Julia 是一种新的同像函数式编程语言(homoiconic functional language)，它专注于科学计算领域。 虽然拥有同像宏(homoiconic macros)、一级函数(first-class functions)和底层控制等全部功能，但 Julia 依旧和 Python 一样易于学习和使用。
示例代码基于 Julia 1.0.0
x# 单行注释只需要一个井号「#」#= 多行注释 只需要以「#=」开始「=#」结束 还可以嵌套.=####################################################### 1. 原始类型与操作符##################################################### Julia 中一切皆为表达式# 这是一些基本数字类型typeof(3) # => Int64typeof(3.2) # => Float64typeof(2 + 1im) # => Complex{Int64}typeof(2 // 3) # => Rational{Int64}# 支持所有的普通中缀操作符1 + 1 # => 28 - 1 # => 710 * 2 # => 2035 / 5 # => 7.010 / 2 # => 5.0 # 整数除法总是返回浮点数div(5, 2) # => 2 # 使用 div 可以获得整除的结果5 \ 35 # => 7.02^2 # => 4 # 幂运算，不是异或 (xor)12 % 10 # => 2# 用括号提高优先级(1 + 3) * 2 # => 8# 位操作符~2 # => -3 # 按位非 (not)3 & 5 # => 1 # 按位与 (and)2 | 4 # => 6 # 按位或 (or)xor(2, 4) # => 6 # 按位异或 (xor)2 >>> 1 # => 1 # 逻辑右移2 >> 1 # => 1 # 算术右移2 << 1 # => 4 # 逻辑/算术左移# 可以用函数 bitstring 查看二进制数。bitstring(12345)# => "0000000000000000000000000000000000000000000000000011000000111001"bitstring(12345.0)# => "0100000011001000000111001000000000000000000000000000000000000000"# 布尔值是原始类型truefalse# 布尔操作符!true # => false!false # => true1 == 1 # => true2 == 1 # => false1 != 1 # => false2 != 1 # => true1 < 10 # => true1 > 10 # => false2 <= 2 # => true2 >= 2 # => true# 链式比较1 < 2 < 3 # => true2 < 3 < 2 # => false# 字符串可以由「"」创建"This is a string."# 字符字面量可用「'」创建'a'# 字符串使用 UTF-8 编码# 可以像取数组取值一样用 index 取出对应字符ascii("This is a string")[1]# => 'T': ASCII/Unicode U+0054 (category Lu: Letter, uppercase) # Julia 的 index 从 1 开始 :(# 但只有在字符串仅由 ASCII 字符构成时，字符串才能够被安全的引索# 因此建议使用遍历器 (map, for loops, 等)# $ 可用于字符插值:"2 + 2 = $(2 + 2)" # => "2 + 2 = 4"# 可以将任何 Julia 表达式放入括号。# 另一种输出格式化字符串的方法是使用标准库 Printf 中的 Printf 宏using Printf "%d is less than %f\n" 4.5 5.3 # => 5 is less than 5.300000# 打印字符串很容易println("I'm Julia. Nice to meet you!") # => I'm Julia. Nice to meet you!# 字符串可以按字典序进行比较"good" > "bye" # => true"good" == "good" # => true"1 + 2 = 3" == "1 + 2 = $(1 + 2)" # => true###################################################### 2. 变量与集合##################################################### 给变量赋值就是声明变量some_var = 5 # => 5some_var # => 5# 访问未声明变量会抛出异常try some_other_var # => ERROR: UndefVarError: some_other_var not definedcatch e println(e)end# 变量名必须以下划线或字母开头# 之后任何字母，数字，下划线，叹号都是合法的。SomeOtherVar123! = 6 # => 6# 甚至可以用 unicode 字符☃ = 8 # => 8# 用数学符号非常方便2 * π # => 6.283185307179586# 注意 Julia 的命名规约:## * 名称可以用下划线「_」分割。# 不过一般不推荐使用下划线，除非不用变量名就会变得难于理解## * 类型名以大写字母开头，单词以 CamelCase 方式连接，无下划线。## * 函数与宏的名字小写，无下划线。## * 会改变输入的函数名末位为「!」。# 这类函数有时被称为 mutating functions 或 in-place functions.# 数组存储一列值，index 从 1 开始a = Int64[] # => 0-element Array{Int64,1}# 一维数组可以以逗号分隔值的方式声明b = [4, 5, 6] # => 3-element Array{Int64,1}: [4, 5, 6]b = [4; 5; 6] # => 3-element Array{Int64,1}: [4, 5, 6]b[1] # => 4b[end] # => 6# 二维数组以分号分隔维度matrix = [1 2; 3 4] # => 2×2 Array{Int64,2}: [1 2; 3 4]# 指定数组的类型b = Int8[4, 5, 6] # => 3-element Array{Int8,1}: [4, 5, 6]# 使用 push! 和 append! 往数组末尾添加元素push!(a, 1) # => [1]push!(a, 2) # => [1,2]push!(a, 4) # => [1,2,4]push!(a, 3) # => [1,2,4,3]append!(a, b) # => [1,2,4,3,4,5,6]# 用 pop 弹出尾部的元素pop!(b) # => 6b # => [4,5]# 再放回去push!(b, 6) # => [4,5,6]b # => [4,5,6]a[1] # => 1 # 永远记住 Julia 的引索从 1 开始！而不是 0！# 用 end 可以直接取到最后索引。它可以用在任何索引表达式中a[end] # => 6# 数组还支持 popfirst! 和 pushfirst!popfirst!(a) # => 1 a # => [2,4,3,4,5,6]pushfirst!(a, 7) # => [7,2,4,3,4,5,6]a # => [7,2,4,3,4,5,6]# 以叹号结尾的函数名表示它会改变参数的值arr = [5,4,6] # => 3-element Array{Int64,1}: [5,4,6]sort(arr) # => [4,5,6]arr # => [5,4,6]sort!(arr) # => [4,5,6]arr # => [4,5,6]# 数组越界会抛出 BoundsErrortry a[0] # => ERROR: BoundsError: attempt to access 7-element Array{Int64,1} at # index [0] # => Stacktrace: # => [1] getindex(::Array{Int64,1}, ::Int64) at .\array.jl:731 # => [2] top-level scope at none:0 # => [3] ... # => in expression starting at ...\LearnJulia.jl:203 a[end + 1] # => ERROR: BoundsError: attempt to access 7-element Array{Int64,1} at # index [8] # => Stacktrace: # => [1] getindex(::Array{Int64,1}, ::Int64) at .\array.jl:731 # => [2] top-level scope at none:0 # => [3] ... # => in expression starting at ...\LearnJulia.jl:211catch e println(e)end# 报错时错误会指出出错的文件位置以及行号，标准库也一样# 你可以在 Julia 安装目录下的 share/julia 文件夹里找到这些标准库# 可以用 range 初始化数组a = [1:5;] # => 5-element Array{Int64,1}: [1,2,3,4,5]# 注意！分号不可省略a2 = [1:5] # => 1-element Array{UnitRange{Int64},1}: [1:5]# 可以切割数组a[1:3] # => [1, 2, 3]a[2:end] # => [2, 3, 4, 5]# 用 splice! 切割原数组arr = [3,4,5]splice!(arr, 2) # => 4 arr # => [3,5]# 用 append! 连接数组b = [1,2,3]append!(a, b) # => [1, 2, 3, 4, 5, 1, 2, 3]a # => [1, 2, 3, 4, 5, 1, 2, 3]# 检查元素是否在数组中in(1, a) # => true# 用 length 获得数组长度length(a) # => 8# 元组(Tuples)是不可变的tup = (1, 2, 3) # => (1,2,3)typeof(tup) # => Tuple{Int64,Int64,Int64}tup[1] # => 1try tup[1] = 3 # => ERROR: MethodError: no method matching # setindex!(::Tuple{Int64,Int64,Int64}, ::Int64, ::Int64)catch e println(e)end# 大多数组的函数同样支持元组length(tup) # => 3tup[1:2] # => (1,2)in(2, tup) # => true# 可以将元组的元素解包赋给变量a, b, c = (1, 2, 3) # => (1,2,3) a # => 1b # => 2c # => 3# 不用括号也可以d, e, f = 4, 5, 6 # => (4,5,6)d # => 4e # => 5f # => 6# 单元素 tuple 不等于其元素值(1,) == 1 # => false(1) == 1 # => true# 交换值e, d = d, e # => (5,4) d # => 5e # => 4# 字典用于储存映射(mappings)（键值对）empty_dict = Dict() # => Dict{Any,Any} with 0 entries# 也可以用字面量创建字典filled_dict = Dict("one" => 1, "two" => 2, "three" => 3)# => Dict{String,Int64} with 3 entries:# => "two" => 2, "one" => 1, "three" => 3# 用 [] 获得键值filled_dict["one"] # => 1# 获得所有键keys(filled_dict)# => Base.KeySet for a Dict{String,Int64} with 3 entries. Keys:# => "two", "one", "three"# 注意，键的顺序不是插入时的顺序# 获得所有值values(filled_dict)# => Base.ValueIterator for a Dict{String,Int64} with 3 entries. Values: # => 2, 1, 3# 注意，值的顺序也一样# 用 in 检查键值是否已存在，用 haskey 检查键是否存在in(("one" => 1), filled_dict) # => truein(("two" => 3), filled_dict) # => falsehaskey(filled_dict, "one") # => truehaskey(filled_dict, 1) # => false# 获取不存在的键的值会抛出异常try filled_dict["four"] # => ERROR: KeyError: key "four" not foundcatch e println(e)end# 使用 get 可以提供默认值来避免异常# get(dictionary,key,default_value)get(filled_dict, "one", 4) # => 1get(filled_dict, "four", 4) # => 4# Set 表示无序不可重复的值的集合empty_set = Set() # => Set(Any[])# 初始化一个带初值的 Setfilled_set = Set([1, 2, 2, 3, 4]) # => Set([4, 2, 3, 1])# 新增值push!(filled_set, 5) # => Set([4, 2, 3, 5, 1])# 检查 Set 中是否存在某值in(2, filled_set) # => truein(10, filled_set) # => false# 交集，并集，差集other_set = Set([3, 4, 5, 6]) # => Set([4, 3, 5, 6])intersect(filled_set, other_set) # => Set([4, 3, 5])union(filled_set, other_set) # => Set([4, 2, 3, 5, 6, 1])setdiff(Set([1,2,3,4]), Set([2,3,5])) # => Set([4, 1])###################################################### 3. 控制语句##################################################### 声明一个变量some_var = 5# 这是一个 if 语句块，其中的缩进不是必须的if some_var > 10 println("some_var is totally bigger than 10.")elseif some_var < 10 # elseif 是可选的 println("some_var is smaller than 10.")else # else 也是可选的 println("some_var is indeed 10.")end# => some_var is smaller than 10.# For 循环遍历# 可迭代的类型包括：Range, Array, Set, Dict 和 AbstractStringfor animal = ["dog", "cat", "mouse"] println("$animal is a mammal") # 你可以用 $ 将变量或表达式插入字符串中 end# => dog is a mammal# => cat is a mammal# => mouse is a mammal# 你也可以不用「=」而使用「in」for animal in ["dog", "cat", "mouse"] println("$animal is a mammal")end# => dog is a mammal# => cat is a mammal# => mouse is a mammalfor pair in Dict("dog" => "mammal", "cat" => "mammal", "mouse" => "mammal") from, to = pair println("$from is a $to")end# => mouse is a mammal# => cat is a mammal# => dog is a mammal# 注意！这里的输出顺序和上面的不同for (k, v) in Dict("dog" => "mammal", "cat" => "mammal", "mouse" => "mammal") println("$k is a $v")end# => mouse is a mammal# => cat is a mammal# => dog is a mammal# While 循环let x = 0 while x < 4 println(x) x += 1 # x = x + 1 的缩写 endend# => 0# => 1# => 2# => 3# 用 try/catch 处理异常try error("help")catch e println("caught it $e")end# => caught it ErrorException("help")###################################################### 4. 函数##################################################### 关键字 function 用于定义函数# function name(arglist)# body...# endfunction add(x, y) println("x is $x and y is $y") # 函数会返回最后一行的值 x + yendadd(5, 6)# => x is 5 and y is 6# => 11# 更紧凑的定义函数f_add(x, y) = x + y # => f_add (generic function with 1 method)f_add(3, 4) # => 7# 函数可以将多个值作为元组返回fn(x, y) = x + y, x - y # => fn (generic function with 1 method)fn(3, 4) # => (7, -1)# 还可以定义接收可变长参数的函数function varargs(args...) return args # 使用 return 可以在函数内的任何地方返回end# => varargs (generic function with 1 method)varargs(1,2,3) # => (1,2,3)# 省略号「...」称为 splat# 刚刚用在了函数定义中# 在调用函数时也可以使用它，此时它会把数组或元组解包为参数列表add([5,6]...) # 等价于 add(5,6)x = (5, 6) # => (5,6)add(x...) # 等价于 add(5,6)# 可定义带可选参数的函数function defaults(a, b, x=5, y=6) return "$a $b and $x $y"end# => defaults (generic function with 3 methods)defaults('h', 'g') # => "h g and 5 6"defaults('h', 'g', 'j') # => "h g and j 6"defaults('h', 'g', 'j', 'k') # => "h g and j k"try defaults('h') # => ERROR: MethodError: no method matching defaults(::Char) defaults() # => ERROR: MethodError: no method matching defaults()catch e println(e)end# 还可以定义带关键字参数的函数function keyword_args(;k1=4, name2="hello") # 注意分号 ';' return Dict("k1" => k1, "name2" => name2)end# => keyword_args (generic function with 1 method)keyword_args(name2="ness") # => ["name2"=>"ness", "k1"=>4]keyword_args(k1="mine") # => ["name2"=>"hello", "k1"=>"mine"]keyword_args() # => ["name2"=>"hello", "k1"=>4]# 可以在一个函数中组合各种类型的参数function all_the_args(normal_arg, optional_positional_arg=2; keyword_arg="foo") println("normal arg: $normal_arg") println("optional arg: $optional_positional_arg") println("keyword arg: $keyword_arg")end# => all_the_args (generic function with 2 methods)all_the_args(1, 3, keyword_arg=4)# => normal arg: 1# => optional arg: 3# => keyword arg: 4# Julia 有一等函数function create_adder(x) adder = function (y) return x + y end return adderend# => create_adder (generic function with 1 method)# 这是用 "stabby lambda syntax" 创建的匿名函数(x -> x > 2)(3) # => true# 这个函数和上面的 create_adder 是等价的function create_adder(x) y -> x + yend# => create_adder (generic function with 1 method)# 你也可以给内部函数起个名字function create_adder(x) function adder(y) x + y end adderend# => create_adder (generic function with 1 method)add_10 = create_adder(10) # => (::getfield(Main, Symbol("#adder#11")){Int64}) # (generic function with 1 method)add_10(3) # => 13# 内置的高阶函数有map(add_10, [1,2,3]) # => [11, 12, 13]filter(x -> x > 5, [3, 4, 5, 6, 7]) # => [6, 7]# 还可以使用 list comprehensions 让 map 更美观[add_10(i) for i = [1, 2, 3]] # => [11, 12, 13][add_10(i) for i in [1, 2, 3]] # => [11, 12, 13]###################################################### 5. 类型##################################################### Julia 有类型系统# 所有的值都有类型；但变量本身没有类型# 你可以用 `typeof` 函数获得值的类型typeof(5) # => Int64# 类型是一等值typeof(Int64) # => DataTypetypeof(DataType) # => DataType# DataType 是代表类型的类型，也代表他自己的类型# 类型可用于文档化代码、执行优化以及多重派分(dispatch)# Julia 并不只是静态的检查类型# 用户还可以自定义类型# 就跟其它语言的 record 或 struct 一样# 用 `struct` 关键字定义新的类型# struct Name# field::OptionalType# ...# endstruct Tiger taillength::Float64 coatcolor # 不带类型标注相当于 `::Any`end# 默认构造函数的参数是类型的属性，按类型定义中的顺序排列tigger = Tiger(3.5, "orange") # => Tiger(3.5, "orange")# 用新类型作为构造函数还会创建一个类型sherekhan = typeof(tigger)(5.6, "fire") # => Tiger(5.6, "fire")# 类似 struct 的类型被称为具体类型# 它们可被实例化，但不能有子类型# 另一种类型是抽象类型# 抽象类型名abstract type Cat end # 仅仅是指向类型结构层次的一个名称# 抽象类型不能被实例化，但可以有子类型# 例如，Number 就是抽象类型subtypes(Number) # => 2-element Array{Any,1}: # => Complex # => Realsubtypes(Cat) # => 0-element Array{Any,1}# AbstractString，类如其名，也是一个抽象类型subtypes(AbstractString) # => 4-element Array{Any,1}: # => String # => SubString # => SubstitutionString # => Test.GenericString# 所有的类型都有父类型。可以用函数 `supertype` 得到父类型typeof(5) # => Int64supertype(Int64) # => Signedsupertype(Signed) # => Integersupertype(Integer) # => Realsupertype(Real) # => Numbersupertype(Number) # => Anysupertype(supertype(Signed)) # => Realsupertype(Any) # => Any# 除了 Int64 外，其余的类型都是抽象类型typeof("fire") # => Stringsupertype(String) # => AbstractStringsupertype(AbstractString) # => Anysupertype(SubString) # => AbstractString# <: 是子类型化操作符struct Lion <: Cat # Lion 是 Cat 的子类型 mane_color roar::AbstractStringend# 可以继续为你的类型定义构造函数# 只需要定义一个与类型同名的函数，并调用已有的构造函数得到正确的类型Lion(roar::AbstractString) = Lion("green", roar) # => Lion# 这是一个外部构造函数，因为它在类型定义之外struct Panther <: Cat # Panther 也是 Cat 的子类型 eye_color Panther() = new("green") # Panthers 只有这个构造函数，没有默认构造函数end# 像 Panther 一样使用内置构造函数，让你可以控制如何构建类型的值# 应该尽量使用外部构造函数，而不是内部构造函数###################################################### 6. 多分派##################################################### Julia 中所有的函数都是通用函数，或者叫做泛型函数(generic functions)# 也就是说这些函数都是由许多小方法组合而成的# Lion 的每一种构造函数都是通用函数 Lion 的一个方法# 我们来看一个非构造函数的例子# 首先，让我们定义一个函数 meow# Lion, Panther, Tiger 的 meow 定义分别为function meow(animal::Lion) animal.roar # 使用点记号「.」访问属性end# => meow (generic function with 1 method)function meow(animal::Panther) "grrr"end# => meow (generic function with 2 methods)function meow(animal::Tiger) "rawwwr"end# => meow (generic function with 3 methods)# 试试 meow 函数meow(tigger) # => "rawwwr"meow(Lion("brown", "ROAAR")) # => "ROAAR"meow(Panther()) # => "grrr"# 回顾类型的层次结构Tiger <: Cat # => falseLion <: Cat # => truePanther <: Cat # => true# 定义一个接收 Cat 类型的函数function pet_cat(cat::Cat) println("The cat says $(meow(cat))")end# => pet_cat (generic function with 1 method)pet_cat(Lion("42")) # => The cat says 42try pet_cat(tigger) # => ERROR: MethodError: no method matching pet_cat(::Tiger)catch e println(e)end# 在面向对象语言中，通常都是单分派# 这意味着使用的方法取决于第一个参数的类型# 而 Julia 中选择方法时会考虑到所有参数的类型# 让我们定义一个有更多参数的函数，这样我们就能看出区别function fight(t::Tiger, c::Cat) println("The $(t.coatcolor) tiger wins!")end# => fight (generic function with 1 method)fight(tigger, Panther()) # => The orange tiger wins!fight(tigger, Lion("ROAR")) # => fight(tigger, Lion("ROAR"))# 让我们修改一下传入 Lion 类型时的行为fight(t::Tiger, l::Lion) = println("The $(l.mane_color)-maned lion wins!")# => fight (generic function with 2 methods)fight(tigger, Panther()) # => The orange tiger wins!fight(tigger, Lion("ROAR")) # => The green-maned lion wins!# 我们不需要一只老虎参与战斗fight(l::Lion, c::Cat) = println("The victorious cat says $(meow(c))")# => fight (generic function with 3 methods)fight(Lion("balooga!"), Panther()) # => The victorious cat says grrrtry fight(Panther(), Lion("RAWR")) # => ERROR: MethodError: no method matching fight(::Panther, ::Lion) # => Closest candidates are: # => fight(::Tiger, ::Lion) at ... # => fight(::Tiger, ::Cat) at ... # => fight(::Lion, ::Cat) at ... # => ...catch e println(e)end# 试试把 Cat 放在前面fight(c::Cat, l::Lion) = println("The cat beats the Lion")# => fight (generic function with 4 methods)# 由于无法判断该使用哪个 fight 方法，而产生了错误try fight(Lion("RAR"), Lion("brown", "rarrr")) # => ERROR: MethodError: fight(::Lion, ::Lion) is ambiguous. Candidates: # => fight(c::Cat, l::Lion) in Main at ... # => fight(l::Lion, c::Cat) in Main at ... # => Possible fix, define # => fight(::Lion, ::Lion) # => ...catch e println(e)end# 在不同版本的 Julia 中错误信息可能有所不同fight(l::Lion, l2::Lion) = println("The lions come to a tie") # => fight (generic function with 5 methods)fight(Lion("RAR"), Lion("brown", "rarrr")) # => The lions come to a tie# 深入编译器之后# 你还可以看看 llvm 以及它生成的汇编代码square_area(l) = l * l # => square_area (generic function with 1 method)square_area(5) # => 25# 当我们喂给 square_area 一个整数时会发生什么？code_native(square_area, (Int32,), syntax = :intel) # .text # ; Function square_area { # ; Location: REPL[116]:1 # 函数序言 (Prologue) # push rbp # mov rbp, rsp # ; Function *; { # ; Location: int.jl:54 # imul ecx, ecx # 求 l 的平方，并把结果放在 ECX 中 # ;} # mov eax, ecx # pop rbp # 还原旧的基址指针(base pointer) # ret # 返回值放在 EAX 中 # nop dword ptr [rax + rax] # ;}# 使用 syntax 参数指定输出语法。默认为 AT&T 格式，这里指定为 Intel 格式code_native(square_area, (Float32,), syntax = :intel) # .text # ; Function square_area { # ; Location: REPL[116]:1 # push rbp # mov rbp, rsp # ; Function *; { # ; Location: float.jl:398 # vmulss xmm0, xmm0, xmm0 # 标量双精度乘法 (AVX) # ;} # pop rbp # ret # nop word ptr [rax + rax] # ;}code_native(square_area, (Float64,), syntax = :intel) # .text # ; Function square_area { # ; Location: REPL[116]:1 # push rbp # mov rbp, rsp # ; Function *; { # ; Location: float.jl:399 # vmulsd xmm0, xmm0, xmm0 # 标量双精度乘法 (AVX) # ;} # pop rbp # ret # nop word ptr [rax + rax] # ;}# 注意！只要参数中有浮点数，Julia 就会使用浮点指令# 让我们计算一下圆的面积circle_area(r) = pi * r * r # => circle_area (generic function with 1 method)circle_area(5) # => 78.53981633974483code_native(circle_area, (Int32,), syntax = :intel) # .text # ; Function circle_area { # ; Location: REPL[121]:1 # push rbp # mov rbp, rsp # ; Function *; { # ; Location: operators.jl:502 # ; Function *; { # ; Location: promotion.jl:314 # ; Function promote; { # ; Location: promotion.jl:284 # ; Function _promote; { # ; Location: promotion.jl:261 # ; Function convert; { # ; Location: number.jl:7 # ; Function Type; { # ; Location: float.jl:60 # vcvtsi2sd xmm0, xmm0, ecx # 从内存中读取整数 r # movabs rax, 497710928 # 读取 pi # ;}}}}} # ; Function *; { # ; Location: float.jl:399 # vmulsd xmm1, xmm0, qword ptr [rax] # pi * r # vmulsd xmm0, xmm1, xmm0 # (pi * r) * r # ;}} # pop rbp # ret # nop dword ptr [rax] # ;}code_native(circle_area, (Float64,), syntax = :intel) # .text # ; Function circle_area { # ; Location: REPL[121]:1 # push rbp # mov rbp, rsp # movabs rax, 497711048 # ; Function *; { # ; Location: operators.jl:502 # ; Function *; { # ; Location: promotion.jl:314 # ; Function *; { # ; Location: float.jl:399 # vmulsd xmm1, xmm0, qword ptr [rax] # ;}}} # ; Function *; { # ; Location: float.jl:399 # vmulsd xmm0, xmm1, xmm0 # ;} # pop rbp # ret # nop dword ptr [rax + rax] # ;}你可以在 Julia 中文文档 / Julia 文档(en) 中获得关于 Julia 的更多细节。
如果有任何问题可以去 Julia 中文社区 / 官方社区(en) 提问，大家对待新手都非常的友好。