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| -rw-r--r-- | julia.html.markdown | 628 |
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diff --git a/julia.html.markdown b/julia.html.markdown index 1023e303..66329feb 100644 --- a/julia.html.markdown +++ b/julia.html.markdown @@ -1,5 +1,5 @@ --- -language: julia +language: Julia contributors: - ["Leah Hanson", "http://leahhanson.us"] filename: learnjulia.jl @@ -8,11 +8,15 @@ filename: learnjulia.jl Julia is a new homoiconic functional language focused on technical computing. While having the full power of homoiconic macros, first-class functions, and low-level control, Julia is as easy to learn and use as Python. -This is based on the current development version of Julia, as of June 29th, 2013. +This is based on Julia 0.3. ```ruby -# Single line comments start with a hash. +# Single line comments start with a hash (pound) symbol. +#= Multiline comments can be written + by putting '#=' before the text and '=#' + after the text. They can also be nested. +=# #################################################### ## 1. Primitive Datatypes and Operators @@ -20,105 +24,110 @@ This is based on the current development version of Julia, as of June 29th, 2013 # Everything in Julia is a expression. -# You have numbers -3 #=> 3 (Int64) -3.2 #=> 3.2 (Float64) -2 + 1im #=> 2 + 1im (Complex{Int64}) -2//3 #=> 2//3 (Rational{Int64}) - -# Math is what you would expect -1 + 1 #=> 2 -8 - 1 #=> 7 -10 * 2 #=> 20 -35 / 5 #=> 7.0 -5 \ 35 #=> 7.0 -5 / 2 #=> 2.5 -div(5, 2) #=> 2 -2 ^ 2 #=> 4 # power, not bitwise xor -12 % 10 #=> 2 +# There are several basic types of numbers. +3 # => 3 (Int64) +3.2 # => 3.2 (Float64) +2 + 1im # => 2 + 1im (Complex{Int64}) +2//3 # => 2//3 (Rational{Int64}) + +# All of the normal infix operators are available. +1 + 1 # => 2 +8 - 1 # => 7 +10 * 2 # => 20 +35 / 5 # => 7.0 +5 / 2 # => 2.5 # dividing an Int by an Int always results in a Float +div(5, 2) # => 2 # for a truncated result, use div +5 \ 35 # => 7.0 +2 ^ 2 # => 4 # power, not bitwise xor +12 % 10 # => 2 # Enforce precedence with parentheses -(1 + 3) * 2 #=> 8 +(1 + 3) * 2 # => 8 # Bitwise Operators -~2 #=> -3 # bitwise not -3 & 5 #=> 1 # bitwise and -2 | 4 #=> 6 # bitwise or -2 $ 4 #=> 6 # bitwise xor -2 >>> 1 #=> 1 # logical shift right -2 >> 1 #=> 1 # arithmetic shift right -2 << 1 #=> 4 # logical/arithmetic shift left +~2 # => -3 # bitwise not +3 & 5 # => 1 # bitwise and +2 | 4 # => 6 # bitwise or +2 $ 4 # => 6 # bitwise xor +2 >>> 1 # => 1 # logical shift right +2 >> 1 # => 1 # arithmetic shift right +2 << 1 # => 4 # logical/arithmetic shift left # You can use the bits function to see the binary representation of a number. bits(12345) -#=> "0000000000000000000000000000000000000000000000000011000000111001" +# => "0000000000000000000000000000000000000000000000000011000000111001" bits(12345.0) -#=> "0100000011001000000111001000000000000000000000000000000000000000" +# => "0100000011001000000111001000000000000000000000000000000000000000" # Boolean values are primitives true false # Boolean operators -!true #=> false -!false #=> true -1 == 1 #=> true -2 == 1 #=> false -1 != 1 #=> false -2 != 1 #=> true -1 < 10 #=> true -1 > 10 #=> false -2 <= 2 #=> true -2 >= 2 #=> true +!true # => false +!false # => true +1 == 1 # => true +2 == 1 # => false +1 != 1 # => false +2 != 1 # => true +1 < 10 # => true +1 > 10 # => false +2 <= 2 # => true +2 >= 2 # => true # Comparisons can be chained -1 < 2 < 3 #=> true -2 < 3 < 2 #=> false +1 < 2 < 3 # => true +2 < 3 < 2 # => false # Strings are created with " "This is a string." -# Character literals