For holding data values during processing, variables are the basic storage units .
A D V E R T I S E M E N T
A problem arises, however, with the large collections of data values for which it is impractical to declare a separate variable for each of them.
Consider a need to store the names or codes for all the states.
It would require declaring and assigning values to the 50 different variables.
Fortunately, though, there is a way to handle these large sets of similar data items through use of collections and arrays.
An array is a collection of data values and all of which are accessible through the same variable name.
An array can be thought of as a "table" and data values occupying its "cells."
For instance, you can visualize an array named Letters containing the first five letters of the Greek alphabet as in the table below:
A major advantage of using an array to store a collection of values is that all values can be referenced through the single array name and it is not necessary to individually declare and name separate variables for all the items. Rather, the items in an array are indexed by their position in the array, numbered beginning with 0 (zero) for the first element in the array and thus, the value "gamma" in the above array is identified by the reference Letters(2).
In parentheses,the name of the array is followed by an index value enclosed.
An array is declared with a Dim statement which is similar to declaring variables. The array name is followed by a pair of parentheses to indicate an array variable and for a single dimension array (one column) the parentheses are empty; for a multidimensional array commas are used for each additional column.To declare the type of data,an array is typed in the same way as variables to occupy the array. The following statements declare a single- and a two-dimensional array, respectively, each of which contain string data.
Dim Letters(4) As String
Dim Letters(4,1) As String
to indicate an array with 5 rows, and an array with 2 columns and 5 rows, respectively and also note that, because array indexing begins with 0, the dimensions of an array are always 1 less than the actual number of array elements. You can determine the actual size of an array through the arrayName.Length property, which returns the total number of elements in the array.
There are numerous ways to assign values to the elements of an array and if the values are known, and they are modest in size, you can populate an array during its declaration.
By enclosing them within braces, separated by commas,values are assigned .
The following statement is one way to initially load to the single-dimension Letters array. (A dimension size is not required, being determined from the number of values.)
Dim Letters(4) As String
Letters(0) = "x"
Letters(1) = "y"
Letters(2) = "z"
Letters(3) = "p"
Letters(4) = "q"
Similar to arrays for storing sets of data items are Visual Basic collections and a collection is a one-dimensional storage area in which values of mixed data types can be placed, although there are few occasions for doing so. Collections are increased in size simply by adding items to it.
With a Dim statement,a collection is declared that creates a new Collection object.For storing the state codes,the following statement declares a collection.
For example, to establish state names as the keys to them and to load the States collection with state codes, the following code is used.
Dim States As New Collection