Representing algorithms: Understand and explain the term decomposition.

The idea

Decomposition is the technique of taking a large, complex problem and splitting it into smaller parts that can each be tackled separately. Understanding the term means recognising why programmers do it: smaller parts are easier to design, write, test and debug, and different people can work on different parts. Explaining it means describing the process in your own words and applying it to a scenario. For example, a school timetable program could be split into reading pupil data, allocating lessons to rooms, detecting clashes and printing the timetable. Each part is a manageable job with a clear purpose, and the parts can be solved one at a time before being combined into the whole solution.

What earns the marks

• Defines decomposition as breaking a large problem into smaller, more manageable parts or sub-problems.
• Explains a benefit, such as each part being easier to design, code, test or debug.
• Applies decomposition to a given scenario by naming sensible smaller parts of the overall problem.
• Recognises that the smaller parts together still solve the original whole problem.

Common mistakes and how to put them right

• Confusing decomposition with abstraction.

  Put it right: State that decomposition is about dividing the problem, not hiding detail.

• Thinking decomposition means writing code in a random order.

  Put it right: The sub-problems are identified deliberately and each has a clear, identifiable task.

• Believing the sub-problems must be completely independent.

  Put it right: They are separate tasks that are later combined to form the whole solution.

Exam tips

• Learn a one-sentence definition you can reproduce quickly, then add a benefit to gain the explanation mark.
• When given a scenario, name two or three concrete sub-problems rather than repeating the definition.
• Link each sub-problem to a clear task so the examiner can see it accomplishes something identifiable.