Overview of Arrays

Arrays:

• a group of related elements given a common name.
• each element is referenced by its position in the group (index).
• an array is similar to a list of numbered items

 

Justification:

• How data is organized plays an important role in the design process.
• It is sometimes necessary to show relationships among different variables or store and reference variables as a group.
• Many problems have so many components that it is difficult to process them if each one must have a unique field name.
  

 

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Declaration:
        Dim array_name (upper_bound) As component_type      

Example:
        Dim grade (35) As Integer

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Component_type indicates what can be stored in each element of the array.

The upper_bound is the highest valid index.

The array has upper_bound + 1 elements, because the lower_bound defaults to 0.

Individual numeric array elements are initialized to zero by default.

When specifying the upper_bound, using a constant  like SIZE instead of a number makes it easier to change the array size: only one program line needs to be changed to change the array size, loop limits, etc.

Attempting to access an index that is smaller than the lower bound or larger than the upper bound is a run-time error.

 

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To access a particular element in an array use the following syntax:
        Array_name(constant or variable) 

Example:
        Print (Grade(ctr)) 

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Example:
        Dim grade (35) As Integer

 

• Grade is of type Integer -- a group of 36 memory locations, each of which contains a data item of type Integer.
• Each element is accessed by its position in the group of 36.

 

 

 

                                          

 

                                             

 

                                             

 

 

 

• Grade(0) refers to the first element of in the group of 36.

• Grade(I-1) refers to the Ith element of in the group of 36.

• The index must be a Long or Long expression between 0 and upper_bound.

• Since each element is an Integer it occupies two bytes.

 

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Processing arrays

To process an array element by element, you can use a For loop.

 

For ctr = 0 to 35

     Print Grade(ctr) & Space$(2);

Next ctr

 

Or

 

For ctr = LBound(Grade)  to UBound(Grade)

     Print Grade(ctr) & Space$(2);

Next ctr

 

Function LBound returns the lowest-numbered index value.

 

Function UBound returns the highest_numbered index value.

 

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Accessing array elements

 

To access an element of an array, use the array name and an index.

 

The index can be a variable, constant, or expression of type Long , and indicates the location of the element that will be referenced.

 

 

A For loop can be used to process all elements of an array.  If an application requires either processing all elements in the array or stopping if a certain condition is detected, one of the forms of While loops must be used.

 

The following example checks all students in a class to determine whether everyone is passing.  If a failing student is detected, processing can be terminated because it has been determined that everyone is not passing.  Therefore a While loop is used.

 

All_Passing = True

Ctr = 0

While (Ctr <= 35) And All_Passing

    If (Grade(Ctr) < 70) Then

           All_Passing = False

    Else

            Ctr = Ctr + 1

    EndIf

Wend

 

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Example:

 

Dim studentGrade (9) As String

 

Sample contents after some processing:

 

studentGrade

 

"F"

"B"

"C"

"A"

"F"

"C"

"A"

"A"

"C"

"B"

 

studentGrade(4) = "F"

Assigns "F" to the fifth cell in the array studentGrade.

 

 

ID = 6

studentGrade(ID) = "A"

Assigns "A" to the seventh cell in the array studentGrade.

 

For ID = 0 to 9

      Print studentGrade(ID);

Next ID

The loop prints all the values in studentGrade:  FBCAFCAACB

 

For ID = 0 to 9

      Print "Student " & ID+1 & "Grade " & studentGrade(ID)

Next ID

Student 1  Grade F

Student 2  Grade B

  :

  :

Student 10 Grade B

 

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Option Base

• The default lower bound is 0 unless Option Base specifies otherwise.

• If the general declarations contain the line Option Base 1, the lower bound is set to 1 instead of 0.

• Placing an array declaration before Option Base is an error.

• Option Base can be followed only by 0 or 1.

 

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Declaring Arrays with the To Keyword

 

Arrays can be declared by using the To keyword within the subscript syntax.

For example, if you want to create an array of five Integer variables in which the first element has index 1 and the last element has index 5, you could use

Dim fiveArray(1 To 5) as Integer

For example, if you want to create an array of Integer variables to track the number of students scheduled to graduate during the first six years after the year 2000, you could use

 Dim arrayY2K(2000 To 2005) as Integer

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Determining array size

 

The number of elements in an array can be determined using the following formula:

 

      UBound (array) - LBound (array) + 1

 

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Bounds checking in arrays

 

If an attempt is made to access an element outside of the array bounds, it is a run-time error. i.e., Visual Basic performs bounds checking on array subscripts.

 

For example, given the declaration Dim grade (35) As Integer, an attempt to access grade(ctr) when ctr has a value of 36 would be a run-time error.