In
computer programming
Computer programming or coding is the composition of sequences of instructions, called computer program, programs, that computers can follow to perform tasks. It involves designing and implementing algorithms, step-by-step specifications of proc ...
, a precondition is a condition or
predicate that must always be true just prior to the execution of some section of
code
In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communicati ...
or before an operation in a
formal specification.
If a precondition is violated, the effect of the section of
code
In communications and information processing, code is a system of rules to convert information—such as a letter, word, sound, image, or gesture—into another form, sometimes shortened or secret, for communication through a communicati ...
becomes undefined and thus may or may not carry out its intended work. Preconditions that are missing, insufficient, or not formally proved (or have an incorrect attempted proof), or are not checked statically or dynamically, can give rise to
Security problems, particularly in unsafe languages that are not strongly typed.
Often, preconditions are simply included in the documentation of the affected section of code. Preconditions are sometimes tested using
guards or
assertions within the code itself, and some languages have specific syntactic constructions for doing so.
Example
The
factorial
In mathematics, the factorial of a non-negative denoted is the Product (mathematics), product of all positive integers less than or equal The factorial also equals the product of n with the next smaller factorial:
\begin
n! &= n \times ...
function is only defined where its parameter is an integer greater than or equal to zero. So an implementation of the factorial function would have a precondition that its parameter be an integer ''and'' that the parameter be greater than or equal to zero. Alternatively the type system of the language may be used to specify that the parameter of the factorial function is a natural number (unsigned integer), which can be formally verified automatically by a compiler's type checker.
In addition where numeric types have a limited range (as they do in most programming languages) the precondition must also specify the maximum value that the parameter may have if overflow is not to occur. (e.g. if an implementation of factorial returns the result in a 64-bit unsigned integer then the parameter must be less than 21 because factorial(21) is larger than the maximum unsigned integer that can be stored in 64 bits).
Where the language supports range sub-types (e.g.
Ada) such constraints can be automatically verified by the type system. More complex constraints can be formally verified interactively with a
proof assistant.
In object-oriented programming
Preconditions in
object-oriented software development are an essential part of
design by contract. Design by contract also includes notions of
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 w ...
and
class invariant.
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 obliged to ensure that the precondition holds prior to calling the routine. The reward for the caller's effort is expressed in the called routine's
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 w ...
.
Eiffel example
The routine in the following example written in
Eiffel 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: 0 <= a_hour 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.
[Meyer, 1997, pp. 570–573.] That is, the redefined routine may lessen the obligation of the client, but not increase it.
See also
*
Design by contract
*
Guard (computer science)
*
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 w ...
*
Hoare logic
Hoare logic (also known as Floyd–Hoare logic or Hoare rules) 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 l ...
*
Invariants maintained by conditions
*
Database trigger
References
{{reflist
Programming constructs
Formal methods
Logic in computer science