precondition

A precondition is a condition that must be true before the execution of an operation or function begins. In logic and computer science, preconditions encode the assumptions under which a statement, rule, or program is considered valid. In program verification, a precondition specifies when a command may be executed to guarantee a desired outcome described by a postcondition.

In design by contract, preconditions form part of the agreement between a caller and a callee. They state what the caller must guarantee when invoking a procedure and what the callee is assured to receive. The corresponding postcondition describes what will be true after the operation completes, assuming the precondition held. Invariants are conditions that must remain true throughout the execution of a program or within a data structure.

Common examples illustrate the idea: a division operation typically has a precondition that the divisor is not zero; an array access requires the index to be within bounds; a function that returns a non-null result requires certain properties of the input. Precondition concepts also appear in formal methods, where they delineate the permissible starting state for reasoning about program behavior.

Handling preconditions involves input validation and contract checking. If a precondition is violated, the system may raise an exception, abort the operation, or signal an error. Good practice includes documenting preconditions in public interfaces, using assertions or contract checks, and leveraging static analysis where possible. Designers should distinguish between what callers must guarantee and what the callee guarantees, aiming for robust, maintainable interfaces with clear, verifiable preconditions.