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Variables
What
are variables?
The dictionary provides these following
definitions:
variable
_adj.
1 a that can
be varied or adapted (a rod of variable length; the pressure is
variable). b (of a gear) designed to give varying speeds.
2 apt to vary; not
constant; unsteady (a variable mood; variable fortunes).
3 Math. (of a
quantity) indeterminate; able to assume different numerical values.
4 (of wind or
currents) tending to change direction.
5 Astron. (of a star)
periodically varying in brightness.
6 Bot. & Zool.
(of a species) including individuals or groups that depart from the
type.
7 Biol. (of an
organism or part of it) tending to change in structure or function.
_n.
1 a variable thing or
quantity.
2 Math. a variable
quantity.
3 Naut. a a
shifting wind. b (in pl.) the region between the NE and SE
trade winds.
Like the dictionary says a variable is
a portion of memory that holds some data, this data is supposed to be
changed during the program flow.
Let’s see now what a variable does
and how it works.
First, a variable must have a name so
we can manipulate it form the code. In Visual Basic you can declare
the variables you use or just use the variable without declaration and
Visual Basic assumes a default type of variable. Aldo you can specify
an variable type by using a prefix like:
$ for string, & for long
etc. (consult the Visual Basic help for a complete description of
these prefixes)
Declaring
variables in Visual Basic
Dim
[variable name] As [variable type]
This is the syntax for declaring
variables.
- [variable name] – can be any name
you want but it must not start with a number or symbol
(!@#$%^&*()+)
- [variable type] – can be any type
of variable. At beginning we will use only the basic variables
(String, Integer, Long)
e.g.: Dim varName As String,
Dim NrOfEntryes As Long
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Data
type
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Storage
size
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Range
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| Byte
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1
byte
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0
to 255
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| Boolean
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2
bytes
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True
or False
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| Integer
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2
bytes
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-32,768
to 32,767
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Long
(long integer)
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4
bytes
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-2,147,483,648
to 2,147,483,647
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Single
(single-precision floating-point)
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4
bytes
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-3.402823E38
to -1.401298E-45 for negative values; 1.401298E-45 to
3.402823E38 for positive values
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Double
(double-precision floating-point)
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8
bytes
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-1.79769313486231E308
to
-4.94065645841247E-324 for negative values;
4.94065645841247E-324 to 1.79769313486232E308 for positive
values
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Currency
(scaled integer)
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8
bytes
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-922,337,203,685,477.5808
to 922,337,203,685,477.5807
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| Decimal
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14
bytes
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+/-79,228,162,514,264,337,593,543,950,335
with no decimal point;
+/-7.9228162514264337593543950335 with 28 places to the right
of the decimal; smallest non-zero number is
+/-0.0000000000000000000000000001
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| Date
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8
bytes
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January
1, 100 to December 31, 9999
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| Object
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4
bytes
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Any
Object reference
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String
(variable-length)
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10
bytes + string length
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0
to approximately 2 billion
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String
(fixed-length)
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Length
of string
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1
to approximately 65,400
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Variant
(with numbers)
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16
bytes
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Any
numeric value up to the range of a Double
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Variant
(with characters)
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22
bytes + string length
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Same
range as for variable-length String
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User-defined
(using Type)
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Number
required by elements
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The
range of each element is the same as the range of its data
type.
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Note Arrays of
any data type require 20 bytes of memory plus 4 bytes for each array
dimension plus the number of bytes occupied by the data itself. The
memory occupied by the data can be calculated by multiplying the
number of data elements by the size of each element. For example, the
data in a single-dimension array consisting of 4 Integer data
elements of 2 bytes each occupies 8 bytes. The 8 bytes required for
the data plus the 24 bytes of overhead brings the total memory
requirement for the array to 32 bytes.
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