Constant (computer programming)

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Constant (computer programming)

In computer programming, a constant is a value that cannot be altered by the program during normal execution, i.e., the value is constant. When associated with an identifier, a constant is said to be "named," although the terms "constant" and "named constant" are often used interchangeably. This is contrasted with a variable, which is an identifier with a value that can be changed during normal execution, i.e., the value is variable. Constants are useful for both programmers and compilers: For programmers they are a form of self-documenting code and allow reasoning about correctness, while for compilers they allow compile-time andrun-time checks that verify that constancy assumptions are not violated, and allow or simplify some compiler optimizations. There are various specific realizations of the general notion of a constant, with subtle distinctions that are often overlooked. The most significant are: compile-time (statically-valued) constants, run-time (dynamically-valued) constants, immutable objects, and constant types (const). Typical examples of compile-time constants include mathematical constants, values from standards (here maximum transmission unit), or internal configuration values (here characters per line), such as these C examples: const float PI = 3.1415927; // maximal single float precisionconst unsigned int MTU = 1500; // Ethernet v2, RFC 894const unsigned int COLUMNS = 80; Typical examples of run-time constants are values calculated based on inputs to a function, such as this C++ example: void f(std::string s) { const size_t l = s.length(); // ...}

Spanish wikipedia http://es.wikipedia.org/wiki/Constante_(informática)

English Wikipedia http://en.wikipedia.org/wiki/Constant_(computer_programming)