Edexcel GCSE Computer Science

## Decomposition **Decomposition** means breaking a large problem into smaller parts, each of which is easier to understand, solve and test than the whole. The parts are then broken down again until every one is small enough to write as a single piece of code. It is applied to the problem before any program is written, and the result is a list of separate jobs rather than one tangle. Suppose a school wants a program to run a library. The whole problem is too big to hold in your head, but it splits naturally: - looking up a book by title or author - lending a book to a member and recording the return date - working out fines for late returns - adding new members and removing old ones Each of these can be split further. Working out fines needs the number of days late, the fine per day and a cap on the total, and each of those is a small question with a definite answer. The benefits are practical ones. Different people can write different parts at the same time, so a team finishes sooner. Each part can be tested on its own, so a fault is found in a small piece of code and not somewhere in thousands of lines. A part that already exists, such as the fines calculation, can be reused or replaced without touching the rest. And a large problem stops being intimidating once it is a set of small ones. ## Abstraction **Abstraction** means removing the detail that does not matter for the problem and keeping the detail that does, so that a model of the real world is simple enough to work…