamically Sizing an Array
ReDim Preserve Arrayname(UpperSubscript)

• ReDim is a new keyword
• Used to change the number of elements at run time
• If Preserve is specified, the existing contents
  of the array are preserved
• Arrayname names the existing array
• UpperSubscript specifies the new highest
  subscript value
• Can declare an array with no subscript and state
  number of elements later with ReDim

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Resizing Example

• Array sngScores declared with no elements
• User prompted for number of elements
• ReDim resizes array based on user input

Dim sngScores() As Single ' Declared with no
elements Dim intNumScores as Integer ' Obtain
number of elements from the user intNumScores
CInt(InputBox("Enter number of test scores")) If
intNumScores gt 0 Then ReDim sngScores(intNumScore
s - 1) Else MessageBox.Show("You must enter 1 or
greater.") End If
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Passing Arrays as Arguments to Procedures

• Array intNumbers passed to DisplaySum sub
• Sub computes shows sum of array elements
• Can pass any integer array to DisplaySum

Dim intNumbers() As Integer 2, 4, 7, 9, 8, 12
DisplaySum(intNumbers) Sub DisplaySum(ByVal
intArray() As Integer) Dim intTotal As Integer
0 ' Accumulator Dim intCount As Integer ' Loop
counter For intCount 0 To (intArray.Length -
1) intTotal intArray(intCount) Next Message
Box.Show("Total is " intTotal.ToString) End Sub
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Passing Arrays ByVal and ByRef

• When passing an array ByVal
• Calling procedure sees sub procedure changes to
  element values
• Simple variables dont work this way
• If sub assigns array argument to another array,
  no effect on array in calling procedure
• When passing an array ByRef
• Calling procedure sees sub procedure changes to
  element values
• If sub assigns array argument to another array,
  calling procedure array values affected

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Passing Arrays Example

• After ResetValues procedure executes, intNumbers
  array still contains 1, 2, 3, 4, 5
• If array passed ByRef, intNumbers array will
  contain 0, 0, 0, 0, 0 after procedure runs

Dim intNumbers() As Integer 1, 2, 3, 4, 5
ResetValues(intNumbers) Sub ResetValues(ByVal
intArray() As Integer) Dim newArray() As
Integer 0, 0, 0, 0, 0 intArray
newArray End Sub
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An Array Returned From a Function

• Return type String() indicates array of strings
  returned
• Thus the function result must be assigned to an
  array of strings

Function GetNames() As String() ' Get four names
from user ' Return them as an array of
strings. Dim strNames(3) As String Dim strInput
As String Dim intCount As Integer For intCount
0 To 3 strInput InputBox("Enter name "
_ (intCount 1).ToString)
strNames(intCount) strInput Next Return
strNames End Function
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A Two Dimensional Array Picture

• Thus far, arrays have been one-dimensional
• Arrays can also be two-dimensional
• A two-dimensional array looks like a spreadsheet
  with rows and columns

Column 0 Column 1 Column 2 Column 3 Row 0 Row
1 Row 2
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Two Dimensional Array Syntax
Dim ArrayName(UpperRow, UpperColumn) As DataType

• UpperRow and UpperColumn give the highest
  subscript for the row and column indices of the
  array
• The array on the previous slide could be
• Defines three rows 0, 1, and 2
• And four columns 0, 1, 2, and 3

Dim array(2,3) As Single
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Two Dimensional Array Subscripts
Dim array(2,3) As Single
Column 0 Column 1 Column 2 Column 3 Row
0 sngScores(0,0) sngScores(0,1) sngScores(0,2) sng
Scores(0,3) Row 1 sngScores(1,0) sngScores(1,1) s
ngScores(1,2) sngScores(1,3) Row
2 sngScores(2,0) sngScores(2,1) sngScores(2,2) sng
Scores(2,3)
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Nested Loops And Two Dimensions

• Must use two subscripts to indicate a single
  element in a two dimensional array
• Nested loops are often used to specify subscripts
  for two-dimensional array processing
• For example, a nested loop below is used to
  insert a value into every element of array scores

For intRow 0 To 2 For intCol 0 To 3 intNum
Val(InputBox("Enter a score.")) sngScores(intR
ow, intCol) intNum Next intCol Next intRow
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Implicit Sizing and Initialization

• Can be used with multi-dimensional arrays
• Row 0 values
• Row 1 values
• Row 2 values
• Initializes array intNumberswith the following
  values

