Using Loops

 Loops are used to execute a block of program statement a number of times, or until a condition occurs.  The For…Next Loop is used to execute a number of times; the Do Loop and Do While Loop is used to loop until a condition occurs. The Do Loop tests for the condition after completing the loop statements once, the Do While loop tests for the condition before doing the loop, even on the first pass.

What is the drawback of loops?  You can accidentally create program code that cannot release itself from the loop!  You must consider how the loop will end, and make sure that condition will occur!

Syntax:

For variable = start to end

     Program Statements

Next

The variable must be an integer.  The any program statements can be used between the For and the Next lines; do indent them though. VB will take care of incrementing the variable for you automatically.

 

In the top section, declare the counter variable, so that it will carry a value independent of any subroutine. 

Dim X as integer = 1

Program Code for button 1 click

Picturebox1.image = system.drawing.image.fromfile _

(“c:\picture\face” & X & “.gif”)

X += 1

If X = 5 then X = 1

What it does: 

When the user clicks button 1, face1.gif will appear.  X will become 2. The value of X will be inspected to see if it is 5.  If it is 5, then it will be reset to 1.

When the user clicks button 1 again, face2.gif will appear.  X will become 3. The value of X will be inspected to see if it is 5.  If it is 5, then it will be reset to 1.

 The 4 pictures will be shown, with the user setting the pace by clicking button 1.

Note: 

X += 1  This is the new way to increment X in Visual Basic 2005. 

It is the same as X = X + 1

 

The Do While loop tests a condition then proceeds through through the loop if the condition is true.

Syntax:

Do While condition

    Block of program statements go here

Loop

Example:

This does the same thing as the Counter example.  It opens four image files. I added the msgBox so that you can see each picture.  Otherwise it would do them all very fast, and you might not even see it.

           

 The Do loop tests for the condition after completing the loop once.  The condition will not be evaluated until the loop is completed. 

Syntax:

Do

    Block of program statements go here

Loop While condition

Example: