# Variables

A variable, in Julia, is a name associated (or bound) to a value. It’s useful when you want to store a value (that you obtained after some math, for example) for later use. For example:

    # Assign the value 10 to the variable x
    julia> x = 10
    10

    # Doing math with x's value
    julia> x + 1
    11

    # Reassign x's value
    julia> x = 1 + 1
    2

    # You can assign values of other types, like strings of text
    julia> x = "Hello World!"
    "Hello World!"

Julia provides an extremely flexible system for naming variables. Variable names are case-sensitive, and have no semantic meaning (that is, the language will not treat variables differently based on their names).

    julia> x = 1.0
    1.0

    julia> y = -3
    -3

    julia> Z = "My string"
    "My string"

    julia> customary_phrase = "Hello world!"
    "Hello world!"

    julia> UniversalDeclarationOfHumanRightsStart = "人人生而自由，在尊严和权利上一律平等。"
    "人人生而自由，在尊严和权利上一律平等。"

Unicode names (in UTF-8 encoding) are allowed:

    julia> δ = 0.00001
    1.0e-5

    julia> 안녕하세요 = "Hello"
    "Hello"

In the Julia REPL and several other Julia editing environments, you can type many Unicode math symbols by typing the backslashed LaTeX symbol name followed by tab. For example, the variable name `δ` can be entered by typing `\delta`-tab, or even `α̂₂` by `\alpha`-tab-`\hat`-tab-`\_2`-tab.

Julia will even let you redefine built-in constants and functions if needed:

    julia> pi
    π = 3.1415926535897...

    julia> pi = 3
    WARNING: imported binding for pi overwritten in module Main
    3

    julia> pi
    3

    julia> sqrt(100)
    10.0

    julia> sqrt = 4
    WARNING: imported binding for sqrt overwritten in module Main
    4

However, this is obviously not recommended to avoid potential confusion.

## Allowed Variable Names

Variable names must begin with a letter (A-Z or a-z), underscore, or a subset of Unicode code points greater than 00A0; in particular, Unicode character categories Lu/Ll/Lt/Lm/Lo/Nl (letters), Sc/So (currency and other symbols), and a few other letter-like characters (e.g. a subset of the Sm math symbols) are allowed. Subsequent characters may also include ! and digits (0-9 and other characters in categories Nd/No), as well as other Unicode code points: diacritics and other modifying marks (categories Mn/Mc/Me/Sk), some punctuation connectors (category Pc), primes, and a few other characters.

Operators like `+` are also valid identifiers, but are parsed specially. In some contexts, operators can be used just like variables; for example `(+)` refers to the addition function, and `(+) = f` will reassign it. Most of the Unicode infix operators (in category Sm), such as `⊕`, are parsed as infix operators and are available for user-defined methods (e.g. you can use `const ⊗ = kron` to define `⊗` as an infix Kronecker product).

The only explicitly disallowed names for variables are the names of built-in statements:

    julia> else = false
    ERROR: syntax: unexpected "else"
     ...

    julia> try = "No"
    ERROR: syntax: unexpected "="
     ...

## Stylistic Conventions

While Julia imposes few restrictions on valid names, it has become useful to adopt the following conventions:

- Names of variables are in lower case.
- Word separation can be indicated by underscores (`'_'`), but use of underscores is discouraged unless the name would be hard to read otherwise.
- Names of `Type`s and `Module`s begin with a capital letter and word separation is shown with upper camel case instead of underscores.
- Names of `function`s and `macro`s are in lower case, without underscores.
- Functions that write to their arguments have names that end in `!`. These are sometimes called “mutating” or “in-place” functions because they are intended to produce changes in their arguments after the function is called, not just return a value.