Arrays

An array is a way to store data in a set to save space and to have an easier name for individual items that form a set of variables.  An example of a set is quarters of a game – 1^st quarter, 2^nd quarter, 3^rd quarter and 4^th quarter.  For a program to keep track of the final score in each quarter, four separate variables would be required, and they all would have a similar name.

Quarter can be declared as an array, given one name.  Each individual quarter can be referred to an element within the array.

Syntax:

Dim ArrayName(number) as type

The ArrayName is what you name the array.

Number is the number of elements the array will hold.

Type is the type of data the elements will be.

Example:

Dim Quarter(4) as Integer

Visual Basic uses Zero Based arrays, the following integer variables are created:  Quarters(0), Quarter(1), Quarter(2) and Quarter(3)

The default is for arrays to be public.  If you declare an array in a module, you will need to use the keyword Public rather than Dim for it to be treated as public.

 The problem is immediately visible.  Quarter(3) will be the name for the score in the 4^th quarter.  To get around this, I declare an array with one extra element, so that I can refer to the elements in a way that makes sense.  I end up with a variable I’ll never use, Quarter(0), but that’s OK with me.  So I’d do it like this:

Dim Quarter(5) as Integer

Here’s where you really see the power of an array. The array element number can be substituted with another variable and action can be taken on it as a group using looping statements.

Example:

For x = 1 to 4

      TotalScore = TotalScore + quarter(x)

Next

A one dimensional array is a list, a two dimensional array is a table. Three dimensional arrays are also possible. This kind of an array would be a mathematical model of some kind.

Syntax:

Dim Scoreboard(1,3) as integer

This would create a table with 2 rows and 3 columns.  You could use this to record the scores in each quarter for 2 teams.

When working with the individual elements, you can think of these elements in terms of a table.  This one would have 2 rows – one for each team, and 4 columns, one each quarter.

Reference these table cells as Scoreboard(row, column).

Many years ago, there was a soft drink called RC cola.  We used this to remember row column referencing when programming arrays.

A sample of a scoreboard program is here.

In some cases, you will not know the dimensions of the array you are creating.  An example is the Scoreboard.  What happens if you go into overtime?

When you find that a condition occurs where you will need to reconfigure your list or table by adding more cells, use the ReDim statement.

Syntax:

ReDim ArrayName(number)

Example:

If OverTime = “yes” then

      ReDim Scoreboard(1,4)

End if

 There are additional methods that can be used with arrays including Array.Sort, Array.Find, Array.Reverse, Array.Copy and Array.Clear.

 Syntax:

 Array.Sort(ArrayName)

Many textbooks explain arrays in terms of memory allocation. This may be a tradition, but I never found it helpful to think about how the memory is used to store an array.  I just need to know how to reference the elements. Thinking about the one dimension array as a linear set of boxes works for me.  Thinking about the two dimensional array as a table with rows and columns works for me too. I know they are in memory, and it isn’t useful to bring memory into story.  I’m not sure exactly how items are placed in memory, but I think they all follow each other one after the other and that no table form is ever created in memory.  The analogy of referencing a table cells is useful; thinking about memory is not.