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Arrays are types of variables used in Visual Basic. All Visual
Basic programmers, ranging in experience from beginner to
advanced, need to have an understanding of arrays. An advantage
to using arrays is the ability to refer to a group or series
of variables by one name. This can make programming more efficient.
The following set of questions will help determine the level
of expertise a programmer has with arrays. A mid level programmer
should be able to answer all of these questions correctly.
A junior level programmer should be able to answer at least
7 of these questions correctly.
In
the following example:
Dim Z(3)
How
many elements are in the array?
Answer: 4, elements
0 through 3.
The above array is a one-dimensional array consisting of 4
elements. Arrays can be multi-dimensional, as shown in the
following question.
In the following example:
Dim X(1,2)
How
many elements can be stored?
Answer: 6
This is a 2 dimensional array. A graphical representation
of the array looks like:
(each box represents
an element in the array) You can have up to 32 dimensions
in an array [x(1,2,3,...32)].
Arrays can also
be declared using the following syntax:
Dim
X(100 To 200) As Integers
What
will happen if the following code is executed?
Dim X(100 To 200) As Integer
X(5) = 4
Answer: This would
yield a subscript out of range error. The lower bound of the
array is 100 and 5 is less than that. Declaring an array using
the to syntax creates a lower and upper boundary. If the candidate
answers that the 5th element is set to 4 this is incorrect.
I
need to use an array to hold information but I'm not sure
of the size. What should I do?
Answer:
Use a dynamic array. Dynamic arrays can be resized programmatically
using the ReDim statement.
How
do I resize a dynamic array?
Answer:
Use the ReDim statement. The ReDim statement
allows you to resize an array on the fly. The following example
demonstrates resizing an array first to 100 elements then
to 1000 elements:
Dim
X() as Integer
ReDim X(100)
'
'
'
ReDim X(1000)
What
is the statement LBound(x) used for?
Answer:
The LBound statement is used with an array to determine
the lower boundary of an array. This is particularly useful
with dynamic arrays.
In the following
code fragment LBound(X) = 10
DIM
X(10 to 400)
RMsgbox LBound(X)
What
is the statement UBound(x) used for?
Answer:
The UBound statement is used with an array to determine
the upper boundary of an array. This is particularly useful
with dynamic arrays.
In the following
code fragment LBound(X) = 400
DIM
X(10 to 400)
RMsgbox UBound(X)
Whenever
I re-dimension an array with the ReDim statement all
of the information in the array is lost. How can I prevent
this from happening?
Answer:
Use the preserve keyword with the ReDim statement.
The preserve statement will keep the information from being
erased. Note that in a multi-dimensional array only the last
element can be resized if you use the preserve statement.
What
is the Option Base statement used for?
Answer:
Option Base sets the default lower boundary of an array.
It can be set to either 1 or 0. If Option Base isn't
set in Visual Basic, the default is 0. Setting the Option
Base to anything other than 0 or 1 will yield an error.
Question
10: (somewhat advanced) I need to allocate an array for some
information, it may contain as many as 10000 elements or as
few as 2 elements. Should I just use an array of 10000 or
use a dynamic array?
Answer:
Since Visual Basic allocates memory for static arrays when
they are created, in this case it would be best to use a dynamic
array. There are some exceptions to this, as it does take
time (processing time) to resize an array, but the general
rule of thumb for this case is to use dynamic arrays.
About the author
Mark Horninger, A+, MCSE+I, MCSE, MCSD, MCDBA, is President
and founder of Haverford Consultants Inc. (http://www.haverford-consultants.com),
located in the suburbs of Philadelphia, PA. He develops custom
applications and system engineering solutions, specializing
primarily in Microsoft operating systems and Microsoft BackOffice
products. He has over 12 years of computer consulting experience
and has passed 31 Microsoft Certification Exams. During his
career, Mark has worked on many extensive and diverse projects
including database development, client server and web-based
application development, training, embedded systems development
and Windows NT and 2000 project rollout planning and implementations.
Mark is a contributing author to the books: MCSE
Windows 2000 Professional Study Guide, Designing
SQL Server 2000 Databases for .NET Enterprise Servers,
VB .NET Developers Guide
and Configuring and Troubleshooting
Windows XP Professional.
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