To effectively design computer programs, tools have been
developed to assist in the process. The required tools for this
course take two forms. The first is known as Flow Charts
and the second is known as pseudocode. All programs can
be expressed in either of these forms. You are required to know
both.
All computer programs can be expressed using three basic
constructs - these are
Process
Iteration or Repetition
Decision Making
Both Flow Charts and pseudocode allow you to easilly express
these forms.
Flow Charts
Flow charts consist of shapes connected by lines that represent different
constructs. These can easily be draw with a pencil in a free-hand
form. During the construction process, many changes will need to
be made.
Inside the boxes, you can put any of the other processes shown
below.
The most simplest program just consists
of a series of steps that the computer will follow one
after the other. This is drawn as a series of trapeziums for I/O and rectangles with
each process described inside.
The I/O might include: Write Amount to accounts file
Read data from user
Get Age of customer
The processes might
include: Set counter to counter + 1
Calculate the diameter of a circle
Common processes and other structures
can be grouped together into a common module or
subprocess. These can then be given a name and then
refered to by that name in a mai process module.
Such
sub-processes migh include: Write Report to Printer
Calculate Tax
Determine amount owed
This is the first of the three basic
constructs for iteration. It says:
Do the process/processes in the box while some condition
is true. Of course, these processes may consist of one or
more processes, sub-processes or other constructs.
The
implication with this is the condition must be set before
the block is entered and it must have the potential to be
changed somewhere in the inside processes. As well, the
process may not be entered at all.
This is the second of the three basic
constructs for iteration. It says:
Do the process/processes in the box until some condition
is true. Of course, these processes may consist of one or
more processes, sub-processes or other constructs.
The
implication with this is the condition is not
set before the block is entered and it must have the
potential to be changed somewhere in the inside processes.
As well, the process must always be carried out at least
once.
This is the third of the three basic
constructs for iteration. It says:
Do the process/processes in the box a set number of times
while the value of the counter changes from the start
number to the finish number in increments of 1. Of
course, these processes may consist of one or more
processes, sub-processes or other constructs.
(Some
languages, such as Basic, allow a step in the counting of
values other than just 1)
This is the first of the decision making
constructs. The condition is a predicate, the same as
above and is described below.
This says to do the
processes in the left box if the condition is true and
the right box if the condition is false. The process in
the right box can be omitted. This is the same as saying
"Do nothing if false". Of course, the process
that is done may be any construct defined on this page,
so another "If-Then-Else" block may be the
process described. When this happens, the situation is
known as "nesting" and can become quite complex.
To overcome the problem and complexity
of nesting If-Then-Else statements, the construct of Case
was provided in more recent implementations of language.
It
allows multiple values to be tested and then the
appropriate process to be carried out based on the result.
If none return a true value, the default process is then
carried out. This is usually "do nothing".
Pseudocode is an alternate method of
representing the same structures as seen with N-S diagrams. The
basics of this is that key words are written in uppercase.
Examples of processes in pseudocode are as
follows. The uppercase words are key words
SET value TO total + 3
WRITE name TO printer
GET balance FROM keyboard
Iteration - Test First
DO WHILE <predicate>
processes
ENDWHILE
Iteration - Test Last
DO
processes
UNTIL <predicate>
Iteration - Fixed Repetition
DO FOR value <- start TO finish
processes
ENDFOR
Decision Making - IF-Then-Else
IF <predicate> THEN
processes
ENDIF
and
IF <predicate> THEN
processes
ELSE
processes
ENDIF
Decision Making - Case
CASE selection OF
item 1 : processes
item 2 : processes
..
item n : processes
ELSE
processes
ENDCASE
Predicates
Predicates are condition statements and rely on Boolean Logic
and Algebra. They consist of three parts and always results in
the value of True or False.
<Value to test> <Boolean Operator>
<Value to test against>
An example might be
Age > 6 where
Age is Value to test
> is Boolean Operator
6 is Value to test against
Predicates can also consist of multiple conditions joined by
AND, OR, XOR (and other boolean operators such as NAND and NOR).
The following tables will help interpret the final value of
complex predicates.
