Variable
In
computer science and
mathematics, a
variable (sometimes called a
pronumeral) is a symbol denoting a
quantity or
symbolic representation. In mathematics, a variable often represents an
unknown quantity that has the potential to
change; in computer science, it represents a place where a quantity can be stored. Variables are often contrasted with
constants, which are known and unchanging.
In
applied statistics, a
variable is a measurable factor, characteristic, or attribute of an individual or a system – in other words, something that might be expected to vary over time or between individuals.
In
mathematical statistics, 'variable' has a technical meaning -
random variables are defined in the mathematical context of
measure theory as
measurable functions from a
probability space to a measurable space.
Variables are used in
open sentences. For instance, in the formula:
x + 1 = 5,
x is a variable which represents an "unknown"
number. Variables are often represented by
letters of the
Roman alphabet, but are also represented by letters of other
alphabets, such as the
Greek alphabet, as well as various other
symbols.
In this sense, variables are used as a "fill the blank" within many fields - mathematics, linguistics, and so on.
In statistics, variables refer to measurable attributes, as these typically vary over time or between individuals. Variables can be
continuous (taking values from a continuum) or
discrete (taking values from a defined set). Temperature is a continuous variable, while number of legs of an animal is a discrete variable. This concept of a variable is widely used in the
natural,
medical and
social sciences.
In
causal models, a distinction is made between
"independent variables" and
"dependent variables", the latter being expected to vary in value in response to changes in the former. In other words, an independent variable is presumed to potentially affect a dependent one. In experiments, independent variables include factors that can be altered or chosen by the researcher independent of other factors.
For example, in an experiment to test whether the
boiling point of
water changes with
altitude, the altitude is under direct control and is the
independent variable, and the boiling point is presumed to depend upon it and is therefore the
dependent variable. The collection of results from an experiment, or information to be used to draw conclusions, is known as
data. It is often important to consider which variables to allow for, or to directly control or eliminate, in the
design of experiments.
While independent variables can refer to quantities and qualities that are under experimental control, they can also include extraneous factors that influence results in a confusing or undesired manner.
In general, if strongly confounding variables exist that can substantially affect the result, then this makes it more difficult to interpret the results. For example, a study into the incidence of
cancer with
age will also have to take into account
variables such as
income (poor people may have less healthy lives),
location (some cancers vary depending on diet and sunlight),
stress and
lifestyle issues (cancer may be related to these more than age), and so on. Failure to at least consider these factors can lead to grossly inaccurate
deductions. For this reason controlling unwanted variables is important in research.
Variables are useful in mathematics because they allow instructions to be specified in a general way. If one were forced to use actual values, then the instructions would only apply in a more narrow, and specific set of situations. For example:specify a mathematical definition for finding the number twice that of ANY number: double(
x) =
x +
x.
Now, all we need to do to find the double of a number is replace
x with any number we want.
*double(1) = 1 + 1 = 2
*double(3) = 3 + 3 = 6
*double(55) = 55 + 55 = 110
*:etc...
In the above example, the variable
x is a "
placeholder" for ANY number. One important thing we are assuming is that the value of each occurrence of
x is the same -- that
x does not get a new value between the first
x and the second
x.
Note that in
computer programming languages without
referential transparency, changes such as this
can occur. Variables in
computer programming are also useful for this reason. The term variable, as used by programmers, is much more vague than the term as used by mathematicians. Variables allow the programmer to specify relationships between
data.
In
computer programming, variables are usually represented by
alphanumeric strings. A
variable can be thought of as a place to store a
value in
computer memory.
More precisely, a variable binds (associates) a
name (sometimes called an
identifier) with the location; a value in turn is stored as a data
object in that location so that the object can be accessed later via the variable, much like referring to someone by name. Variables in
computer programming are analogous to variables in mathematics. Put in another way, an object could exist without being bound to a variable, but without such a referent, it would be inaccessible from code.
Variable names
Variables are denoted by
identifiers.
In some programming languages, specific characters are prepended 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 prefixes
$,
@,
%, and
& indicate scalar, array, hash, and subroutine variables, respectively.
