Cohesion is the first principle to consider when doing design. At thi level this is the concept that each procedure is responsible for only one thing. My earlier post about commenting rules was really all bout how to identify the cohesion level for a procedure.
Note: Cohesion can also be applied to higher order groupings, such as classes.
There is a ranked taxonomy of cohesion (from worst to best).
Coincidental - There really isn't any.
Logical - It all does the same kind of thing (more applicable to higher order groupings).
Temporal - It all happens at the same tiime.
Procedural - One thing is required to follow another.
Communicational - They all work on the same stuff.
Sequential - The results of one part are the input to the next.
Functional - It just does one thing
It is not always possible to get good cohesion in a procedure (eg an initialization routine). It is important when confronting that situation to simplify the procedure as much as possible. A low cohesion routine which is a list of function calls (each of which is a higher cohesion routine) will be part of a better system than one which is full of if statements and loops.
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