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PHP has hundreds of base functions and thousands more from extensions. Prior to PHP version 5.3.0, functions are not first-class functions and can only be referenced by their name, whereas PHP 5.3.0 introduces closures. [35] User-defined functions can be created at any time and without being prototyped. [35]
An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been declared by that code. [1]In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time.
Predicate variables should be distinguished from predicate constants, which could be represented either with a different (exclusive) set of predicate letters, or by their own symbols which really do have their own specific meaning in their domain of discourse: e.g. =, , , <, ,....
A third way is by declaring and defining a variable as being "constant". A global variable or static variable can be declared (or a symbol defined in assembly) with a keyword qualifier such as const, constant, or final, meaning that its value will be set at compile time and should not be changeable at runtime. Compilers generally put static ...
because the argument to f must be a variable integer, but i is a constant integer. This matching is a form of program correctness, and is known as const-correctness.This allows a form of programming by contract, where functions specify as part of their type signature whether they modify their arguments or not, and whether their return value is modifiable or not.
In relational databases a virtual column is a table column whose value(s) is automatically computed using other columns values, or another deterministic expression. Virtual columns are defined of SQL:2003 as Generated Column, [1] and are only implemented by some DBMSs, like MariaDB, SQL Server, Oracle, PostgreSQL, SQLite and Firebird (database server) (COMPUTED BY syntax).
Defining logical constants is a major part of the work of Gottlob Frege and Bertrand Russell. Russell returned to the subject of logical constants in the preface to the second edition (1937) of The Principles of Mathematics noting that logic becomes linguistic: "If we are to say anything definite about them, [they] must be treated as part of ...
The language of congruence arithmetic [1]: 140 is defined as the est of Boolean combinations, where the atomic predicates are: the addition of a constant + =, with an integral constant, the order relation , the modular relations, with a fixed modular value.