MadSci Network: Computer Science

Re: what is the structure way to write a big program?

Date: Mon Aug 30 21:14:37 1999
Posted By: David Taylor, Staff, programming, Testronics
Area of science: Computer Science
ID: 935736379.Cs

What is the structure(d) way to write a big program?

Well, this question has spawned "religious" wars ( spirited arguments of 
belief or prejudice rather than objectively provable fact ). While there 
are some things you can learn from a book, programming requires more 
hands-on practice than formal teaching. 

Many rules-of-thumb have been found to be useful aids in programming.

One important rule-of-thumb is to minimize interelationships between 
different sections of code. This allows one to pretend that sections of 
code are 'black boxes' - each one doesn't care how the others work. If one 
has to be rewritten for some reason ( happens fairly often ), it doesn't 
really bother anything else. Real life involves compromises of this 
principle - and the price is paid - to change one thing, you have to check, 
and possibly change, several others. 

Let's say a function is written so that it's output is strictly dependant 
on its explicit parameters. No global variables are referenced in the 
function. You can rest easy that changes in unrelated sections of the 
program should not bother the function. You may also cut and paste that 
function into another program with very little trouble.

If that function references any global variables, then it will be more 
effort to re-use the function in another program. There is also the 
considerably increased possibility that another 'unrelated' section of the 
program will interact negatively with this function.

Accordingly, global variables should be used sparingly. My favorite way to 
use them is for a few pieces of information that the whole program needs to 
be able to read, but only ONE part of the program writes (preferably only 
on program initialization). 

One of my personal rules-of-thumb is that if you will deal with three or 
more of some data structure, then it is time to write a 'class' for it. A 
class is a combination of data and functions to operate on that data. 
Everything outside that class needs to use a 'public' set of interface 
functions that you write. Yes, it takes a bit more time to do things this 
way. It helps keep a control on the way parts of a program are interrelated 
though. Languages that don't directly support classes can still benefit 
from this technique - it just requires more programmer discipline.

Another rule-of-thumb is to prefer a lot of relatively simple functions to 
a very few, really complex functions. Smaller functions are more flexible 
and can be reused effectively (even within the same program ). If a 
function is much longer than about 120 lines of substantive code, than you 
really need to at least examine the possibility of breaking it into smaller 
pieces. Ideally, you should be able to write one ordinary sentence to 
describe what each function does. At the head of each function, put a 
comment containing a brief overall description of the function. This helps 
later when you have to maintain something you haven't thought about in 

There are all sort's of books about how to structure a program. While you 
might take a look and experiment with some of them, take it all with a 
grain of salt. Practice is the real teacher here.

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