1. Arrays

2. Introduction
This chapter introduces basic concepts of data structures—collections of related data items.
Arrays are simple data structures consisting only of data items of the same type.
Arrays normally are “static” entities, in that they typically remain the same size once they’re created, although they can be resized

3. Arrays
An array is a group of variables (called elements) containing values that all have the same type.
To refer to a particular element, we specify the array’s name and the position number of the element to which we refer.
The first element in every array is the zeroth element.
The highest position number in array is called the array’s upper bound
The position number in parentheses more formally is called an index and must be a nonnegative integer or integer expression.

5. Declaring and Allocating Arrays
The following statement can be used to declare the array in Fig. 7.1:
Dim c(11) As Integer
The parentheses that follow the variable name indicate that c is an array.
Arrays can be declared to contain elements of any type; every element of the array is of that type.
The number in parentheses in the array declaration helps the compiler allocate memory for the array c.

6. Declaring and Allocating Arrays
Initializer Lists
You can follow an array declaration with an equal sign and an initializer list in braces, { and }, to specify the initial values of the array’s elements.
For instance,
Dim numbers() As Integer = {1, 2, 3, 6}
declares and allocates an array containing four Integer values.
Default Initialization
When you do not provide an initializer list, the elements in the array are initialized to the default value for the array’s type—0 for numeric primitive data-type variables, False for Boolean variables and Nothing for String and other class types.

7. Arrays
Array Length
Every array “knows” its own length (that is, number of elements), which is determined by the array’s Length property, as in:
c.Length
All arrays have the methods and properties of class System.Array.
Array method GetUpperBound returns the index of the last element in the array.
The value returned by method GetUpperBound is one less than the value of the array’s Length property.
For one-dimensional arrays, the argument passed to GetUpperBound is always 0.

8. Summing the Elements of an Array
Often, the elements of an array represent a series of related values that are used in a calculation.

9. Exceptions with Arrays
An exception indicates a problem that occurs while an app executes.
The name “exception” suggests that the problem occurs infrequently—if the “rule” is that a statement normally executes correctly, then the problem represents the “exception to the rule.”
Exception handling enables you to create fault-tolerant programs that can resolve (or handle) exceptions.

10. Using Array Elements for Accumulators
SCOUTS 1 EXAMPLE
Array elements are regular variables and perform in the same ways as many other variables.
Subscripted variables can be used in any way chosen, such as for counters or total accumulators.
The statement
Dim TotalInteger(7) As Integer
declares the eight accumulators with subscripts 0 to 7.

11. Using List Boxes With Arrays
Allow the user to select from a list and the SelectedIndex property can be used as the subscript of the total array.
IndexInteger = GroupListBox.SelectedIndex
If IndexInteger <> –1 Then

12. Passing an Array to a Method
To pass an array argument to a method, specify the name of the array without using parentheses.
For example, if array hourlyTemperatures has been declared as
Dim hourlyTemperatures(24) As Integer
the method call
DisplayDayData(hourlyTemperatures)
passes array hourlyTemperatures to method
DisplayDayData.

13. Passing an Array to a Method
Every array object “knows” its own upper bound (that is, the value returned by the method GetUpperBound), so when you pass an array object to a method, you do not need to pass the upper bound of the array as a separate argument.
For a method to receive an array through a method call, the method’s parameter list must specify that an array will be received.
For example, the method header for DisplayDayData might be written as
Sub DisplayDayData(temperatureData() As Integer)
indicating that DisplayDayData expects to receive an Integer array in parameter temperatureData.

14. Passing an Array to a Method
When you pass an array to method DisplayDayData, a copy of the array’s reference is passed and the method can change the original array’s element values.
Individual array elements can be passed by value or by reference like simple variables of that type.
For instance, array element values of primitive types, such as Integer, can be passed either by value or by reference, depending on the method definition.

15. For Each…Next Repetition Statement
The For Each…Next repetition statement iterates through the values in a data structure, such as an array, without using a loop counter.
VB references EACH element of the array and assigns its value to ElementName.
Variable used for ElementName must be same datatype as array elements or an Object datatype.
Best to declare the variable for ElementName as part of the For Each statement to create a block-level variable
Makes one pass through the loop per element
Use Exit For statement within loop to exit early.

16. The For Each and Next Statements
For Each ElementName [As Datatype] In ArrayName ' Statement(s) in loop. Next [ElementName]
For Each OneNameString As String In NameString
' Write one element of the array.
Debug.WriteLine(OneNameString)
Next OneNameString
If the array has 12 elements, the loop will execute 12 times.
*The next slide provides an example of the For Each and Next Statements.

17. Multidimensional Arrays
The arrays we’ve studied so far are one-dimensional arrays—they contain a list of values and use only one index to access each element.
Multidimensional arrays, also know as rectangular arrays, require two or more indices to identify particular elements.
Each row is the same size, and each column is the same size (hence the term “rectangular”).
To identify a particular table element, we specify two indices—by convention, the first identifies the element’s row, the second the element’s column.

19. The Dim Statement for Two-Dimensional Arrays — Example(s)
Dim NameString(2, 3) As String
Dim NameString( , ) As String = {{"James", "Mary", "Sammie", "Sean"}, _ {"Tom", "Lee", "Leon", "Larry"}, {"Maria", "Margaret", "Jill", "John"}}
' Both statements establish an array of 12 elements.
(0, 0)
James
(0, 1)
Mary
(0, 2)
Sammie
(0, 3)
Sean
(1, 0)
Tom
(1, 1)
Lee
(1, 2)
Leon
(1, 3)
Larry
(2, 0)
Maria
(2, 1)
Margaret
(2, 2)
Jill
(2, 3)
John
The elements of the array may be used in the same way as any other variable — in accumulators, counts, and reference fields for look up, or in statements like assignment and printing, and as conditions.
Must always use two subscripts when referring to individual elements of the table. Specify the row with the first subscript and the column with the second subscript.

20. Nested For/Next Example
For RowInteger As Integer= 0 To 2
For ColumnInteger As Integer= 0 To 3
' Initialize each element.
NameString(RowInteger, ColumnInteger) = " "
Next ColumnInteger
Next RowInteger
The assignment statement in the inner loop will be executed 12 times, once for each element of NameString.

21. Resizing an Array with the ReDim Statement
An array’s size cannot be changed, so a new array must be created if you need to change the size of an existing array.
The ReDim statement “resizes” an array at execution time by creating a new array and assigning it to the specified array variable.
The old array’s memory is eventually reclaimed by the runtime.
To save the original data stored in an array, follow the ReDim keyword with the optional Preserve.