From problem to specification: inputs, outputs and constraints

Step 1 Warm-up recall

Think of a task you do with fixed steps, such as making tea. List the steps and say what must be ready before you start.

Your answer

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Step 2 Learning goal

By the end of this lesson you can:

• define a computational problem as inputs, a required output and constraints (Foundation);
• spot what is missing from a vague request (Core);
• write a precise specification for a simple task (Core).

Step 3 A familiar example

You ask a colleague to “tidy the list”. One person sorts it, another deletes duplicates, a third makes it shorter. The request did not say what a tidy list is, so nobody could be wrong or right.

Step 4 Clear explanation

A computational problem is a question with defined inputs and a required output. A specification states three things: what kind of input is allowed, what the output must be, and any constraints, such as “the list is never empty”. A machine cannot ask what you meant, so anything left open is either guessed or fails.

Compare vague requests with precise specifications:

A good specification says what the answer is, not how to find it. “Return the lowest price” is a specification. “Check each price and keep the smallest” is a method, which is the subject of Lesson 2. Keeping the two apart lets you ask whether a method is correct: does it always produce the output the specification requires?

Step 5 Worked example (4 marks)

Write a specification for “find the oldest person in a team”.

1. Input: a list of people, each with a date of birth. (1 mark)
2. Output: the person with the earliest date of birth. (1 mark)
3. Constraint: the list has at least one person. (1 mark)
4. Open question stated: if two people share the earliest date, return the first in the list. (1 mark)

Step 6 Learner practice (instant-marked)

Step 7 Recap

• A computational problem has defined inputs and a required output.
• A specification states the input, the output and the constraints.
• Say what the answer is before saying how to find it.

Step 8 Exit ticket (ungraded)

Write a one-sentence specification for “find the longest word in a sentence”, including what happens in a tie.

Your answer

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Step 9 Go further (Extension)

Choose a spreadsheet function you use, such as a sum or a lookup, and write its specification: input, output and one constraint.