Chapter 7 – Arrays
7.1 Creating and Accessing Arrays
7.4 Two-Dimensional Arrays
1
Simple and Array Variables
• A scalar variable (or simple variable) is
a name to which Visual Basic can
assign a single value.
• These are the variables we have used to
date
• An array variable is a collection of
simple variables of the same type to
which Visual Basic can efficiently assign
a list of values.
2
Example
Suppose you want to evaluate the exam grades for 30
students and to display the names of the students whose
scores are above average. We need a total of 60 simple
variables!
Private Sub btnDisplay_Click(...) _
Handles btnDisplay.Click
Dim student0 As String, score0 As Double
Dim student1 As String, score1 As Double
.
.
Dim student29 As String,score29 As Double
How practical is this?
3
Using Arrays
Upper bound of subscripts
in the array
Dim students(29) As String
Dim scores(29) As Double
Array name
Data type
Only two array variables needed
for 30 data items
4
Putting Values into an Array
students(0) = "Tom Brown"
Subscript
(index)
Read: "students sub zero equals Tom Brown"
Which means that the string "Tom Brown" is being
stored at the first location in the array called
students because all arrays begin counting at 0.
5
Array Terminology
• Dim arrayName(n) As DataType
• 0 is the lower bound of the array
• n is the upper bound of the array–the
last available subscript in this array
• The number of elements, n + 1, is the
size of the array.
6
Example 7.1.1: Form
mtbNumber
txtWinner
7
Example 7.1.1: Code
Subscript
(also called Index)
8
Example 7.1.1: Code and Output
Subscript
starts at 0
Count
starts at 1
Assigning the selected element of the array into the TextBox’s Text property. The
array is local, and is assigned values every time the button is clicked.
9
Load Event Procedure
Occurs as the Form loads in memory
(when the program first executes)
Private Sub frmName_Load(...) _
Handles MyBase.Load
The keyword MyBase refers to the form
being loaded. This event procedure is a
good place to assign values to an array.
Why should array variables be
assigned when form loaded?
10
Form Load
Example 7.1.2: Code
Class variable, not local variable.
Array loads when form is run
By making the array a class variable and
loading it in the Form Load, you only
need to create the array once.
11
Initializing Arrays
Arrays may be initialized when created:
Dim arrayName() As DataType =
{value0, value1, value2, ..., valueN}
declares an array having upper bound N and assigns value0 to
arrayName(0), value1 to arrayName(1), ..., and valueN to arrayName(N).
Rather than explicitly specifying the array’s size, it is determined based
on the number of initial values that are assigned.
Example: Dim teamNames() As String =
{"Packers", "Packers", "Jets", "Chiefs"}
12
Array Methods
arrayName.Count number of elements
arrayName.Max
highest value
arrayName.Min
lowest value
arrayName.First
first element
arrayName.First is same as arrayName(0)
arrayName.Last
last element
13
Methods for Numeric Arrays
numArrayName.Average average value of
elements
numArrayName.Sum
sum of values of
elements
14
Using Loops Instead of
Methods
• In Example 7.1.4 the greatest value in a
numeric array ages is determined.
• The value of the variable max is set to the
first element of the array.
• Then a For…Next loop successively
examines each element of the array and
resets the value of max when appropriate.
15
Loop reading array
Example 7.1.4: Code
Dim ages() As Integer = {55, 56, 61, 52, 69,
64, 46, 54, 47}
'last 9 presidents
Dim max As Integer = ages(0)
For i As Integer = 1 To ages.Count - 1
If ages(i) > max Then
Start at position 0
max = ages(i)
The loop counter is the subscript
End If
Next
txtOutput.Text = "Greatest age: " & max
Output: Greatest
age: 69
16
For Each Loops
For i As Integer = 1 To ages.Count - 1
If ages(i) > max Then
max = ages(i)
End If
Next
can be replaced with
For Each age As Integer In ages
If age > max Then
max = age
End If
Next
17
For Each Loops (continued)
• In the For…Next loop, the counter variable
i can have any name.
• In the For Each loop, the looping variable
age can have any name.
