computer science

Postcondition

Postcondition is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Postcondition rather than just read about it. In short: In computer programming, a postcondition is a condition or predicate that must always be true just after the execution of some section of code or after an operation in a formal specification. Postconditions are sometimes tested using assertions within the code itself.

Key takeaways

Postcondition belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Postcondition to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Postcondition from memory before moving on to harder problems.

Reference excerpt

In computer programming, a postcondition is a condition or predicate that must always be true just after the execution of some section of code or after an operation in a formal specification. Postconditions are sometimes tested using assertions within the code itself. Often, postconditions are simply included in the documentation of the affected section of code. For example: The result of a factorial is always an integer and greater than or equal to 1. So a program that calculates the factorial of an input number would have postconditions that the result after the calculation be an integer and that it be greater than or equal to 1. Another example: a program that calculates the square root of an input number might have the postconditions that the result be a number and that its square be equal to the input.

Postconditions in object-oriented programming In some software design approaches, postconditions, along with preconditions and class invariants, are components of the software construction method design by contract. The postcondition for any routine is a declaration of the properties which are guaranteed upon completion of the routine's execution. As it relates to the routine's contract, the postcondition offers assurance to potential callers that in cases in which the routine is called in a state in which its precondition holds, the properties declared by the postcondition are assured.

Eiffel example The following example written in Eiffel sets the value of a class attribute hour based on a caller-provided argument a_hour. The postcondition follows the keyword ensure. In this example, the postcondition guarantees, in cases in which the precondition holds (i.e., when a_hour represents a valid hour of the day), that after the execution of set_hour, the class attribute hour will have the same value as a_hour. The tag hour_set describes this postcondition clause and serves to identify it in case of a runtime postcondition violation.

Postconditions and inheritance In the presence of inheritance, the routines inherited by descendant classes (subclasses) do so with their contracts, that is their preconditions and postconditions, in force. This means that any implementations or redefinitions of inherited routines also have to be written to comply with their inherited contracts. Postconditions can be modified in redefined routines, but they may only be strengthened. That is, the redefined routine may increase the benefits it provides to the client, but may not decrease those benefits.

See also Precondition Design by contract Hoare logic Invariants maintained by conditions Database trigger

References

Worked examples

Example 1 — a first encounter with Postcondition

Start with the simplest possible case. Write down what Postcondition claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Postcondition before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Postcondition ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Postcondition

In research: Postcondition appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Postcondition in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Postcondition is common in secondary-school and first-year university syllabi. It links to neighbouring topics Formal methods, Logic in computer science, Mathematics of computing, so understanding it makes those chapters shorter.
In everyday life: Look for Postcondition outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Postcondition in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Postcondition means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Postcondition out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Postcondition in simple terms?

Why does Postcondition matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Postcondition?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Postcondition.