Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
, a precondition is a condition or predicate that must always be true just prior to the execution of some section of code
Code
A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....
In computer science, a formal specification is a mathematical description of software or hardware that may be used to develop an implementation. It describes what the system should do, not how the system should do it...
.
If a precondition is violated, the effect of the section of code
Code
A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....
becomes undefined and thus may or may not carry out its intended work. Security
Computer security
Computer security is a branch of computer technology known as information security as applied to computers and networks. The objective of computer security includes protection of information and property from theft, corruption, or natural disaster, while allowing the information and property to...
problems can arise due to incorrect preconditions.
Often, preconditions are simply included in the documentation of the affected section of code. Preconditions are sometimes tested using assertions
Assertion (computing)
In computer programming, an assertion is a predicate placed in a program to indicate that the developer thinks that the predicate is always true at that place.For example, the following code contains two assertions:...
within the code itself, and some languages have specific syntactic constructions for doing so.
In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n...
is only defined for integers greater than or equal to zero. So a program that calculates the factorial of an input number would have preconditions that the number be an integer and that it be greater than or equal to zero.
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
Design by contract , also known as programming by contract and design-by-contract programming, is an approach to designing computer software...
. Design by contract also includes notions of postcondition
Postcondition
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...
In computer science, a predicate is called an invariant to a sequence of operations provided that: if the predicate is true before starting the sequence, then it is true at the end of the sequence.-Use:...
.
The precondition for any routine defines any constraints on object state which are necessary for successful execution. From the program developer's viewpoint, this constitutes the routine caller's portion of the contract. The caller then is obligated to ensure that the precondition holds prior to calling the routine. The reward for the caller's effort is expressed in the called routines postcondition
Postcondition
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...
.
Eiffel example
The routine in the following example written in Eiffel
Eiffel (programming language)
Eiffel is an ISO-standardized, object-oriented programming language designed by Bertrand Meyer and Eiffel Software. The design of the language is closely connected with the Eiffel programming method...
takes as an argument an integer which must be a valid value for an hour of the day, i. e., 0 through 23, inclusively. The precondition follows the keyword require. It specifies that the argument must be greater than or equal to zero and less than or equal to 23. The tag "valid_argument:" describes this precondition clause and serves to identify it in case of a runtime precondition violation.
set_hour (a_hour: INTEGER)
-- Set `hour' to `a_hour'
require
valid_argument: a_hour >= 0 and a_hour <= 23
do
hour := a_hour
ensure
hour_set: hour = a_hour
end
Preconditions and inheritance
In the presence of inheritance, the routines inherited by descendant classes (subclasses) do so with their preconditions in force. This means that any implementations or redefinitions of inherited routines also have to be written to comply with their inherited contract. Preconditions can be modified in redefined routines, but they may only be weakened. That is, the redefined routine may lessen the obligation of the client, but not increase it.
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...
Hoare logic is a formal system with a set of logical rules for reasoning rigorously about the correctness of computer programs. It was proposed in 1969 by the British computer scientist and logician C. A. R. Hoare, and subsequently refined by Hoare and other researchers...
In computer science, a predicate is called an invariant to a sequence of operations provided that: if the predicate is true before starting the sequence, then it is true at the end of the sequence.-Use:...
A database trigger is procedural code that is automatically executed in response to certain events on a particular table or view in a database. The trigger is mostly used for keeping the integrity of the information on the database...
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