• The primary difference between the two
types of loops is that in a For Each loop
no changes can be made in the values
of elements of the array.
18
Passing an Array Element
A single element of an array can be
passed to a procedure just like any
ordinary numeric or string variable.
Private Sub btnDisplay_Click(...) Handles _
btnDisplay.Click
Dim num(20) As Integer
num(5) = 10
lstOutput.Items.Add(Triple(num(5)))
End Sub
Function Triple(ByVal x As Integer) As Integer
Return 3 * x
End Function
19
Passing Arrays to Procedures
• An array declared in a procedure is local to
that procedure.
• An entire array can be passed to a Sub or
Function procedure.
• The calling statement uses the name of the
array without parentheses.
• The header of the Sub or Function
procedure uses the name with an empty
set of parentheses.
20
Example: 7.1.6
Variation of Example 7.1.4
Function procedure used to find the largest
number in an array.
The loop counter is the subscript
What is the scope of the array?
21
Input assigned to array
Example 7.1.7
3-Use of Average Method
1-Set size of array based on user input
2-Loop saving each grade to the array
22
User-Defined Array-Valued
Functions
Headers have the form
Function FunctionName(ByVal var1 As Type1,
ByVal var2 As Type2, ...) As DataType()
23
Searching for an Element in
an Array
A statement of the form
numVar = Array.IndexOf(arrayName, value)
assigns to numVar the index of the first
occurrence of value in arrayName. Or assigns
-1 if the value is not found.
24
Searching an Array:
Example 7.1.8
-1 if item not found
No error checking – name must be in correct case
25
Copying an Array
If arrayOne and arrayTwo have been
declared with the same data type, then the
statement
arrayOne = arrayTwo
makes arrayOne an exact duplicate of
arrayTwo. Actually, they share the same
location in memory.
26
Split Method
• Facilitates working with text files.
• Split can convert a string containing commaseparated data into a string array.
• The 0th element of the array contains the text
preceding the first comma, the 1st element
contains the text between the first and
second commas, ..., and the last element
contains the text following the last comma.
27
Split Example
For instance, suppose the string array
employees has been declared without an
upper bound, and the string variable line has
the value “Bob,23.50,45”.
employees = line.Split(","c)
•
•
•
•
sets the size of employees to 3
sets employees(0) = “Bob”
sets employees(1) = “23.50”
sets employees(2) = “45”
28
Split Comments
employees = line.Split(","c)
• In this example, the character comma is
called the delimiter for the Split method,
and the letter c specifies that the comma
has data type Character instead of String
• Any character can be used as a delimiter.
If no character is specified, the space
character will be used as the delimiter.
29
Example
Private Sub btnConvert_Click(...) _
Handles btnConvert.Click
Dim stateData(), line As String
line = "California,1850,Sacramento,Eureka"
stateData = line.Split(","c)
For Each entry As String In stateData
lstOutput.Items.Add(entry)
Next
End Sub
30
Example Output
California
1850
Sacramento
Eureka
31
Example 7.1.9
Letter ‘c’ designates character
instead of string
32
Join Function
The reverse of Split method is Join function. Join
concatenates the elements of a string array into a
string containing the elements separated by a
specified delimiter.
Sets up Array
Dim greatLakes() As String = {"Huron",
"Ontario", "Michigan", "Erie", "Superior"}
Dim lakes As String
lakes = Join(greatLakes, ",") Join & assign to lakes
txtOutput.Text = lakes
Output: Huron,Ontario,Michigan,Erie,Superior
33
Out of Range Error
The following code references an array element
that doesn't exist. This will cause an error.
How many items are in the Tree array? What are the subscripts?
34
Text Files
• Hold data to be processed by programs.
• Can be created, viewed, and managed by
word processors, Notepad, or by the Visual
Basic IDE.
• Have the extension .txt
• Normally placed in the bin\Debug folder in
the Solution Explorer
• Can be used to “load” array with loop
35
A Text File Displayed in the
Visual Basic IDE (7.1.3)
The file contains the ages of the first 44
U.S. presidents when they assumed office.