written with ' +# Character literals are written with ' 'a' -# A string can be treated like a list of characters -"This is a string"[1] #=> 'T' # Julia indexes from 1 +# A string can be indexed like an array of characters +"This is a string"[1] # => 'T' # Julia indexes from 1 +# However, this is will not work well for UTF8 strings, +# so iterating over strings is recommended (map, for loops, etc). # $ can be used for string interpolation: -"2 + 2 = $(2 + 2)" #=> "2 + 2 = 4" -# You can put any Julia expression inside the parenthesis. +"2 + 2 = $(2 + 2)" # => "2 + 2 = 4" +# You can put any Julia expression inside the parentheses. # Another way to format strings is the printf macro. @printf "%d is less than %f" 4.5 5.3 # 5 is less than 5.300000 +# Printing is easy +println("I'm Julia. Nice to meet you!") + #################################################### ## 2. Variables and Collections #################################################### -# Printing is pretty easy -println("I'm Julia. Nice to meet you!") - -# No need to declare variables before assigning to them. -some_var = 5 #=> 5 -some_var #=> 5 +# You don't declare variables before assigning to them. +some_var = 5 # => 5 +some_var # => 5 # Accessing a previously unassigned variable is an error try - some_other_var #=> ERROR: some_other_var not defined + some_other_var # => ERROR: some_other_var not defined catch e println(e) end -# Variable name start with a letter. You can use uppercase letters, digits, -# and exclamation points as well after the initial alphabetic character. -SomeOtherVar123! = 6 #=> 6 +# Variable names start with a letter. +# After that, you can use letters, digits, underscores, and exclamation points. +SomeOtherVar123! = 6 # => 6 # You can also use unicode characters -☃ = 8 #=> 8 +☃ = 8 # => 8 +# These are especially handy for mathematical notation +2 * π # => 6.283185307179586 # A note on naming conventions in Julia: # -# * Names of variables are in lower case, with word separation indicated by -# underscores ('\_'). +# * 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 begin with a capital letter and word separation is shown # with CamelCase instead of underscores. @@ -129,45 +138,49 @@ SomeOtherVar123! = 6 #=> 6 # functions are sometimes called mutating functions or in-place functions. # Arrays store a sequence of values indexed by integers 1 through n: -a = Int64[] #=> 0-element Int64 Array +a = Int64[] # => 0-element Int64 Array # 1-dimensional array literals can be written with comma-separated values. -b = [4, 5, 6] #=> 3-element Int64 Array: [4, 5, 6] -b[1] #=> 4 -b[end] #=> 6 +b = [4, 5, 6] # => 3-element Int64 Array: [4, 5, 6] +b[1] # => 4 +b[end] # => 6 # 2-dimentional arrays use space-separated values and semicolon-separated rows. -matrix = [1 2; 3 4] #=> 2x2 Int64 Array: [1 2; 3 4] +matrix = [1 2; 3 4] # => 2x2 Int64 Array: [1 2; 3 4] # Add stuff to the end of a list with push! and append! -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] +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] # Remove from the end with pop -pop!(a) #=> 6 and b is now [4,5] +pop!(b) # => 6 and b is now [4,5] # Let's put it back push!(b,6) # b is now [4,5,6] again. -a[1] #=> 1 # remember that Julia indexes from 1, not 0! +a[1] # => 1 # remember that Julia indexes from 1, not 0! # end is a shorthand for the last index. It can be used in any # indexing expression -a[end] #=> 6 +a[end] # => 6 + +# we also have shift and unshift +shift!(a) # => 1 and a is now [2,4,3,4,5,6] +unshift!