In our last tutorial we looked at Visual Basic looping and
advanced conditional statements. In this tutorial we complete our overview
of Visual Basic statements by examining operators and assignments. One
cannot talk about assignment statements without also mentioning operators.
There are two types of assignment statement - simple replacement and computational
assignments. Here are examples.
simple replacement: IValue = 322 or Xdbl = val(i)
computational assignments: FV = (1+rate)^ nyears * PV or FinalString =
Prefix & " starts the string " & Suffix
It is the computational assignments that are of the most interest to Visual
Basic Programmers. In simple replacements as shown in the examples, Ivalue
takes the value 322 and Xdbl's current value is replaced by the value
of the array element val(i). However, in computational assignments a simple
to quite complicated calculation is first performed and the resulting
value is assigned to the variable.
Simple Assignments and
Simple replacement assignments are most often used to initialize variables.
A research study showed that more programming errors are due to failure
to initialize variables properly than any other cause. Part of the difficulty
is that when two variables in an assignment are not of the same type automatic
conversions are invoked that can change the value assigned in unexpected
ways. In general, when a smaller variable type is assigned to a larger
variable type (Integer to Double or Byte to String) no information is
lost. But that is not always the case, as when a Date variable is assigned
to just about any other variable type. Therefore Table 1 which lists common
data conversions, is worth knowing well:
The most commonly required conversions are to and from Strings. Programmers
are constantly converting out for display as strings on forms. likewise
data input from forms must be converted strings to the proper data type
as in our calculator program. So paying attention to data conversions
on replacement assignments can eliminate a lot of time consuming bugs.
Common Data Conversions in Visual Basic
|Integer ANSI to String
||Chr(66) ' returns "B"
|Number to String
||Format(99e2, "###,###") '
|Date to String
||Format(Now, "mmmm yyyy") '
returns "August 1999"
|String to Date
||CDate("09/99") ' returns 09/1/99
|String to Number
||Val("-1.098e3") ' returns -1098
Computational assignments most often involve String or numeric operations
but also can involve date manipulations, object references or other operations.
We shall use the VB's Immediate Window to illustrate some of the rules
of numeric and String manipulations.
Table 2 lists the major operators in Visual Basic - numeric or arithmetic,
comparison operators (which we have already used in conditional statements)
and logical operators(also used mainly in conditional statements). This
also happens to be the order of precedence of the operators.
Why is precedence important ? Enter Print 990 + 3 + 7 / 3 in VB's Immediate
Window. The answer is 995.333333333333 not a number close to 330 which
some might have expected. Now try to guess what the value of Print -21
- 2 ^ 3 * - 3 will be.
If you guessed 3 then you know the rules of operator precedence. The computation
is equivalent to (-21)-((2 ^3) * (-3)) = (-21)-(8 * (-3)) = (-21) - (-24)=
-21 + 24 = 3. Note because ^-exponentiation is the highest precedence
operator you do the operation 2^3 before any other. likewise because two
operators come together, "* -", VB assumes the second operator
is negation. In general arithmetic operations proceed comparison and then
comparison come before logical.
Visual Basic's Operators and their Precedence
||Not - not
||<> not equals
||And - and
|*, / multiplication and division
||< less than
||Or - or
|\ - integer division
||> greater than
||Xor - exclusive or
|mod modulus arithmetic
||<= less than or equal
||Eqv - equivalence
|+, - addition and subtraction
||>= greater than or equal to
||Imp - implication
|& or + String concatenation
||Like a pattern matching operator
an object type comparer
Try to guess what these operations will do in the Immediate Window:
Print I > 0 or i mod 3 > 99 and 3 > i ^ -5
Print Not I > 0 imp False
Figure 1 -VB's Immediate Window
|I can't stress enough the utility of VB's Immediate Window
for checking out your calculations and computational assignment statements.
It really is worthwhile to verify logical comparisons you are not sure
of in the Immediate Window. Also it easy to cut and paste to and from
the Immediate Window. Finally you can assign values to all the variables
used in an expression then test the expression. If you don't get the results
expected just edit the expression right in the Immediate Window, press
return and get new results.
Finally, the Like comparison operator is very useful when looking
up filenames that match a pattern. The test fname like "b?t*.bak"
matches "b1t1.bak" to "bztzzzzz.bak". It can be most
In this tutorial we have completed our overview of Visual Basic syntax
with a review of operators and assignment statements. VB operators are,
with the possible exception of logical operators, fairly simple and easy
to understand. Yet sophisticated commercial applications like SHL's Transform
and Primavera's SureTrack have been built in Visual Basic. Indeed, with
Microsoft Office 2000 and scores of other programs exposing their inner
workings through VBA-Visual Basic for Applications and compatible components,
expect to see more VB/VBA programs working directly with such programs
as Microsoft Word or Excel and Visio 2000 and Corel Presentations or Draw,
etc. Our next tutorial which returns to visual programming explores these
prebuilt VB components.
Jacques Surveyer is a consultant and writer - he can be reached at theOpensourcery.com
The book Beginning VB6/Wrox covers statement on pages 113-173; VB6 Black
Book/Coriolis does the same on pages 36-96. Also check VB