TOPIC 3.2. FEATURES OF PROCEDURAL PROGRAMMING LANGUAGES

TOP-DOWN DESIGN

• Top-down design is the technique of breaking down a problem into the major tasks to be performed; each of these tasks is then further broken into separate subtasks, and so on until each subtask is sufficiently simple to be written as a self-contained module or procedure.
• The program then consists of a series of calls to these modules, which may themselves call other modules.

JACKSON STRUCTURE DIAGRAMS

• It is useful to have some way of representing the structure of a program - how the modules all relate to form the whole solution and a Jackson structure diagram is one way of doing this.
• When a program is large and complex it becomes especially important to plan out the solution before doing any coding and a structure diagram also serves a useful purpose as documentation when the program is complete.
• The diagram below resembles a family tree, with the main program statements written across the top line.

THE BUILDING BLOCKS OF A STRUCTURED PROGRAM

Only three different 'building blocks' or program constructs are needed to write a structured program. These are:

1. Sequence - in which one statement follows another and the program executes them in the sequence given.
2. Selection - 'if..then..else' is an example of selection, where the next statement to be executed depends on the value of an expression.
3. Iteration - a section of the program is repeated many times, as for example in the 'while..do' statement.

• Programs that are written using top-down techniques and suing only the three constructs described are called structured programs.

REPRESENTATION OF A LOOP

• Within the loop written, it is used to indicate a section of code repeated may times.
• For example,

REPRESENTATION OF SELECTION

• Selection is represented by a small circle in each of the boxes representing the alternative paths.
• For example,

MODULAR PROGRAMMING

• Any program that you write is likely to be relatively short; however, in industry and commerce some problems will require thousands or even tens of thousands of lines of code to solve.
• The importance of splitting up the problem into a series of self-contained modules then becomes obvious.
• A module should not exceed 100 or so lines, and preferably be short enough to fit on a single page; some modules may be only a few lines.

ADVANTAGES OF MODULAR PROGRAMMING

1. Some modules will be standard procedures used again and again in different programs or parts of the same program; for example, a routine to display a standard opening screen.
2. A module is small enough o be understandable as a unit of code. It is therefore easier to understand and debug, especially if its purpose is clearly defined and documented.
3. Program maintenance becomes easier because the affected modules can be quickly identified and changed.
4. In a very large project, several programmers may be working on a single program. Using a modular approach, each programmer can be given a specific set of modules to work on. This enables the whole program to be finished sooner.
5. More experienced programmers can be given the more complex modules to write, and the junior programmers can work on the simpler modules.
6. Modules can be tested independently, thereby shortening the time taken to get the whole program working.
7. If a programmer leaves part way through a project, it is easier for someone else to take over a set of self contained modules.
8. A large project becomes easier to monitor and control.

BOTTOM-UP DESIGN

• When faced with a large and complex problem it may be difficult to see how the whole thing can be down.
• It may be easier to attack parts of the problem individually, taking the easier aspects first and thereby gaining the insight and experience to tackle the more difficult tasks and finally to try and bolt them all together to form the complete solution.
• This is called as a bottom-up approach.
• It suffers from the disadvantage that the parts of the program may not fit together very easily, there may be a lack of consistency between modules, and considerable re-programming may have to be done.

RESOURCE:

P M Heatcote, [A level Computing, 3rd Edition], Ashford Colour Press Ltd, 1996.