Transcript Iteration (Looping Constructs in VB) Iteration: Groups of

Iteration (Looping Constructs in VB)
Iteration: Groups of statements which are repeatedly
executed until a certain test is satisfied
Carrying out Iteration in VB programming, requires the
use of Looping Constructs
There are 2 main types of Looping Constructs:
Determinate Loops: will repeat themselves a known
(specific) number of times (For..Next Loops)
Indeterminate Loops: will repeat themselves an
unknown number of times (Do Loops) [Until; While]
Determinate Loops
A group of statement is repeated a specific number of
times
For..Next Loops
Repeat the statements in a loop a specific number of
times
Each For statement has a corresponding Next
statement
For..Next
LoopTosyntax:
For Counter/LoopIndex
= Start
End [step step]
Statements (Body of Loop)
Next Counter/LoopIndex
The For..Next loop uses the For and Next statements
and a Counter Variable [LoopIndex]
The elements of a For..Next:
The Counter/LoopIndex must be a numeric variable,
determines the number of times the statements inside
the loop will be executed
Start and End may be Constants, Variables, Numeric
Property Values, or Numeric Expressions, and further
determine the initial and final value of the counter
The optional word Step may be included, along with the
value to be added to the LoopIndex (positive/negative)
for each iteration of the loop, and if omitted, the default
value is 1 for each increment of the loop
Dim iLoopIndex As Integer
Dim iMaximum As Integer
iMaximum = Inputbox(“Enter the value”, Number of
Entries)
For iLoopIndex = 0 To iMaximum
‘The statements inside of the loop are indented, and
referred to as
‘the body of the loop
Next iLoopIndex
A Counter-Controlled Loop generally has 3 elements:
Initialise the Counter
Increment the Counter [step]
Test the Counter to determine when it is time to
Terminate the loop
For iIndex = 2 To 100 Step 2
will count from 2 to 100 by 2
The statements in the body of the loop will be executed
50 times, with iIndex = 2, 4, 6, ….
The program checks for greater than the test value and
not equal to
Exiting For..Next Loops
If you enter an Endless Loop, the program execution
will have to be broken manually
Therefore, you will need to enter Break Time
Ctrl + Break
With For..Next loops, you may need to terminate the
loop before the loop index reaches its final value
VB provides an Exit For statement for this situation
Generally, an Exit For statement is part of an If
statement
For iLoopIndex = 1 To 10
If txtInput.Text = “ ” Then ‘nothing was entered into the input
textbox
MsgBox “You must enter something”
Exit For
End If
……….
……….
‘statements in the loop
Next iLoopIndex