Following is an example of variable declarations and assignments in
C++ and
C#:
int a = 10;
string b = "hello";
Scope and extent
The
scope of a variable describes where in a program's text a variable may be used, while
extent (or
lifetime) describes when in a program's execution a variable has a value. A variable's scope affects its extent.
Scope is a
lexical aspect 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 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 life-time and exit from that scope typically ends its life-time. For instance, a variable with
lexical scope is meaningful only within a certain block of statements or
subroutine. A
global variable, or one with
indefinite scope, may be referred to anywhere in the program. It is erroneous to refer to a variable where it is out of scope.
Lexical analysis of a program can determine whether variables are used out of scope. In compiled languages, such analysis can be performed statically, at compile-time.
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 a 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 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 life-time 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.
Typed and untyped variables
In
statically-typed languages such as
Java or
ML, a variable also has
type, meaning that only values of a given class (or set of classes) can be stored in it. In
dynamically-typed languages such as
Python or
Lisp, it is values and not variables which carry type.
See type system.Typing of variables also allows
polymorphisms to be resolved at compile time.
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. Such 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 since the amount of elements in the list is independent of the elements' types.
Parameters
The
arguments or
formal parameters of functions are also referred to as variables. For instance, in these equivalent functions in Python and Lisp
def addtwo(x): return x + 2
(defun addtwo (x) (+ x 2))
the variable named
x is an argument. It is given a value when the function is called. In most languages, function arguments 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 any given language. Many language implementations allocate space for
local variables, whose extent lasts for a single function call, on the
call stack, 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 value have large or unknown sizes when the code is compiled. Such variables reference the location of (i.e. the
pointer to) 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 re-use when the object is no longer needed to represent some variable's value.
Objects allocated from the
heap must be reclaimed specially when the objects are no longer needed. In a
garbage-collected language, such as C#, Java, and Lisp, the runtime environment automatically "reaps" objects when extant variables can no longer refer to them. In
non-garbage-collected languages, such as C, the program (and thus the programmer) must explicitly
allocate memory and 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 of 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 the case where only a portion of the memory reachable from a variable needs to be reclaimed.
Mutable vs. immutable
Variables can be either
mutable or
immutable. Mutable variables are said to have both an
l-value and an
r-value, but immutable have only an
r-value. In functional programming, all variables are immutable. Because immutable variables are semantically the same as named constants or constant functions, the term "variable" generally usually indicates a mutable variable.
In C++, but not in C, "
mutable" is a keyword to allow a mutable member to be modified by a
const member function.
Constants
A
constant variable is similar to an immutable variable, but while the value referenced by an immutable variable cannot change during program execution, with a constant it is the location referenced by the constant that cannot change. Since constant variables are often bound to literal values, which are themselves immutable, constant variables are often themselves immutable (e.g.
const int HoursPerDay=24). Although a constant value is specified only once, the constant variable can be referenced multiple times in a program. Using a constant instead of specifying a value multiple times in the program can simplify code maintenance, not only to simplify changing its value but also to supply a meaningful name for it and to consolidate such constant assignments to a standard code location, for example at the beginning.
Programming languages provide one of two kinds of constant variables:
Static constant or Manifest constant: Languages such as Visual Basic allow assigning to static constant a fixed value, which will be known before the program starts. Such a constant has the same value each time its program runs. Changing the value is accomplished by changing (and possibly recompiling) the code. E.g.: CONST a = 60
;Dynamic constant: Languages such as Java allow assigning to a dynamic constant an expression, possibly involving non-constant operands. The value of such constants may rely on variables defined while a computer program is running. Thus, unlike static constants, the values of dynamic constants cannot be determined at compile time. E.g.: final int a = b + 20;.
Although such dynamic constants have fixed values if they are primitive types (e.g. int), their values may change if they contain a reference to an object, as shown in the following example:
final StringBuffer sampleDynamicConstant = new StringBuffer ("InitialValueOfDynamicConstant"); sampleDynamicConstant.append("_AppendedText"); System.out.println(sampleDynamicConstant);
The above code produces the following output:
InitialValueOfDynamicConstant_AppendedText
*
Undefined variable