36
Using a Text File to Populate
a String Array
• Assume that the previous text file is in the
program’s bin\Debug folder.
• The text file can be used to fill a string array
with the statement
Dim strAges() As String =
IO.File.ReadAllLines("AgesAtInaugural.txt")
• The array strAges will have size 44 and
upper bound 43.
37
Populating a Numeric Array
with a Text File
Dim strAges() As String =
IO.File.ReadAllLines("AgesAtInaugural.txt")
Dim ages(43) As Integer
For i As Integer = 0 To 43
ages(i) = CInt(strAges(i))
Next
Class
The File class’s ReadAllLines
method opens the designated file,
reads all lines, and returns a string
array containing these values.
(definition of object with
properties and methods)
Argument (data
passed to a method)
IO.File.ReadAllLines("AgesAtInaugural.txt")
Namespace
(a collection of related classes)
Method
(function or sub of a class)
38
Example 7.1.3
39
Example 7.1.3
Read data from file
40
Example 7.1.3
Convert from string to integer, and place
into integer array
41
Example 7.1.3
Various
methods of
the Array
class
42
Example 7.1.5
• This example illustrates several topics
• Reading from a text file
• Manipulating Form’s properties in the code
• Looping through an array, using a flag to
determine if something has been found
• Resizing an array using the ReDim
Preserve statement
43
44
Example 7.1.5 – Form Load
Read from text file into array
45
Example 7.1.5 – Form Load
Determine the number of elements
in the array
46
Example 7.1.5 – Form Load
Dynamically set the form’s title text
47
Example 7.1.5 – Form Load
Dynamically set the button’s text
48
Example 7.1.5 – btnDisplay
49
Example 7.1.5 – btnDisplay
Looping through the array to display the game numbers that the specified
team has won.
Note the use of the loop counter
variable to index into the array
50
Flag Variables
• Have type Boolean
• Used when looping through an array
• Provide information to be used after loop
terminates. Or, allows for the early
termination of the loop.
51
Example 7.1.5 – btnDisplay
Use of the flag variable noWins to decide whether to display a
no win message
Start by assuming no wins
If a win was found, set the flag to false
Only if we never found a match will we
display the No Games Won message
52
Example 7.1.5 – btnAddWinner
Code for increasing the size of the
array and adding a new winner to the
list of Super Bowl winners
53
ReDim Statement
The size of an array may be changed after
it has been created.
ReDim arrayName(m) or
ReDim Preserve arrayName(m)
where arrayName is the name of an already
declared array and m is an Integer literal,
variable, or expression, changes the upper
bound of the array to m.
Size may change but data type cannot
54
Preserve Keyword
ReDim arrayName(m)
resets all values to their default. This can
be prevented with the keyword Preserve.
ReDim Preserve arrayName(m)
resizes the array and retains as many
values as possible.
55
Example 7.1.5 – btnAddWinner
Use ReDim Preserve to set a
new size to the array and keep
the existing data.
Before ReDim Preserve
After ReDim Preserve
56
Example 7.1.5 – btnAddWinner
Modify the Form’s title and the
text of the button.
Before
After
57
Simple File Write Example
• This example illustrates the following:
• Splitting a string into an array
• Writing to a file
• Catching a runtime error (an exception) and
posting an error message when the
exception occurs
58
Simple File Write Example
59
Simple File Write Example
If it is a valid
file path, the
file will be
written OK.
60
Simple File Write Example
If it is a not a
valid file path,
or the string is
empty, the file
will be not be
written and an
error message
will appear.
61
Simple File Write Example
Splitting the string into an array (delimiter is the
space character)
62
Simple File Write Example
Writing the file from the array. The writeAllLines
method takes two arguments:
1) The file path (including file name)
2) The array to write
63
Exception Handling
• You can gracefully handle runtime errors
(exceptions) using the Try-Catch block
• In the Try portion, you have all the code that
you want to try to execute, some of which
may generate an exception
• In the Catch portion, you have the code that
should run in the event of an error
• If an exception occurs during the Try code,
processing will automatically branch to the
Catch code
64
Simple File Write Example
Exception-handling. If there is any problem
writing the file, the error message will appear.
