Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
, a variable is a symbolic name given to some known or unknown quantity or information, for the purpose of allowing the name to be used independently of the information it represents. A variable name in computer source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
and thus also its contents, and these may change during the course of program execution.
In mathematics, a variable is a value that may change within the scope of a given problem or set of operations. In contrast, a constant is a value that remains unchanged, though often unknown or undetermined. The concepts of constants and variables are fundamental to many areas of mathematics and...
. The value of a computing variable is not necessarily part of an equation
Equation
An equation is a mathematical statement that asserts the equality of two expressions. In modern notation, this is written by placing the expressions on either side of an equals sign , for examplex + 3 = 5\,asserts that x+3 is equal to 5...
In mathematics, a formula is an entity constructed using the symbols and formation rules of a given logical language....
as in mathematics. In computing, a variable may be employed in a repetitive process: assigned a value in one place, then used elsewhere, then reassigned a new value and used again in the same way (see iteration). 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.
Compilers have to replace variables' symbolic names with the actual locations of the data. While the variable name, type, and location generally remain fixed, the data stored in the location may get altered during program execution.
Identifiers referencing a variable
An identifier referencing a variable can be used to access the variable in order to read out the value, or alter the value, or modify the attributes
Attribute (computing)
In computing, an attribute is a specification that defines a property of an object, element, or file. It may also refer to or set the specific value for a given instance of such....
In computer science, a lock is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. Locks are one way of enforcing concurrency control policies.-Types:...
In computer science, a semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel programming environment....
, etc.
For instance, a variable might be referenced by the identifier "total_count" and the variable can contain the number 1956. If the same variable is referenced by the identifier "x" as well, and if using this identifier "x", the value of the variable is altered to 2009, then reading the value using the identifier "total_count", the result is 2009 and not 1956.
If in a particular programming language a variable can only be referenced by a single identifier, that can simply be called the name of the variable. Otherwise we can talk about one of the names of the variable. For instance, in the previous example, the "total_count" is a name of the variable in question, and "x" is another name of the same variable.
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
In computer programming, an assignment statement sets or re-sets the value stored in the storage location denoted by a variable name. In most imperative computer programming languages, assignment statements are one of the basic statements...
In computer programming, a function may be described as pure if both these statements about the function hold:# The function always evaluates the same result value given the same argument value...
In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...
Logic programming is, in its broadest sense, the use of mathematical logic for computer programming. In this view of logic programming, which can be traced at least as far back as John McCarthy's [1958] advice-taker proposal, logic is used as a purely declarative representation language, and a...
In mathematics, and in other disciplines involving formal languages, including mathematical logic and computer science, a free variable is a notation that specifies places in an expression where substitution may take place...
to expressions and keep a single value during their entire lifetime
Scope (programming)
In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
due to the requirements of referential transparency. In imperative languages, the same behavior is exhibited by constant
Constant (programming)
In computer programming, a constant is an identifier whose associated value cannot typically be altered by the program during its execution...
s, which are typically contrasted with normal variables.
A type system associates a type with each computed value. By examining the flow of these values, a type system attempts to ensure or prove that no type errors can occur...
of a programming language, variables may only be able to store a specified datatype (e.g. integer
Integer (computer science)
In computer science, an integer is a datum of integral data type, a data type which represents some finite subset of the mathematical integers. Integral data types may be of different sizes and may or may not be allowed to contain negative values....
In formal languages, which are used in mathematical logic and theoretical computer science, a string is a finite sequence of symbols that are chosen from a set or alphabet....
). Alternatively a datatype may be associated only with the current value, allowing a single variable to store anything supported by the programming language.
Naming conventions
Unlike their mathematical counterparts, programming variables and constants commonly take multiple-character names, e.g. COST or total. Single-character names are most commonly used only for auxiliary variables; for instance, i, j, k for array
Array data type
In computer science, an array type is a data type that is meant to describe a collection of elements , each selected by one or more indices that can be computed at run time by the program. Such a collection is usually called an array variable, array value, or simply array...
index variables.
Some naming conventions are enforced at the language level as part of the language syntax and involve the format of valid identifiers. In almost all languages, variable names cannot start with a digit (0-9) and cannot contain whitespace characters. Whether, which, and when punctuation marks are permitted in variable names varies from language to language; many languages only permit the underscore (_) in variable names and forbid all other punctuation. In some programming languages, specific (often punctuation) characters (known as sigil
Sigil (computer programming)
In computer programming, a sigil is a symbol attached to a variable name, showing the variable's datatype or scope. In 1999 Philip Gwyn adopted the term "to mean the funny character at the front of a Perl variable".- Historical context:...
s) are prefixed or appended to variable identifiers to indicate the variable's type.
Case-sensitivity of variable names also varies between languages and some languages require the use of a certain case in naming certain entities;For example, Haskell
Haskell (programming language)
Haskell is a standardized, general-purpose purely functional programming language, with non-strict semantics and strong static typing. It is named after logician Haskell Curry. In Haskell, "a function is a first-class citizen" of the programming language. As a functional programming language, the...
requires that names of types start with a capital letter. Most modern languages are case-sensitive; some older languages are not. Some languages reserve certain forms of variable names for their own internal use; in many languages, names beginning with 2 underscores ("__") often fall under this category.
However, beyond the basic restrictions imposed by a language, the naming of variables is largely a matter of style. At the machine code level, variable names are not used, so the exact names chosen do not matter to the computer. Thus names of variables identify them, for the rest they are just a tool for programmers to make programs easier to write and understand.
Programmers often create and adhere to code style guidelines which offer guidance on naming variables or impose a precise naming scheme. Shorter names are faster to type but are less descriptive; longer names often make programs easier to read and the purpose of variables easier to understand. However, extreme verbosity in variable names can also lead to less comprehensible code.
