20. Arrays

Often we may want to store and manipulate a large set of numbers (for example the marks for a number of students in an exam). It is clearly not convenient to give a new variable name to each student's mark, so arrays are introduced for this purpose. An array is just like a vector; i.e. it has many components. Below is an example of a program which uses arrays.

Dim Marks(100) As Double
Dim I, N As Long

' First the Marks are read in from a file

Open "Exam.dat" For Input As #2

N = 0

While Not EOF(2)
N = N + 1
Input #2, Marks(N)
Wend

' Then the sum of the Marks is calculated and the average found

Sum = 0

For I = 1 To N
Sum = Sum + Marks(I)
Next I

Print "The average mark is ", Sum / N

The very first instruction Dim Marks(100) As Double tells the computer to set aside memory for an array called Marks which will have (a maximum of) 100 real-valued elements. Then the marks are read from the file Exam.dat. (See for an explanation of reading in data from a file using the While Not EOF statement.) The next step in the code is to calculate the sum of all the N students marks using a For/Next loop. (See for an explanation of using the For/Next loop to sum up some numbers). Finally the average mark Sum / N is outputted to the screen.

CHECKPOINT: Using the above examples, write a program which finds the largest mark from data in the file Exam.dat.

Note that it is possible to have more than one dimension in the array. Consider the following:

Dim A(20,100,5) As Long

For I = 1 To 10
    For J = 1 To 100
        For K = 1 To 5
        A(I,J,K) = ...
        Next K
    Next J
Next I

In this case, the array A is a 3-dimensional array, and the elements of A are accessed within the three nested For/Next loops. Note that the I loop only goes up to 10, whereas the corresponding dimension of A (i.e. the first dimension) has a maximum value of 20, i.e. you don't have to access all the elements of the array.

You can pass arrays to subroutines using the following. (Note, for information on subroutines, see sec..)

.
.
.
call add( x(), n, total )
.
.
.

sub add( x(), n, sum )

Dim i as Long

sum = 0.

For i = 1 To n
sum = sum + x(i)
Next i

end sub

Note that in the above example that

• the arguments of the subroutine add include the array x and it appears with empty parentheses.

• the last argument of the subroutine is called total in the calling programme, but sum in the subroutine itself. i.e. you are allowed to call the arguments by a new name within the subroutine.

I've included a small programme which uses a subroutine call with an array as an argument to find the minimum of that array. Click here to download it.

Arrays can be used as a convenient way of storing more than one piece of data within a single variable name. They are the computing equivalent of vectors or matrices. You can have arrays of any data type: e.g. Double, Long, String see .