Variable (high-level programming)
In high-level programming, a variable is an abstract storage or indirection location paired with an associated symbolic name, which contains some known or unknown quantity of data or object referred to as a value; or in simpler terms, a variable is a named container for a particular set of bits or type of data or undefined. A variable can eventually be associated with or identified by a memory address. The variable name is the usual way to reference the stored value, in addition to referring to the variable itself, depending on the context. This separation of name and content allows the name to be used independently of the exact information it represents. The identifier in computer source code can be bound to a value during run time, and the value of the variable may thus change during the course of program execution.
Variables in programming may not directly correspond to the concept of variables in mathematics. The latter is abstract, having no reference to a physical object such as storage location. The value of a computing variable is not necessarily part of an equation or formula as in mathematics. Furthermore, the variables can also be constants if the value is defined statically. Variables in computer programming are frequently given long names to make them relatively descriptive of their use, whereas variables in mathematics often have terse, one- or two-character names for brevity in transcription and manipulation.
A variable's storage location may be referenced by several different identifiers, a situation known as aliasing. Assigning a value to the variable using one of the identifiers will change the value that can be accessed through the other identifiers.
Compilers have to replace variables' symbolic names with the actual locations of the data. While a variable's name, type, and location often remain fixed, the data stored in the location may be changed during program execution.
Actions on a variable
In imperative programming languages, values can generally be accessed or changed at any time. 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 variables.Depending on the type system of a programming language, variables may only be able to store a specified data type. Alternatively, a datatype may be associated only with the current value, allowing a single variable to store anything supported by the programming language. Variables are the containers for storing the values.
Variables and scope:
- Automatic variables: Each local variable in a function comes into existence only when the function is called, and disappears when the function is exited. Such variables are known as automatic variables.
- External variables: These are variables that are external to a function and can be accessed by name by any function. These variables remain in existence permanently; rather than appearing and disappearing as functions are called and exited, they retain their values even after the functions that set them have returned.
Identifiers referencing a variable
For instance, a variable might be referenced by the identifier "" and the variable can contain the number 1956. If the same variable is referenced by the identifier "" as well, and if using this identifier "", the value of the variable is altered to 2009, then reading the value using the identifier "" will yield a result of 2009 and not 1956.
If a variable is only referenced by a single identifier, that identifier can simply be called the name of the variable; otherwise, we can speak of it as one of the names of the variable. For instance, in the previous example the identifier "" is the name of the variable in question, and "" is another name of the same variable.
Scope and extent
The scope of a variable describes where in a program's text the variable may be used, while the extent of a variable describes when in a program's execution the variable has a value. The scope of a variable affects its extent. The scope of a variable is actually a property of the name of the variable, and the extent is a property of the storage location of the variable. These should not be confused with context, which is a property of the program, and varies by point in the program's text or execution—see scope: an overview. Further, object lifetime may coincide with variable lifetime, but in many cases is not tied to it.Scope is an important part of the name resolution of a variable. Most languages define a specific scope for each variable, which may differ within a given program. The scope of a variable is the portion of the program's text 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 function/subroutine, or more finely within a block of expressions/statements ; this is static resolution, performable at parse-time or compile-time. Alternatively, a variable with dynamic scope is resolved at run-time, based on a global binding stack that depends on the specific control flow. 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 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.
Unless the programming language features garbage collection, a variable whose extent permanently outlasts its scope can result in a memory leak, whereby the memory allocated for the variable can never be freed since the variable which would be used to reference it for deallocation purposes is no longer accessible. However, it can be 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. A variable whose scope begins before its extent does is said to be uninitialized and often has an undefined, arbitrary value if accessed, since it has yet to be explicitly given a particular value. A variable whose extent ends before its scope may become a dangling pointer and deemed uninitialized once more since its value has been destroyed. Variables described by the previous two cases may be said to be 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.
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, thus 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 name spaces, or to make individual variables "private" through either dynamic variable scoping or lexical variable scoping.
Many programming languages employ a reserved value to indicate an invalid or uninitialized variable.
Typing
In statically typed languages such as C, C++, Java or C#, a variable also has a type, meaning that only certain kinds of values can be stored in it. For example, a variable of type "integer" is prohibited from storing text values.In dynamically typed languages such as Python, a variable's type is inferred by its value, and can change according to its value. In Common Lisp, both situations exist simultaneously: A variable is given a type 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 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 number 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,def add_two -> int:
return x + 2
- prints: 7
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.