(a,7) # => [7,2,4,3,4,5,6] # Function names that end in exclamations points indicate that they modify # their argument. -arr = [5,4,6] #=> 3-element Int64 Array: [5,4,6] -sort(arr) #=> [4,5,6]; arr is still [5,4,6] -sort!(arr) #=> [4,5,6]; arr is now [4,5,6] +arr = [5,4,6] # => 3-element Int64 Array: [5,4,6] +sort(arr) # => [4,5,6]; arr is still [5,4,6] +sort!(arr) # => [4,5,6]; arr is now [4,5,6] # Looking out of bounds is a BoundsError try - a[0] #=> ERROR: BoundsError() in getindex at array.jl:270 - a[end+1] #=> ERROR: BoundsError() in getindex at array.jl:270 + a[0] # => ERROR: BoundsError() in getindex at array.jl:270 + a[end+1] # => ERROR: BoundsError() in getindex at array.jl:270 catch e println(e) end @@ -177,105 +190,109 @@ end # inside the julia folder to find these files. # You can initialize arrays from ranges -a = [1:5] #=> 5-element Int64 Array: [1,2,3,4,5] +a = [1:5;] # => 5-element Int64 Array: [1,2,3,4,5] # You can look at ranges with slice syntax. -a[1:3] #=> [1, 2, 3] -a[2:] #=> [2, 3, 4, 5] +a[1:3] # => [1, 2, 3] +a[2:end] # => [2, 3, 4, 5] -# Remove arbitrary elements from a list with splice! +# Remove elements from an array by index with splice! arr = [3,4,5] -splice!(arr,2) #=> 4 ; arr is now [3,5] +splice!(arr,2) # => 4 ; arr is now [3,5] # Concatenate lists with append! b = [1,2,3] -append!(a,b) # Now a is [1, 3, 4, 5, 1, 2, 3] +append!(a,b) # Now a is [1, 2, 3, 4, 5, 1, 2, 3] -# Check for existence in a list with contains -contains(a,1) #=> true +# Check for existence in a list with in +in(1, a) # => true # Examine the length with length -length(a) #=> 7 +length(a) # => 8 # Tuples are immutable. -tup = (1, 2, 3) #=>(1,2,3) # an (Int64,Int64,Int64) tuple. -tup[1] #=> 1 +tup = (1, 2, 3) # => (1,2,3) # an (Int64,Int64,Int64) tuple. +tup[1] # => 1 try: - tup[0] = 3 #=> ERROR: no method setindex!((Int64,Int64,Int64),Int64,Int64) + tup[1] = 3 # => ERROR: no method setindex!((Int64,Int64,Int64),Int64,Int64) catch e println(e) end # Many list functions also work on tuples -length(tup) #=> 3 -tup[1:2] #=> (1,2) -contains(tup,2) #=> true +length(tup) # => 3 +tup[1:2] # => (1,2) +in(2, tup) # => true # You can unpack tuples into variables -a, b, c = (1, 2, 3) #=> (1,2,3) # a is now 1, b is now 2 and c is now 3 +a, b, c = (1, 2, 3) # => (1,2,3) # a is now 1, b is now 2 and c is now 3 + +# Tuples are created even if you leave out the parentheses +d, e, f = 4, 5, 6 # => (4,5,6) -# Tuples are created by default if you leave out the parentheses -d, e, f = 4, 5, 6 #=> (4,5,6) +# A 1-element tuple is distinct from the value it contains +(1,) == 1 # => false +(1) == 1 # => true -# Now look how easy it is to swap two values -e, d = d, e #=> (5,4) # d is now 5 and e is now 4 +# Look how easy it is to swap two values +e, d = d, e # => (5,4) # d is now 5 and e is now 4 # Dictionaries store mappings -empty_dict = Dict() #=> Dict{Any,Any}() +empty_dict = Dict() # => Dict{Any,Any}() -# Here is a prefilled dictionary +# You can create a dictionary using a literal filled_dict = ["one"=> 1, "two"=> 2, "three"=> 3] # => Dict{ASCIIString,Int64} # Look up values with [] -filled_dict["one"] #=> 1 +filled_dict["one"] # => 1 # Get all keys keys(filled_dict) -#=> KeyIterator{Dict{ASCIIString,Int64}}(["three"=>3,"one"=>1,"two"=>2]) +# => KeyIterator{Dict{ASCIIString,Int64}}(["three"=>3,"one"=>1,"two"=>2]) # Note - dictionary keys are not sorted or in the order you inserted them. -# Get all values +# Get all values values(filled_dict) -#=> ValueIterator{Dict{ASCIIString,Int64}}(["three"=>3,"one"=>1,"two"=>2]) +# => ValueIterator{Dict{ASCIIString,Int64}}(["three"=>3,"one"=>1,"two"=>2]) # Note - Same as above regarding key ordering. -# Check