Variable (programming)

In computer programming, a variable is an identifier (usually a letter, word, or phrase) that is linked to a value stored in the system's memory or an expression that can be evaluated. For instance, a variable might be called " " and contain a number.

In imperative programming languages, values can generally be accessed or changed at any time. However, in pure functional and logic languages, variables are bound to expressions and keep a single value during their entire lifetime due to the requirements of referential transparency. In imperative languages, the same behavior is exhibited by constants, which are typically contrasted with normal variables.

Depending on the type system of a programming language, variables may only be able to store a specified datatype (e.g. integer or string). Alternatively a datatype may be associated only with the current value, allowing a single variable to store anything supported by the programming language.

The use of the term "variable" is based on the similar mathematical concept of the same name.

Naming conventions
Unlike their mathematical counterparts, programming variables and constants commonly take multiple-character names, e.g.  or. Single-character names are most commonly used only for auxiliary variables; for instance,,  ,   for array index variables.

In some programming languages, specific characters (known as sigils) are prefixed or appended to variable identifiers to indicate the variable's type. For example:
 * In BASIC, the suffix $ on a variable name indicates that its value is a string;
 * In Perl, the sigils $, @, %, and & indicate scalar, array, hash, and subroutine variables, respectively.
 * In spreadsheets variables can refer to cells (e.g. ), named ranges, or values in associated source code or functions.

In source code
In computer source code, a variable name is one way to bind a variable to a memory location; the corresponding value is stored as a data object in that location so that the object can be accessed and manipulated later via the variable's name.

In spreadsheets
In a spreadsheet, a cell may contain a formula with references to other cells. Such a cell reference is a kind of variable; its value is the value of the referenced cell (see also: reference (computer science)).

Scope and extent
The scope of a variable describes where in a program's text, the variable may be used, while the extent (or lifetime) describes when in a program's execution a variable has a value. The scope of a variable is actually a property of the name of the variable, and the extent is a property of the variable itself.

A variable name's scope affects its extent.

Scope is a lexical aspect of a variable. Most languages define a specific scope for each variable (as well as any other named entity), which may differ within a given program. The scope of a variable is the portion of the program code for which the variable's name has meaning and for which the variable is said to be "visible". Entrance into that scope typically begins a variable's lifetime and exit from that scope typically ends its lifetime. For instance, a variable with "lexical scope" is meaningful only within a certain block of statements or subroutine. Variables only accessible within a certain functions are termed "local variables". A "global variable", or one with indefinite scope, may be referred to anywhere in the program.

Extent, on the other hand, is a runtime (dynamic) aspect of a variable. Each binding of a variable to a value can have its own extent at runtime. The extent of the binding is the portion of the program's execution time during which the variable continues to refer to the same value or memory location. A running program may enter and leave a given extent many times, as in the case of a closure.

In portions of code, a variable in scope may never have been given a value, or its value may have been destroyed. Such variables are described as "out of extent" or "unbound". In many languages, it is an error to try to use the value of a variable when it is out of extent. In other languages, doing so may yield unpredictable results. Such a variable may, however, be assigned a new value, which gives it a new extent. By contrast, it is permissible for a variable binding to extend beyond its scope, as occurs in Lisp closures and C static local variables. When execution passes back into the variable's scope, the variable may once again be used.

For space efficiency, a memory space needed for a variable may be allocated only when the variable is first used and freed when it is no longer needed. A variable is only needed when it is in scope, but beginning each variable's lifetime when it enters scope may give space to unused variables. To avoid wasting such space, compilers often warn programmers if a variable is declared but not used.

It is considered good programming practice to make the scope of variables as narrow as feasible so that different parts of a program do not accidentally interact with each other by modifying each other's variables. Doing so also prevents action at a distance. Common techniques for doing so are to have different sections of a program use different namespaces, or to make individual variables "private" through either dynamic variable scoping or lexical variable scoping.