Dim intNumbers(,) As Integer _ 1, 2, 3,
4, 5, 6, 7, 8, 9
Col 0 Col 1 Col 2
Row 0 1 2 3
Row 1 4 5 6
Row 2 7 8 9
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Sum Columns, 2-Dimensional Array

• Code below uses nested loops to compute sum of
  elements in each column of a 2 dimensional array
• Outer loop controls column subscript

Dim intValues(,) As Integer 1, 2, 3, 4,
_ 5, 6, 7, 8, _ 9, 10, 11, 12 For
intCol 0 to 3 intTotal 0 'Initialize
accumulator For intRow 0 to 2 'Sum rows in
this column intTotal intValues(intRow,
intCol) next intRow 'Display the sum of the
column MessageBox.Show("Column "
intCol.ToString() _ " sum is "
intTotal.ToString) Next intCol
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Three-Dimensional Arrays Beyond

• VB allows arrays of up to 32 dimensions
• Can think of a 3 dimensional arrayas having
  multiple pages of two dimensional arrays
• Arrays beyond three dimensions are difficult to
  visualize
• But, all one needs to do is to be consistent in
  the use of the different indices

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Enabled Property

• Most controls have an Enabled property
• If this Boolean property is set to false, the
  control is disabled, meaning the control
• Cannot receive the focus
• Cannot respond to user generated events
• Will appear dimmed, or grayed out
• Default value for this property is true
• May be set in code when needed as shown

btnExample.Enabled False
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Timer Control

• A Timer control generates Tick events
• Double-click on Timer tool in Toolbox to add
• Appears in component tray
• Standard prefix is tmr
• Can perform a process at regular intervals by
  coding the process in a Tick event procedure
• Two important properties
• Enabled must be true to generate Tick events
• Interval (milliseconds) decides tick frequency
• Interval of 1000 causes 1 timer tick per second
• Tutorial 8-5 demonstrates the Timer control

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Splash Screens

• A form, often with an application logo, that is
  displayed while an application is loading
• A splash screen usually
• Has Topmost property set to True so it is
  displayed over the forms of other applications
• Disappears shortly by using a Timer
• Is modeless so the application continues to load
  while the splash screen is displayed
• Tutorial 8-6 demonstrates a splash screen

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Anchor Property

• Anchor - a property of various form controls
• A control anchored to a form edge keeps the same
  distance to the form edge
• Controls may be anchored to any, all, or none of
  the right, left, top, or bottom edges
• Default is to anchor to top and left edges
• Can just as easily anchor to bottom right
• If anchoring to opposing sides, control is
  resized when form is resized
• Anchoring to all 4 sides causes control to be
  resized proportionally to form

49
Dock Property

• Dock - a property of various form controls
• A control docked to a form edge is placed
  directly against the form edge
• Controls may be docked to
• Any one form edge but not more than one
• No edge or not docked (the default)
• Length or width of docked control changes to
  match length or width of the form edge

50
Initial Random Number Generation

• Often used for games and simulations
• Must create an instance of the Random class to
  use random number methods and properties
• May supply a seed number as below so the same set
  of random numbers are generated every time
• If no seed number is supplied when instantiated
• VB uses its default random number seed
• Causes each sequence to be unique

Private rand as New Random
Private rand as New Random(25)
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Generating More Random Numbers

• The Next method returns the next integer in a
  random number sequence
• Returns integer between 0 and 2,147,483,647
• Repeatedly use a statement such as the following
  to generate additional random numbers

Dim intNum as Integer rand.Next()
intNum rand.Next()
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Random Numbers in a Range

• Can set an upper limit for the number generated
  by specifying the limit as an argument
• Following example generates a random number
  between 0 and 99
• Lower limit of a random number is always 0
• Can add or subtract to change this lower limit
• Following example generates a random number
  between -50 and 49

intNum rand.Next(100)
intNum rand.Next(100) - 50
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Random Numbers of Type Double

• Use NextDouble method to generate random numbers
  of type double
• Returns floating point number from 0.0 to 1.0
• For a random double between 0.0 and 500.0
• For a random double between 100.0 and 600.0
• Tutorial 8-7 shows an interesting use of random
  numbers

Dim dblNum as Double rand.NextDouble()
dblNum rand.NextDouble() 500
dblNum (rand.NextDouble() 500) 100