The big idea: A problem specification writes down exactly what the program must do, before anyone writes any of it.
It is what you check the finished program against — and without it, "finished" is a matter of opinion.
Why it comes first: "Make a system for the library" cannot be built or tested, because nobody can say when it is done.
The specification turns a wish into something with a definite answer.
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Problem statement
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Objectives and goals
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Input specification
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Output specification
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Constraints and limitations
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Evaluation criteria
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Testable
- "Return results within five seconds on 10,000 records"
- "Reject any member ID outside 1 to 99999"
- "Show a message when no loans are overdue"
- Each one can be tried, and the answer is yes or no
Not testable
- "Be fast"
- "Handle errors properly"
- "Be user-friendly"
- Each one is a matter of opinion, so it can never be settled
The test for an objective: Ask: how would I prove this was met?
If you can describe the test in one sentence, the objective is good. If not, rewrite it until you can.
Where the marks usually are: Exam answers lose marks by writing vague objectives and by skipping the input and output specifications.
Naming types and ranges for inputs takes one line each and is the easiest part to get right.
How this is tested — you must write objectives and input-output specifications that could actually be tested. It comes up two ways:
Paper 2 — working with code
- Construct a specification from a scenario, 4-6 marks
- Write objectives, inputs and outputs
- Identify constraints the scenario implies
Paper 2 — the algorithmic-thinking question
- Specify a problem described in words, with no code involved
- Say how the finished solution would be judged
The classic trap: Writing objectives that cannot be checked. "Be efficient" earns nothing; "process 10,000 records in under five seconds" earns the mark, because it can be tried.
IB-style questionConstruct[6 marks]
A school canteen wants a program that lets students pre-order lunch and tells staff how many of each meal to prepare. Construct a problem specification.
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