A spreadsheet is a computer application that simulates a paper accounting worksheet. It displays multiple cells usually in a two-dimensional matrix or grid consisting of rows and columns. Each cell contains alphanumeric text, numeric values or formulas...
In mathematics, a formula is an entity constructed using the symbols and formation rules of a given logical language....
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)
Reference (computer science)
In computer science, a reference is a value that enables a program to indirectly access a particular data item, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the data item, and accessing those data is called...
In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
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 (meaningful) 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
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
. Variables only accessible within a certain functions are termed "local variable
Local variable
In computer science, a local variable is a variable that is given local scope. Such a variable is accessible only from the function or block in which it is declared. In programming languages with only two levels of visibility, local variables are contrasted with global variables...
In computer programming, a global variable is a variable that is accessible in every scope . Interaction mechanisms with global variables are called global environment mechanisms...
", 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
Name binding
In programming languages, name binding is the association of objects with identifiers. An identifier bound to an object is said to reference that object. Machine languages have no built-in notion of identifiers, but name-object bindings as a service and notation for the programmer is implemented...
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
Closure (computer science)
In computer science, a closure is a function together with a referencing environment for the non-local variables of that function. A closure allows a function to access variables outside its typical scope. Such a function is said to be "closed over" its free variables...
.
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.
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
ML is a general-purpose functional programming language developed by Robin Milner and others in the early 1970s at the University of Edinburgh, whose syntax is inspired by ISWIM...
, 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.
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
, it is values, not variables, which carry type. In Common Lisp
Common Lisp
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...
, both situations exist simultaneously: a variable is given a type (if undeclared, it is assumed to be T, the universal supertype
Subtype
In programming language theory, subtyping or subtype polymorphism is a form of type polymorphism in which a subtype is a datatype that is related to another datatype by some notion of substitutability, meaning that program constructs, typically subroutines or functions, written to operate on...
) which exists at compile time. Values also have types, which can be checked and queried at runtime.
In computer science, polymorphism is a programming language feature that allows values of different data types to be handled using a uniform interface. The concept of parametric polymorphism applies to both data types and functions...
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++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
) 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 length may determine the length of a list. Such a length function may be parametric polymorphic by including a type variable in its type signature
Type signature
In computer science, a type signature or type annotation defines the inputs and outputs for a function, subroutine or method. A type signature includes at least the function name and the number of its arguments...
, 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
Python (programming language)
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
code segment,
def addtwo(x):
return x + 2
addtwo(5) # yields 7
The variable named x is a parameter because it is given a value when the function is called. The integer 5 is the argument which gives x its value. In most languages, function parameters have local scope . This specific variable named x can only be referred to within the addtwo function (though of course other functions can also have variables called x).
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 variable
Local variable
In computer science, a local variable is a variable that is given local scope. Such a variable is accessible only from the function or block in which it is declared. In programming languages with only two levels of visibility, local variables are contrasted with global variables...
s, whose extent lasts for a single function call on the call stack
Call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, control stack, run-time stack, or machine stack, and is often shortened to just "the stack"...
, and whose memory is automatically reclaimed when the function returns. More generally, in name binding
Name binding
In programming languages, name binding is the association of objects with identifiers. An identifier bound to an object is said to reference that object. Machine languages have no built-in notion of identifiers, but name-object bindings as a service and notation for the programmer is implemented...
, 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
Reference (computer science)
In computer science, a reference is a value that enables a program to indirectly access a particular data item, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the data item, and accessing those data is called...
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 storing the 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
Object (computer science)
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
, 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—especially when the objects are no longer needed. In a garbage-collected
Garbage collection (computer science)
In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program...
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, 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
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
, the program (and the programmer) must explicitly allocate
Malloc
C dynamic memory allocation refers to performing dynamic memory allocation in the C via a group of functions in the C standard library, namely malloc, realloc, calloc and free....
memory, and then later free it, to reclaim its memory. Failure to do so leads to memory leak
Memory leak
A memory leak, in computer science , occurs when a computer program consumes memory but is unable to release it back to the operating system. In object-oriented programming, a memory leak happens when an object is stored in memory but cannot be accessed by the running code...
s, in which the heap is depleted as the program runs, risking eventual failure from exhausting available memory.
In computer science, a data structure is a particular way of storing and organizing data in a computer so that it can be used efficiently.Different kinds of data structures are suited to different kinds of applications, and some are highly specialized to specific tasks...
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 or 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
Concatenation
In computer programming, string concatenation is the operation of joining two character strings end-to-end. For example, the strings "snow" and "ball" may be concatenated to give "snowball"...
.
Languages that support variable interpolation include Perl
Perl
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
Tcl is a scripting language created by John Ousterhout. Originally "born out of frustration", according to the author, with programmers devising their own languages intended to be embedded into applications, Tcl gained acceptance on its own...
A Unix shell is a command-line interpreter or shell that provides a traditional user interface for the Unix operating system and for Unix-like systems...
s. 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
Sigil (computer programming)
In computer programming, a sigil is a symbol attached to a variable name, showing the variable's datatype or scope. In 1999 Philip Gwyn adopted the term "to mean the funny character at the front of a Perl variable".- Historical context:...
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
):
$name = "Alice";
print "${name} said Hello World to the crowd of people.";
Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto...
uses the "#" symbol for interpolation, and lets you interpolate any expression, not just variables. Other languages may support more advanced interpolation with a special formatting function, such as printf
Printf
Printf format string refers to a control parameter used by a class of functions typically associated with some types of programming languages. The format string specifies a method for rendering an arbitrary number of varied data type parameter into a string...
, in which the first argument, the format, specifies the pattern in which the remaining arguments are substituted.
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