- Repetition
- If the same or very similar code is being called several times, it should be put into a subroutine to avoid code duplication.
- This is the most obvious reason.
- Clarity
- You should be able to read the code like English
- Reading the code in a top level subroutine or script should give an overview of what it does, without going into detail
- Each subroutine should be able to be easily described in one or two sentences
- Comments are not sufficient for this, as you would still have to scan through many pages to gain an understanding.
- This reason is controversial; Some people say repetition is the only valid reason (see 'dangers' below)
- Testing
- You should be able to test each conceptual part of the system separately.
- It should be obvious to which part of the system a unit test applies.
Dangers of the second two reasons:
- Too much abstraction, difficult to understand and follow code flow
- Code is slow because of overhead of creating and managing subroutines
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