for existence of keys in a dictionary with contains, haskey -contains(filled_dict, ("one", 1)) #=> true -contains(filled_dict, ("two", 3)) #=> false -haskey(filled_dict, "one") #=> true -haskey(filled_dict, 1) #=> false +# Check for existence of keys in a dictionary with in, haskey +in(("one", 1), filled_dict) # => true +in(("two", 3), filled_dict) # => false +haskey(filled_dict, "one") # => true +haskey(filled_dict, 1) # => false -# Trying to look up a non-existing key will raise an error +# Trying to look up a non-existent key will raise an error try - filled_dict["four"] #=> ERROR: key not found: four in getindex at dict.jl:489 + filled_dict["four"] # => ERROR: key not found: four in getindex at dict.jl:489 catch e println(e) end -# Use get method to avoid the error +# Use the get method to avoid that error by providing a default value # get(dictionary,key,default_value) -get(filled_dict,"one",4) #=> 1 -get(filled_dict,"four",4) #=> 4 +get(filled_dict,"one",4) # => 1 +get(filled_dict,"four",4) # => 4 -# Sets store sets -empty_set = Set() #=> Set{Any}() -# Initialize a set with a bunch of values -filled_set = Set(1,2,2,3,4) #=> Set{Int64}(1,2,3,4) +# Use Sets to represent collections of unordered, unique values +empty_set = Set() # => Set{Any}() +# Initialize a set with values +filled_set = Set(1,2,2,3,4) # => Set{Int64}(1,2,3,4) -# Add more items to a set -add!(filled_set,5) #=> Set{Int64}(5,4,2,3,1) +# Add more values to a set +push!(filled_set,5) # => Set{Int64}(5,4,2,3,1) -# There are functions for set intersection, union, and difference. -other_set = Set(3, 4, 5, 6) #=> Set{Int64}(6,4,5,3) -intersect(filled_set, other_set) #=> Set{Int64}(3,4,5) -union(filled_set, other_set) #=> Set{Int64}(1,2,3,4,5,6) -setdiff(Set(1,2,3,4),Set(2,3,5)) #=> Set{Int64}(1,4) +# Check if the values are in the set +in(2, filled_set) # => true +in(10, filled_set) # => false -# Check for existence in a set with contains -contains(filled_set,2) #=> true -contains(filled_set,10) #=> false +# There are functions for set intersection, union, and difference. +other_set = Set(3, 4, 5, 6) # => Set{Int64}(6,4,5,3) +intersect(filled_set, other_set) # => Set{Int64}(3,4,5) +union(filled_set, other_set) # => Set{Int64}(1,2,3,4,5,6) +setdiff(Set(1,2,3,4),Set(2,3,5)) # => Set{Int64}(1,4) #################################################### @@ -285,8 +302,7 @@ contains(filled_set,10) #=> false # Let's make a variable some_var = 5 -# Here is an if statement. Indentation is NOT meaningful in Julia. -# prints "some var is smaller than 10" +# Here is an if statement. Indentation is not meaningful in Julia. if some_var > 10 println("some_var is totally bigger than 10.") elseif some_var < 10 # This elseif clause is optional. @@ -294,85 +310,103 @@ elseif some_var < 10 # This elseif clause is optional. else # The else clause is optional too. println("some_var is indeed 10.") end +# => prints "some var is smaller than 10" -# For loops iterate over iterables, such as ranges, lists, sets, dicts, strings. - +# For loops iterate over iterables. +# Iterable types include Range, Array, Set, Dict, and String. for animal=["dog", "cat", "mouse"] - # You can use $ to interpolate into strings println("$animal is a mammal") + # You can use $ to interpolate variables or expression into strings end # prints: # dog is a mammal # cat is a mammal # mouse is a mammal -# You can use in instead of =, if you want. +# You can use 'in' instead of '='. for animal in ["dog", "cat", "mouse"] println("$animal is a mammal") end +# prints: +# dog is a mammal +# cat is a mammal +# mouse is a mammal for a in ["dog"=>"mammal","cat"=>"mammal","mouse"=>"mammal"] - println("$(a[1]) is $(a[2])") + println("$(a[1]) is a $(a[2])") end +# prints: +# dog is a mammal +# cat is a mammal +# mouse is a mammal for (k,v) in ["dog"=>"mammal","cat"=>"mammal","mouse"=>"mammal"] - println("$k is $v") + println("$k is a $v") end - - -# While loops go until a condition is no longer met. # prints: -# 0 -# 1 -# 2 -# 3 +# dog is a mammal +# cat is a mammal +# mouse is a mammal + +# While loops loop while a condition is true x = 0 while x < 4 println(x) x += 1 # Shorthand for x = x + 1 end +# prints: +# 0 +# 1 +# 2 +# 3 -# Handle exceptions with a try/except block +# Handle exceptions with a try/catch block try error("help") catch e println("caught it $e") end -#=> caught it ErrorException("help") +# => caught it ErrorException("help") #################################################### ## 4. Functions #################################################### -# Use the keyword function to create new functions +# The keyword 'function' creates new functions +#function name(arglist) +# body... +#end function add(x, y) println("x is $x and y is $y") - # Functions implicitly return the value of their last statement + # Functions return the value of their last statement x + y end -add(5, 6) #=> 11 after printing out "x is 5 and y is 6" +add(5, 6) # => 11 after printing out "x is 5 and y is 6" # You can define functions that take a variable number of # positional arguments function varargs(args...) return args + # use the keyword return to return anywhere in the function end +# => varargs (generic function with 1 method) -varargs(1,2,3) #=> (1,2,3) +varargs(1,2,3) # => (1,2,3) # The ... is called a splat. -# It can also be used in a fuction call -# to splat a list or tuple out to be the arguments -Set([1,2,3]) #=> Set{Array{Int64,1}}([1,2,3]) # produces a Set of Arrays -Set([1,2,3]...) #=> Set{Int64}(1,2,3) # this is equivalent to Set(1,2,3) +# We just used it in a function definition. +# It can also be used in a fuction call, +# where it will splat an Array or Tuple's contents into the argument list. +Set([1,2,3]) # => Set{Array{Int64,1}}([1,2,3]) # produces a Set of Arrays +Set([1,2,3]...) # => Set{Int64}(1,2,3) # this is equivalent to Set(1,2,3) -x = (1,2,3) #=> (1,2,3) -Set(x) #=> Set{(Int64,Int64,Int64)}((1,2,3)) # a Set of Tuples -Set(x...) #=> Set{Int64}(2,3,1) +x = (1,2,3) # => (1,2,3) +Set(x) # => Set{(Int64,Int64,Int64)}((1,2,3)) # a Set of Tuples +Set(x...) # => Set{Int64}(2,3,1) # You can define functions with optional positional arguments @@ -380,26 +414,26 @@ function defaults(a,b,x=5,y=6) return "$a $b and $x $y" end -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" +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: no method defaults(Char,) - defaults() #=> ERROR: no methods defaults() + defaults('h') # => ERROR: no method defaults(Char,) + defaults() # => ERROR: no methods defaults() catch e -println(e) + println(e) end # You can define functions that take keyword arguments function keyword_args(;k1=4,name2="hello") # note the ; return ["k1"=>k1,"name2"=>name2] -end +end -keyword_args(name2="ness") #=> ["name2"=>"ness","k1"=>4] -keyword_args(k1="mine") #=> ["k1"=>"mine","name2"=>"hello"] -keyword_args() #=> ["name2"=>"hello","k2"=>4] +keyword_args(name2="ness") # => ["name2"=>"ness","k1"=>4] +keyword_args(k1="mine") # => ["k1"=>"mine","name2"=>"hello"] +keyword_args() # => ["name2"=>"hello","k1"=>4] -# You can also do both at once +# You can combine all kinds of arguments in the same function function all_the_args(normal_arg, optional_positional_arg=2; keyword_arg="foo") println("normal