Programming in
Visual Basic
More Decisions :
The Select Case
Structure
Quite often, as in the
last unit, we need to test the same item
a number of times for different values.
When this is the case, we can replace all
of the If ... Then ... Else statements
that check that particular item with a
Select Case structure. The general format
is
Select case ITEMNAME
Case First
Possibility
Statements
Case Second
Possibility
Statements
...
Case Final
Posibility
Statements
Case Else
Statements
End Select
Each possible value of the item is
checked using a Case statement and is
followed by one or more Basic statements
that will be carried out if there is a
match for that value.
The final part of the structure is
usually a Case Else statement which will
come into operation if none of the other
values match. This is useful for
informing the user that he/she has made
an error.
The Message Box
One Quick and easy way of
displaying messages for the user is the
message box. This brings up a standard
Windows Dialogue Box, with appropriate
buttons for the user to choose from. A
message box is called from Visual Basic
code using the MsgBox function using the
following format:
MsgBox (message, type, title)
where message = text to be sdisplayed,
type = buttons to be displayed, title =
title that appears at the top of the box.
The type and title sections are not
compulsory - if thay are omitted, you
will get a Dialogue Box with just an OK
button and the title of the Form as the
title of the Dialogue Box.
Task Six :
1. Open the project
STOCK.VBP. Double click on the Find Item
command button to bring back the previous
code produces in Task Five.
2. Delete or edit the existing code to
replace the If ... Then ... End If
structures with the following Select Case
structure:
Select Case txtItem
Case
"Radio"
txtPrice = 12.54
Case
"Headphones"
txtPrice = 23.5
Case
"Glasses"
txtPrice = 13.99
Case "Teddy
Bear"
txtPrice = 23.49
Case Else
Msgbox ("Your
item has not been found")
End Select
3. Start the program and
try various correct and incorrect values
for the item to check that your code is
correct.
Exercise Six :
1. Modify the code for
the Find Stock Number command button
within the Stock project to use a Select
Case structure instead of If ... Then ...
Else statements.
2. Open the ADDRESS.VBP project and use
the Select Case structure for the Find
Name command button.
3. Add a new button to the Address
project called cmdFindTelephone, with a
Caption of Find Telephone Number. Write
code code for this command button using
the Select Case structure that will check
a telephone number that is entered and
automatically fill in the name &
address details.
Using Else with an If statement
We saw in the previous tasks, the use of
Else with the Select Case structure. The
Else statement within an If ... Then ...
End If structure performs a similar
operation. This time, instead of checking
a number of possibilities, we only check
one condition and the Else statement
comes in operation immediately if this
condition is not met. The general format
is:
If condtion statement(s) Then
statement(s)
Else
statement(s)
End If
For example, in the Stock project, we
might want to check that a quantity is
entered before carrying out calculations:
If Val(txtQuantity) = 0 Then
MsgBox ("You
have not entered a quantity")
Else
txtSubTotal =
Val(txtPrice) * Val(txtQuantity)
... other
calculations ...
End If
N.B. val(txtQuantity) will also give a
value of 0 if text is accidentally
entered into the quantity box. The above
code will therefore check for a zero
quantity or text incorrectly entered.
Exercise Seven:
1. Open the Stock project
and amend the code for the Calculate
command button to check that a valid
quantity has been entered as described
above.
2. Amend the code for the Find Stock
Number command button to ensure that text
is not entered for the Stock Number.
Remember that val(StockNumber) will give
zero if text is entered. Also, in this
case, the Select Case structure should be
completely contained within the If ...
Else ... End if structure:
If .......... Then
........
Else
Select Case
........
........
........
End Select
End If
Indentation :
Hopefully, you will have
noticed that indentation is widely used
in Visual Basic coding. Indentation is
used to show the various parts of a
structure - the statements that make up
the structure are lined up. For example:
If ..... Then
statement(s)
Else
statement(s)
End If
Notice how the three components of the
structure - If, Else and End If - are
lined up and the statements that are
linked to each part of the structure are
indented. Similary, with the Select Case:
Select Case ...
Case ...
statement(s)
Case ...
statement(s)
Case ...
statement(s)
Case ...
statement(s)
End Select
More indentation is needed for structures
within structures:
Select Case ...
Case ...
If ... Then
statement(s)
Else
statement(s)
End If
Case ...
statement(s)
End Select
The use of indentation makes coding much
easier to read and change, particularly
when the code becomes more and more
complex with the extended use of various
types of structure. It is very difficult,
without the use of indentation, for
instance to match End Ifs, End Selects,
etc. to the appropriate opening
statements, and this type of mismatch is
a very common programming fault.
Comments :
It's easy to remember
what a particular piece of simple code
does immediately after you have written
it. But what happens with very complex
code that you wrotesix months ago that
you have to change for some reason? Quite
often, programmers write code and they
can't remember what its purpose was.
Similarly, they know why and what they've
written, but can't find it!!
The use of comments within programs helps
the original programmer and, more
importantly, other people who have to
read and/or amend the code, understand
what particular sections of code do.
Remarks are used troughout the code as
explanatory notes for the programmer.
Thay do not effect the running of the
code in any way. Each line of comments
needs to start with Rem statement or a
singe apostrophe ( ' ). For example:
Rem The following code checks the item
entered into the Stock form
Rem and enters the appropriate price into
the form
Select Case txtItem
Case
"Radio"
txtPrice = 12.54
Case
"Headphones"
txtPrice = 23.5
Case
"Glasses"
txtPrice = 13.99
Case "Teddy
Bear"
txtPrice = 23.49
'if the item is not
found, display appropriate dialogue box
'with OK button
only
Case Else
MsgBox ("Your
item has not been found")
End Select
Exercise Eight
:
Re-visit all the code you
have written so far. Check that
indentation is used where appropriate and
add comments throughout to describe your
code.
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