4.1 - Repetition
Introduction
Repetition is one of the three basic programming structures. Simply defined, a repetition
structure is a tool to make the computer repeat a single command or series of commands.
Repetition is often described as looping and that word will often be used in this text. Once you
have mastered the type of structures in Visual Basic you will be able to understand their use in
other computer programming languages.
Skills Required
In order to work through this unit, you will need to be able to:
work within the Visual Basic environment
define and use variables correctly
use the IF and SELECT CASE selection structures
use a vertical scroll bar
New Skills
In this unit you will introduced to three types of repetition structures and some introductory
graphics skills.
Outcomes
By the end of this unit students you should be able to:
1. use the FOR...NEXT structure;
2. use the WHILE...WEND structure;
3. use the DO...LOOP structure
4. use a Timer Control as a repetition structure
5. identify as general ranges the number limits of integer, long, single and double variable
types.
6. define the terms twip, pixel and point
7. describe how graphics are positioned on a Visual Basic form
8. import graphics into a program
9. move graphics on the screen using a method and a property
10. use a combo box control
11. use the change event of a control
12. use the form load event
13. assess a programming situation and choose the appropriate programming structure
Description
The use of repetition or structures that repeat is one of the structures that taps the power of
computers. Computers often make the possible impossible by carrying out many operations at an
incredible speed and this often involves repetitive calculations and therefore requires a repetition
or looping structure.
There are three types of looping structures that you will study here:
1. Counter controlled loops
2. Condition controlled loops
3. Timer controlled loops.
We use all of these in our everyday lives:
The instructions on a shampoo bottle to wash, rinse and then repeat the process is an example
of a counter controlled loop.
Stirring a can of paint until the colour is mixed in is an example of a condition controlled
loop.
A grandfather clock that chimes every hour is an example of a timer controlled loop.
Loop structures can be very simple or more complex as you will see in the examples that follow.
You will develop the skills to use each type of repetition structure and as well will learn how to
choose the best structure for a given question. You will as well learn the Visual Basic graphics
system.
Activity 4.1.1 - Repetition Type Identification
In your notebooks, identify each of the following as one of a counter (CTR), condition (CON)
or time (T) controlled loop.
When starting the car push the gas pedal twice before turning the key.
Cut the lawn every weekend
Practise at the piano until you get the song right
Practise your guitar lessons every day
Read from page 50 to page 76 tonight for homework
Polish your shoes until they shine
Make up an three everyday examples of looping, one for each type of repetition.
Activity 4.1.2 - Counter Controlled Repetition
If a person was asked to add the numbers from 1 to 10 without access to a calculator they might
write 1+2+3+4+5+6+7+8+9 or do the question in their head. This would become more
complicated if they were asked to add the numbers from 531 to 1,673. In this section we will
look at how to teach a computer to do this kind of task as a counter controlled repetition.
In a computer the kind of activity like summing a set number of numbers would require a
counter controlled repetition. This simply means repeating an action or actions a set number of
times. If you had a old television that only worked when you hit it three times on the right side
your instruction to someone else who was going to watch it might be "Hit it 3 times on the right
side". The instruction for the computer would be more like:
Counting from 1 to 3
Hit the television on the right side
Next count
The first statement "Counting 1 to 3" indicates that the counting will start at 1 and end at 3. The
next line, "Hit the ..." is indented to show that it is the action being repeated and the last line
completes the structure.
If we go back to the original question, adding the numbers from 1 to 10, the actual Visual Basic
program would look like this:
Dim sum as integer
sum = 0
For ctr=1 to 10
sum=sum + ctr
Next ctr
After this is run the variable sum would hold the sum of the 10 numbers and it could be output to
the screen or used in a program as the programmer wished. The following table explains each
line of the For...Next loop.
Command
Explanation
For ctr=1 to 10
The loop will start the variable at 1, and stop when it is 10, it is assumed
that 1 will be added to the variable each time through the loop.
sum=sum + ctr
The current number (ctr) is added to the sum
Next ctr
The loop ends here, control is passed back to the For command
Write a For...Next for each of the following sums:.
sum the numbers from 100 to 500
sum the odd numbers from 51 to 100
sum every 7th number from 5 to 1 073
sum the numbers from 200 to 100 (in that order, you can use a negative step)
Activity 4.1.3 - Counter Controlled Repetition II
Computing Factorials
A factorial is the product of a number and the numbers that are smaller than the number. The
factorial of 5 would be 5x4x3x2x1. The symbol for factorial is the exclamation mark, 6! would
be read "six factorial".
Construct a simple program that would accept as input an integer, calculate the factorial of the
number and display the factorial in a second text box. A sample screen design is shown below.
Dimension all of your
local variables as integers.
Once you have written the
program, test it with 3!, the
answer the should 6. Try
the numbers 4 to 10 and
record your results. Your
will get an overflow
number for several of
these numbers. Overflow
simply means that the
number cannot be
represented with the given
variable type. Integer
overflow occurs at 32,
768. Once overflow occurs, change the variable type of the result to Long and test to see where
overflow occurs. Once you have found the limit, repeat for single and double to determine their
overflow limits.
HINT: The help for Data Type Summary will be helpful.
Save your file as Factorial