Do/Loops

• A loop repeats a series of instructions.• An iteration is a single execution of the statement(s) in the

loop.• Used when the exact number of iterations is unknown• A Do/Loop terminates based on a specified condition.

• Execution of the loop continues while a condition is True or until a condition is True.

• The condition can be placed at the top or the bottom of the loop.

The Do and Loop Statements — General Form

Do {While |Until} condition ' Statements in loop.

Loop

--OR--

Do ' Statements in loop.

Loop {While | Until} condition

Top of Loop Condition,

Pretest/Entry test

Bottom of Loop

Condition,Posttest/ Exit

Pretest vs. Posttest

• Pretest — loop may never be executed since tested BEFORE running.

Do While … LoopDo Until … Loop

• Posttest — loop will always be executed at least once.

Do … Loop WhileDo … Loop Until

Pretest vs. Posttest Diagram

The Boolean Data Type Revisited

• Can help when searching a list for a specific value• Boolean variable is always in one of two states: True or

False.• When a particular situation occurs, set Boolean variable to

True.• Use a loop to check for True

• Many programmers refer to Boolean variables as switches or flags.

• Switches have two states — on or off.• Flags are considered either up or down.

For/Next Loops

• Used to repeat statements in a loop a specific number of times

• Uses a numeric counter variable, called Loop Index, which is tested to determine the number of times the statements inside the loop will execute

• Loop Index is incremented at the bottom of the loop on each iteration.

• Step value can be included to specify the incrementing amount to increment Loop Index, step can be a negative number.

The For and Next Statements — General Form

For LoopIndex [As DataType] = InitialValue To TestValue [Step Increment]

' Statements in loop.Next [LoopIndex]

A For/Next loop can handle all three elements of a counter-controlled loop.

Initialize the counter.

Increment the counter.

Test the counter to determine when it is time to terminate the loop.

For/Next Loop Diagram

Exiting Loops

• In some situations, you may need to exit the loop prematurely.

• Click on the form’s close box or use the VB menu bar or toolbar to stop the program; or Ctrl+Break.

• Use the Exit For statement inside the loop structure.• Generally, the Exit For statement is part of an If statement.