arg: $normal_arg") println("optional arg: $optional_positional_arg") @@ -420,12 +454,15 @@ function create_adder(x) return adder end -# or equivalently +# This is "stabby lambda syntax" for creating anonymous functions +(x -> x > 2)(3) # => true + +# This function is identical to create_adder implementation above. function create_adder(x) y -> x + y end -# you can also name the internal function, if you want +# You can also name the internal function, if you want function create_adder(x) function adder(y) x + y @@ -434,92 +471,277 @@ function create_adder(x) end add_10 = create_adder(10) -add_10(3) #=> 13 +add_10(3) # => 13 -# The first two inner functions above are anonymous functions -(x -> x > 2)(3) #=> true # There are built-in higher order functions -map(add_10, [1,2,3]) #=> [11, 12, 13] -filter(x -> x > 5, [3, 4, 5, 6, 7]) #=> [6, 7] +map(add_10, [1,2,3]) # => [11, 12, 13] +filter(x -> x > 5, [3, 4, 5, 6, 7]) # => [6, 7] -# We can use list comprehensions for nice maps and filters -[add_10(i) for i=[1, 2, 3]] #=> [11, 12, 13] -[add_10(i) for i in [1, 2, 3]] #=> [11, 12, 13] +# We can use list comprehensions for nicer maps +[add_10(i) for i=[1, 2, 3]] # => [11, 12, 13] +[add_10(i) for i in [1, 2, 3]] # => [11, 12, 13] #################################################### -## 5. Types and Multiple-Dispatch +## 5. Types #################################################### -# Type definition +# Julia has a type system. +# Every value has a type; variables do not have types themselves. +# You can use the `typeof` function to get the type of a value. +typeof(5) # => Int64 + +# Types are first-class values +typeof(Int64) # => DataType +typeof(DataType) # => DataType +# DataType is the type that represents types, including itself. + +# Types are used for documentation, optimizations, and dispatch. +# They are not statically checked. + +# Users can define types +# They are like records or structs in other languages. +# New types are defined using the `type` keyword. + +# type Name +# field::OptionalType +# ... +# end type Tiger taillength::Float64 - coatcolor # no type annotation is implicitly Any + coatcolor # not including a type annotation is the same as `::Any` end -# default constructor is the properties in order -# so, Tiger(taillength,coatcolor) -# Type instantiation -tigger = Tiger(3.5,"orange") # the type doubles as the constructor function +# The default constructor's arguments are the properties +# of the type, in the order they are listed in the definition +tigger = Tiger(3.5,"orange") # => Tiger(3.5,"orange") -# Abtract Types -abstract Cat # just a name and point in the type hierarchy +# The type doubles as the constructor function for values of that type +sherekhan = typeof(tigger)(5.6,"fire") # => Tiger(5.6,"fire") -# * types defined with the type keyword are concrete types; they can be -# instantiated -# -# * types defined with the abstract keyword are abstract types; they can -# have subtypes. -# -# * each type has one supertype; a supertype can have zero or more subtypes. +# These struct-style types are called concrete types +# They can be instantiated, but cannot have subtypes. +# The other kind of types is abstract types. +# abstract Name +abstract Cat # just a name and point in the type hierarchy + +# Abstract types cannot be instantiated, but can have subtypes. +# For example, Number is an abstract type +subtypes(Number) # => 6-element Array{Any,1}: + # Complex{Float16} + # Complex{Float32} + # Complex{Float64} + # Complex{T<:Real} + # ImaginaryUnit + # Real +subtypes(Cat) # => 0-element Array{Any,1} + +# