变量

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 提供了极其灵活的变量命名系统。变量名区分大小写。

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，在尊严和权力上一律平等。"
"人人生而自由，在尊严和权力上一律平等。"

也可以使用 Unicode 字符（UTF-8 编码）来命名：

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 甚至允许重新定义内置的常数和函数：

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

很显然, 不鼓励这样的做法。

可用的变量名

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).

内置的关键字不能当变量名：

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

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

命名规范

尽管 Julia 对命名本身只有很少的限制, 但尽量遵循一定的命名规范吧：

  • 变量名使用小写字母
  • 单词间使用下划线 ('_') 分隔，但不鼓励
  • 类型名首字母大写, 单词间使用驼峰式分隔.
  • 函数名和宏名使用小写字母, 不使用下划线分隔单词.
  • 修改参数的函数结尾使用 ! . 这样的函数被称为 mutating functions 或 in-place functions