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A variable is something which is subject to change; the term is most commonly used in computer science and mathematics, where it denotes a quantity or symbolic representation (one which is often unknown).

 Table of contents 1 General Overview 2 Why Are Variables Useful? 3 Computer Science

## General Overview

Variables are used in open sentences. For instance, in the formula: x + 1 = 5, x is a variable which represents an "unknown" number. In mathematics, variables are usually represented by letters of the Roman alphabet, but are also represented by letters of other alphabets; as well as various other symbols. In computer programming, variables are usually represented by either single letters or alphanumeric stringss.

## Why Are Variables Useful?

Variables are useful in mathematics and computer programming 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 square of ANY number: square(x) = x * x.

Now, all we need to do to find the square of a number is replace x with any number we want.

• square(x) = x * x = y
• square(1) = 1 * 1 = 1
• square(2) = 2 * 2 = 4
• square(3) = 3 * 3 = 9
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. In computer programming languages without referential transparency, such changes can occur.

## Computer Science

In most programming languages, there are 2 types of variables: global and local variables. Global variables exist throughout a program, whereas local variables only exist within a given statement block or function. In some languages, variables are defined via an explicit declaration, in others they are declared implicitly by their first use.