Question: What is the copy-update problem? The copy-update problem is that when a copy is made of an original, and the copy (or original) is modified, and the original (or copy) is not updated, but should be, then copy-update problems result.

That is, a copy that needs an update creates redundancy that, if not computer-checked, can cause problems.

Let us look at this problem in more detail.

Question: What is redundancy? Redundancy is repetition.

This sentence is overly redundant.

Note that the above sentence is self-referential in that it refers to itself.

We are interested in two types of redundancy.

• noncomputer-checked redundancy
• computer-checked redundancy

Question: What is noncomputer-checked redundancy? Noncomputer-checked redundancy is redundancy that is not checked by a computer.

Question: What are some examples of noncomputer-checked redundancy? Some examples of noncomputer-checked redundancy include the following.

• photocopies and original
• databases (not in normal form, or distributed)
• phone list

Is noncomputer-checked redundancy good or bad?

In general, noncomputer-checked redundancy is bad. Avoid if possible/practical.

Programming examples:

• repetition within program code

There are many abstraction techniques to deal with noncomputer-checked redundancy. What are some of these? Here are some abstraction mechanisms that are used to deal with noncomputer-checked redundancy.

• constants
• procedures and functions with parameters
• modules
• classes and objects
• ... and many more ...

Goal: Ideally, every (important) aspect of your program/system should be changeable in one and only one place.

This goal has a huge impact on how one writes code.

Question: What is computer-checked redundancy? Computer-checked redundancy is redundancy that is checked by a computer.

Question: What are some examples of computer-checked redundancy? Some examples of computer-checked redundancy include the following.

• Error detection and correction
• Automated backup systems
• Disaster recovery
• Spelling checkers
• Memory parity checks
• Communication-line protocols
• Type rules and checks in programming languages

Is computer-checked redundancy good or bad?

Computer-checked redundancy is good.

Goal: Work computer-checked redundancy into programs/systems to detect possible errors and other issues.

What does a spelling checker do?

Where is the redundancy?

What are the limitations of a spelling checker?

What does a backup system do?

Where is the redundancy?

What happens if the backup system is not automated?

Where is the redundancy in the following functional definition?

Let y = x+1 to get the following.

Abstraction is looking at similarities and ignoring differences.

In programming terms, for code and data that contain noncomputer-checked redundancy, identify the similarities and differences.

• A constant is a named literal.
• A procedure contains similar code with formal parameters acting as placeholders for actual parameters (the differences) to be supplied later.

The laws of redundancy can be summarized as follows.

• Avoid noncomputer-checked redundancy to avoid problems and issues.


• Use computer-checked redundancy to detect problems and issues.

• Identifying copy-update problems is easy.
• Handling copy-update problems can be done.


• The payoff is significant.
• The payoff is easy to measure.

4 questions omitted (login required)