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:

  1. # Assign the value 10 to the variable x
  2. julia> x = 10
  3. 10
  5. # Doing math with x's value
  6. julia> x + 1
  7. 11
  9. # Reassign x's value
  10. julia> x = 1 + 1
  11. 2
  13. # You can assign values of other types, like strings of text
  14. julia> x = "Hello World!"
  15. "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).

  1. julia> x = 1.0
  2. 1.0
  4. julia> y = -3
  5. -3
  7. julia> Z = "My string"
  8. "My string"
  10. julia> customary_phrase = "Hello world!"
  11. "Hello world!"
  13. julia> UniversalDeclarationOfHumanRightsStart = "人人生而自由，在尊严和权利上一律平等。"
  14. "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:

  1. julia> δ = 0.00001
  2. 1.0e-5
  4. julia> 안녕하세요 = "Hello"
  5. "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 α̂⁽²⁾ by \alpha-tab-\hat- tab-\^(2)-tab. (If you find a symbol somewhere, e.g. in someone else’s code, that you don’t know how to type, the REPL help will tell you: just type ? and then paste the symbol.)

Julia will even let you redefine built-in constants and functions if needed (although this is not recommended to avoid potential confusions):

  1. julia> pi = 3
  2. 3
  4. julia> pi
  5. 3
  7. julia> sqrt = 4
  8. 4

However, if you try to redefine a built-in constant or function already in use, Julia will give you an error:

  1. julia> pi
  2. π = 3.1415926535897...
  4. julia> pi = 3
  5. ERROR: cannot assign a value to imported variable Base.pi from module Main
  7. julia> sqrt(100)
  8. 10.0
  10. julia> sqrt = 4
  11. ERROR: cannot assign a value to imported variable Base.sqrt from module Main

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). Operators can also be suffixed with modifying marks, primes, and sub/superscripts, e.g. +̂ₐ″ is parsed as an infix operator with the same precedence as +. A space is required between an operator that ends with a subscript/superscript letter and a subsequent variable name. For example, if +ᵃ is an operator, then +ᵃx must be written as +ᵃ x to distinguish it from + ᵃx where ᵃx is the variable name.

A particular class of variable names is one that contains only underscores. These identifiers can only be assigned values, which are immediately discarded, and cannot therefore be used to assign values to other variables (i.e., they cannot be used as rvalues#Assignment:_l-values_and_r-values)) or use the last value assigned to them in any way.

  1. julia> x, ___ = size([2 2; 1 1])
  2. (2, 2)
  4. julia> y = ___
  5. ERROR: syntax: all-underscore identifier used as rvalue
  7. julia> println(___)
  8. ERROR: syntax: all-underscore identifier used as rvalue

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

  1. julia> else = false
  2. ERROR: syntax: unexpected "else"
  4. julia> try = "No"
  5. ERROR: syntax: unexpected "="

Some Unicode characters are considered to be equivalent in identifiers. Different ways of entering Unicode combining characters (e.g., accents) are treated as equivalent (specifically, Julia identifiers are NFC. Julia also includes a few non-standard equivalences for characters that are visually similar and are easily entered by some input methods. The Unicode characters ɛ (U+025B: Latin small letter open e) and µ (U+00B5: micro sign) are treated as equivalent to the corresponding Greek letters. The middle dot · (U+00B7) and the Greek interpunct · (U+0387) are both treated as the mathematical dot operator ⋅ (U+22C5). The minus sign − (U+2212) is treated as equivalent to the hyphen-minus sign - (U+002D).

Assignment expressions and assignment versus mutation

An assignment variable = value “binds” the name variable to the value computed on the right-hand side, and the whole assignment is treated by Julia as an expression equal to the right-hand-side value. This means that assignments can be chained (the same value assigned to multiple variables with variable1 = variable2 = value) or used in other expressions, and is also why their result is shown in the REPL as the value of the right-hand side. (In general, the REPL displays the value of whatever expression you evaluate.) For example, here the value 4 of b = 2+2 is used in another arithmetic operation and assignment:

  1. julia> a = (b = 2+2) + 3
  2. 7
  4. julia> a
  5. 7
  7. julia> b
  8. 4

A common confusion is the distinction between assignment (giving a new “name” to a value) and mutation (changing a value). If you run a = 2 followed by a = 3, you have changed the “name” a to refer to a new value 3 … you haven’t changed the number 2, so 2+2 will still give 4 and not 6! This distinction becomes more clear when dealing with mutable types like arrays, whose contents can be changed:

  1. julia> a = [1,2,3] # an array of 3 integers
  2. 3-element Vector{Int64}:
  3.  1
  4.  2
  5.  3
  7. julia> b = a   # both b and a are names for the same array!
  8. 3-element Vector{Int64}:
  9.  1
  10.  2
  11.  3

Here, the line b = a does not make a copy of the array a, it simply binds the name b to the same array a: both b and a “point” to one array [1,2,3] in memory. In contrast, an assignment a[i] = value changes the contents of the array, and the modified array will be visible through both the names a and b:

  1. julia> a[1] = 42     # change the first element
  2. 42
  4. julia> a = 3.14159   # a is now the name of a different object
  5. 3.14159
  7. julia> b   # b refers to the original array object, which has been mutated
  8. 3-element Vector{Int64}:
  9.  42
  10.   2
  11.   3

That is, a[i] = value (an alias for setindex!) mutates an existing array object in memory, accessible via either a or b. Subsequently setting a = 3.14159 does not change this array, it simply binds a to a different object; the array is still accessible via b. The other common syntax to mutate an existing object is a.field = value (an alias for setproperty!), which can be used to change a mutable struct.

When you call a function in Julia, it behaves as if you assigned the argument values to new variable names corresponding to the function arguments, as discussed in Argument-Passing Behavior. (By convention, functions that mutate one or more of their arguments have names ending with !.)

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 Types and Modules begin with a capital letter and word separation is shown with upper camel case instead of underscores.
  • Names of functions and macros 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.

For more information about stylistic conventions, see the Style Guide.