CS0004: Introduction to
Programming
Repetition – Do Loops
Review

A condition is…


Relational Operators…



a statement that can (but does not have to) include relational
or logical operators that results to either True or False
compare two entities. Returns either True or False.
=, <>, <, >, <=, >=
Logical Operators…


combine two or more conditions. Return either True or
False.
And, Or, Not
Loops
So far we have used conditions in making decisions that
execute a sequence of statements once.
If condition Then
statement(s)
End If
 However, we can use conditions to execute a sequence of
statements multiple times.
 A loop is used to repeatedly execute a sequence of statements
a number of times.
 Each repetition of the loop is called a pass, or iteration.
 Loops can be used for validation, computing naturally
repetitive calculations, take in a variable amount of data from
the user, or many other tasks.

Do Loops
Do Loops execute the code they contain until a condition is false.
 Do Loops come in two forms: Pretest and Posttest.
 General Form of Pretest:
Do While condition
statement(s)
Loop
1.
First, it checks if condition is true.

If the condition is false, then the statement(s) are not
executed
If the condition is true, it executes the statement(s) in the
loop’s block.
A.
B.
a.


Then, it goes back to step 1.
Said another way, the statement(s) are executed until
condition is false.
This is called Pretest because the condition comes first.
Pretest Do Loop Flowchart
Is the
condition
True?
Yes
Execute
statements
within the loop
Execute
statements that
follow the loop
No
Pretest Do Loop Examples 1, 2 and 3

New Topics:

Pretest Do Loop



Looping used for validation
Looping used for variable length user input
Notes:

In Example 3:

count is called a counter variable.


sum is called an accumulator variable.



Counter Variables count how many times a loop repeats.
Accumulator Variables reflect the total (accumulation of) work done by all the
repetitions done in the loop.
In this case it was the sum of all numbered entered
The loop in this case is called a sentinel-controlled loop.


A Sentinel-Controlled Loop is broken out of when a variable in its condition
gets a certain value.
When -1 is entered for num, -1 is called an sentinel value.

Sentinel Values create conditions where a sentinel-controlled loop stops repeating.
Pretest Do Loop Example 4

New Topic:


Looping for calculations
Note: Only use looping for calculations when you cannot
easily do the calculations without looping.

How would we do Example 4 without looping?
Posttest Do Loop
General Form of Posttest:
Do
statement(s)
Loop While condition
1. First, it executes the statement(s)
2. Second, it checks the condition

A.
B.

If the condition is false, then the loop ends
If the condition is true, it goes back to step 1.
This is called posttest because it checks the condition at
the end.
Posttest Do Loop Flowchart
Execute
statements
within the loop
Yes
Is the
condition
True?
No
Execute
statements that
follow the loop
Posttest Do Loop Example 1

New Topics:

Posttest Do Loop

When to use Posttest
Until Keyword
In both forms of the Do Loop, you can replace While with
Until
 While loops while the condition is True
 Until loops until the condition is True
Do While num <> -1
statement(s)
Loop
Is the same as…
Do Until num = -1
statement(s)
Loop
 Until may make more sense in some sentinel-controlled loops.

Infinite Loops
An infinite loop is a logical error where a loop has no way of
reaching the condition where the loop stops executing, and
therefore repeats indefinitely.
 For example:
Dim num As Integer = 1
Do While num < 1
num += 1
Loop
 This loop will repeat forever because num will never be less than 1.
 This may be a simple example, but they can be more complex and
harder to spot. If your program seems to stall at about the time a
loop is executing, look to see if you have created an infinite loop.
 To stop a program from running while it is in an infinite loop, hit the
“Stop Debugging” button.

Closing Notes:


You can end a loop prematurely by using the Exit Do
statement in the body of a loop. As soon as Exit Do is
executed, execution jumps immediately to the statement
following the Loop statement.
Just like with an if statement, variables declared inside of a
loop have block-level scope, meaning they cannot be used
outside of the loop. Be careful though, these variables are
then going to be declared EVERY iteration of the loop.