Many programming languages employ a reserved value (often named null or nil) to indicate an invalid or uninitialized variable.

Typing
In statically-typed languages such as Java or ML, a variable also has a type, meaning that only values of a given class (or set of classes) can be stored in it. A variable of a primitive type holds a value of that exact primitive type. A variable of a class type can hold a null reference or a reference to an object whose type is that class type or any subclass of that class type. A variable of an interface type can hold a null reference or a reference to an instance of any class that implements the interface. A variable of an array type can hold a null reference or a reference to an array.

In dynamically-typed languages such as Python, it is values, not variables, which carry type. In Common Lisp, both situations exist simultaneously: a variable is given a type (if undeclared, it is assumed to be, the universal supertype) which exists at compile time. Values also have types, which can be checked and queried at runtime.

Typing of variables also allows polymorphisms to be resolved at compile time. However, this is different from the polymorphism used in object-oriented function calls (referred to as virtual functions in C++) which resolves the call based on the value type as opposed to the supertypes the variable is allowed to have.

Variables often store simple data-like integers and literal strings, but some programming languages allow a variable to store values of other datatypes as well. Such languages may also enable functions to be parametric polymorphic. These functions operate like variables to represent data of multiple types. For example, a function named  may determine the length of a list. Such a  function may be parametric polymorphic by including a type variable in its type signature, since the amount of elements in the list is independent of the elements' types.

Parameters
The formal parameters of functions are also referred to as variables. For instance, in this Python code segment,

The variable named  is a parameter because it is given a value when the function is called. The integer 5 is the argument which gives  its value. In most languages, function parameters have local scope. This specific variable named  can only be referred to within the   function (though of course other functions can also have variables called  ).

Memory allocation
The specifics of variable allocation and the representation of their values vary widely, both among programming languages and among implementations of a given language. Many language implementations allocate space for local variables, whose extent lasts for a single function call on the call stack, and whose memory is automatically reclaimed when the function returns. (More generally, in name binding, the name of a variable is bound to the address of some particular block (contiguous sequence) of bytes in memory, and operations on the variable manipulate that block. Referencing is more common for variables whose values have large or unknown sizes when the code is compiled. Such variables reference the location of the value instead of the storing value itself, which is allocated from a pool of memory called the heap.

Bound variables have values. A value, however, is an abstraction, an idea; in implementation, a value is represented by some data object, which is stored somewhere in computer memory. The program, or the runtime environment, must set aside memory for each data object and, since memory is finite, ensure that this memory is yielded for reuse when the object is no longer needed to represent some variable's value.

Objects allocated from the heap must be reclaimed&mdash;especially when the objects are no longer needed. In a garbage-collected language (such as C#, Java, and Lisp), the runtime environment automatically reclaims objects when extant variables can no longer refer to them. In non-garbage-collected languages, such as C, the program (and the programmer) must explicitly allocate memory, and then later free it, to reclaim its memory. Failure to do so leads to memory leaks, in which the heap is depleted as the program runs, risking eventual failure from exhausting available memory.

When a variable refers to a data structure created dynamically, some of its components may be only indirectly accessed through the variable. In such circumstances, garbage collectors (or analogous program features in languages that lack garbage collectors) must deal with a case where only a portion of the memory reachable from the variable needs to be reclaimed.

Interpolation
Variable interpolation (also variable substitution, variable expansion) is the process of evaluating an expression or string literal containing one or more variables, yielding a result in which the variables are replaced with their corresponding values in memory. It is a specialized instance of concatenation.

Languages that support variable interpolation include Perl, PHP, Ruby, and most Unix shells. In these languages, variable interpolation only occurs when the string literal is double-quoted, but not when it is single-quoted. The variables are recognized because variables start with a sigil (typically "$") in these languages.

For example, the following Perl code: produces the output: Nancy said Hello World to the crowd of people.

Ruby uses the "#" symbol for interpolation, and lets you interpolate any expression, not just variables.