Program Flow
|
| Tips >> Visual
Basic |
|
MAKE WEB APPLICATION WITHOUT KNOWLEDGE OF CODING? CLICK HERE
Structured programming is based around three basic constructs:
1. Sequence,< Selection and Iteration. The Sequence construct
is the simplest, whereby statements are executed in the order they're
found in the code.
Both the Selection and Iteration constructs require a Logical Test.
Logical tests can be performed on Variables, literal values, calculations,
results of functions and controls. The test will result in either
True, False, Null or Empty.
SELECTION CONSTRUCTS
The Selection constructs are used when the flow of execution may
flow down two or more paths.
THE IF STATEMENT
The If statement is used to control the flow of execution
down one of two or more paths, depending on the result of a logical
test. The ElseIf statement may be used to specify conditions
should previous conditions be false, and Else may be used should
none of the above be true.
|
If grade < 40 Then
Debug.Print "Failed"
ElseIf grade < 75 Then
Debug.Print "Passed"
Else
Debug.Print "Excellent Pass"
End If |
|
|
If statements can be nested, and the combined with logical operators.
THE SELECT STATEMENT:
The Select statement can be used when multiple If statements
become messy and difficult to read. Each Case statement is evaluated
from the top down. Should the Case statement be true, the section
of code associated with the condition is executed. Case Else is
used as a catch all.
The following example use the Select Case construct to
move a Shape, shpBlock, according to which arrow key is pressed
on the keyboard.
|
Private Sub Form_KeyDown(KeyCode As Integer,
Shift As Integer)
Select Case KeyCode
Case
vbKeyUp
'
Move the block up the form
shpBlock.Top
= shpBlock.Top - 50
Case
vbKeyDown
'
Move the block down the form
shpBlock.Top
= shpBlock.Top + 50
Case
vbKeyLeft
'
Move the block left on the form
shpBlock.Left
= shpBlock.Left - 50
Case
vbKeyRight
'
Move the block right on the form
shpBlock.Left
= shpBlock.Left + 50
Case
Else
MsgBox
"Incorrect Key", vbExclamation, "KeyDown Demonstration"
End Select
End Sub |
|
|
It's possible to test for ranges using the Is operator. The
following example uses the Is operator to find grades in
a range.
|
Select Case grade
Case Is < 40
Debug.Print
"Failed"
Case Is < 75
Debug.Print
"Passed"
Case Else
Debug.Print
"Excellent Pass"
End Select |
|
|
ITERATION CONSTRUCTS:
The Iteration constructs are used when a block of code is
required to be executed continually until a condition is met.
PRE-CONDITION LOOPS
Pre-condition loops allow for a condition to be tested at
the start of the loop. This type of loop is used when you only want
to execute the block of code should the condition be true. For example,
you would only want to iterate through a RecordSet collection should
the RecordSet contain some records.
|
Do While Not adoRS.EOF
Debug.Print adoRS.Fields("Surname").Value
adoRS.MoveNext
Loop |
|
|
The following example of a pre-condition loop calculate
the factorial of a number. (a number that is multiplied by every
integer number below itself down to 1). The value is taken from
a TextBox, txtFactorial.
|
Private Sub cmdFactorial_Click()
Dim factorial As Long
Dim counter As Integer
Dim strAnswer As String
counter = Val(txtFactorial.Text)
factorial = counter
Do While counter > 1
counter
= counter - 1
factorial
= factorial * counter
Loop
strAnswer = "Factorial of
" & txtFactorial.Text & " is " &
factorial
MsgBox strAnswer, vbInformation,
"Factorial"
End Sub |
|
|
POST-CONDITION LOOPS:
Post-condition loops allow for a condition to be tested
at the end of the loop. This type of loop is used when you want
to execute the block of code at least once. This is useful for validation.
The following example prompts the user for a number between
1 and 10. If a number is not entered in that range, the program
continues to prompt for the number.
|
Dim response As Integer
Do
response = Val(InputBox("Enter
a number between 1 and 10", "Number Cruncher"))
Loop While response < 1 Or response > 10 |
|
|
COUNTED LOOPS:
Sometimes the number of iterations required are known in advance.
In this case a counted loop may be used. Visual Basic provides
a For ... Next construct to handle counted loops.
The following example prints the numbers one to ten.
|
For counter = 1 To 10
Debug.Print counter & vbCrLf
Next counter |
|
|
An optional Step size may be added after the end value,
should you wish the increment to be anything other than 1. The step
size may be positive, negative, integer or fractional. Negative
Step values are used to count backwards.
The following example prints the numbers ten to one.
|
For counter = 10 To 1 Step -1
Debug.Print counter & vbCrLf
Next counter |
|
| |
|
If you don't find what you are looking for. Please click
here to submit your query, our experts will reply soon.
|
