Precondition

Precondition

A precondition is a condition or set of conditions that must be met before a certain process or function can be executed. In computer science, preconditions are often used in the context of software development and programming to ensure that a function or method operates correctly. Preconditions are typically specified in the form of assertions or checks that validate the input parameters or the state of the system before the execution of a function.

Usage in Computer Science

In computer programming, preconditions are used to define the necessary conditions that must be true before a function or method is executed. If the preconditions are not met, the function may not behave as expected, and it may result in errors or undefined behavior. Preconditions are a fundamental concept in design by contract, a methodology introduced by Bertrand Meyer in the context of the Eiffel programming language.

Design by Contract

Design by contract (DbC) is a programming methodology that uses preconditions, postconditions, and invariants to define the contract between a function and its callers. Preconditions specify what must be true before a function is called, postconditions specify what must be true after the function has executed, and invariants specify conditions that must always be true during the execution of a program.

Assertions

In many programming languages, preconditions are implemented using assertions. An assertion is a statement that checks whether a condition is true. If the condition is false, the program typically raises an error or exception. Assertions are used to catch programming errors early in the development process by validating the assumptions made by the programmer.

Examples

Python

In Python, preconditions can be implemented using the `assert` statement:

def divide(a, b):
    assert b != 0, "Denominator must not be zero"
    return a / b

In this example, the precondition `b != 0` ensures that the denominator is not zero before performing the division.

Java

In Java, preconditions can be implemented using the `assert` keyword or by using libraries such as Google Guava:

public int divide(int a, int b) {
    assert b != 0 : "Denominator must not be zero";
    return a / b;
}

Using Google Guava:

import com.google.common.base.Preconditions;

public int divide(int a, int b) {
    Preconditions.checkArgument(b != 0, "Denominator must not be zero");
    return a / b;
}

Related Concepts

• Postcondition
• Invariant (computer science)
• Assertion (software development)
• Exception handling
• Defensive programming

See Also

• Design by contract
• Software testing
• Formal verification
• Programming language

Related Pages

• Postcondition
• Invariant (computer science)
• Assertion (software development)
• Exception handling
• Defensive programming
• Design by contract
• Software testing
• Formal verification
• Programming language