Without the Try-Catch block, an exception would
cause the program to abort…very ugly!
65
7.4 Two-Dimensional Arrays
• Declaring a Two-Dimensional Array Variable
• Implicit Array Sizing and Initialization
• Filling a Two-Dimensional Array with a Text
File
• Various operations on a 2D Array
• Other notes on 2D Arrays
66
Declaring a Two-Dimensional
Array Variable
• One-dimensional arrays store a list of items
of the same type
• Two-dimensional arrays store a table of
items of the same type.
• (i.e. 2D array is an array of 1D arrays)
• Consider the rows of the table as numbered
0, 1, 2, ,,, m and the columns numbered 0, 1,
2, …, n. Then the array is declared with
Dim arrayName(m, n) As DataType
Item in ith row, jth column: arrayName(i,j)
67
Implicit Array Sizing and
Initialization
Arrays can be initialized when they are declared.
Dim arrayName(,) As DataType =
{{ROW0}, {ROW1}, {ROW2}, ..., {ROWn}}
declares a two-dimensional array where ROW0
consists of the entries in the top row of the
corresponding table delimited by commas, and
so on.
R0Wn is a 1D array!
68
Example 7.4.PracProb1
69
Road-Mileage Table
Chicago
LA
NY
Philly
Chicago
0
2054
802
738
LA
2054
0
2786
2706
NY
802
2786
0
100
Philly
738
2706
100
0
70
Road-Mileage Array
Dim rm(,) As Double = {{0, 2054, 802, 738},
{2054, 0, 2786, 2706},
{802, 2786, 0, 100},
{738, 2706, 100, 0}}
declares and initializes an array of roadmileages. Some elements of the array are
rm(0,0)=0, rm(0,1)=2054, rm(1,2)=2786
Note: Always rm(m,n) = rm(n,m). It makes no
difference what city you start with.
ChicagoLA is the same as LA Chicago
71
Filling a Two-Dimensional
Array with a Text File
Dim rm(3, 3) As Double
'road mileage
Dim rowOfNums() As String =
IO.File.ReadAllLines("Distances.txt")
Dim line, data() As String
For i As Integer = 0 To 3
line = rowOfNums(i)
data = line.Split(""c)
For j As Integer = 0 To 3
rm(i, j) = CDbl(data(j))
Next
Next
Text File Distances.txt
0,2054,802,738
2054,0,2786,2706
802,2786,0,100
738,2706,100,0
For each row or the 2D array, create a temporary 1D
array for the corresponding element of the rowOfNums
array array, then fill the row of the 2D array with the
values from the 1D array.
72
Example: TwoDArrayExample
73
Example: TwoDArrayExample
Nested loop produces output
from array. Outer loop goes
row by row. Inner loop goes
column by column.
74
Example: TwoDArrayExample
User inputs to
determine the row
and column to
retrieve.
75
Example: TwoDArrayExample
• NumericUpDown Control
• Displays numbers in a list
that can be scrolled; selected
number can also be typed
• Important properties:
• Minimum – lowest number in
the list
• Maximum – highest number in
the list
• Increment – units of change
(default=1)
76
Example: TwoDArrayExample
Nested loop to search for a
value in the array. Outer
loop counter is the row.
Inner loop counter is the
column.
77
Notes on Two-Dimensional
Arrays
An unsized two-dimensional array can be
declared with a statement of the form
Dim arrayName(,) As varType
78
GetUpperBound Method
After execution of the statement
Dim arrayName(r, s) As varType
the value of arrayName.GetUpperBound(0) is r,
and the value of arrayName.GetUpperBound(1)
is s.
79
ReDim and Two-Dimensional
Arrays
• An already-created array can be resized with
ReDim arrayName(r, s)
which loses the current contents, or with
ReDim Preserve arrayName(r, s)
• When Preserve is used, only the columns
can be resized.
• ReDim cannot change the number of
dimensions in the array.
80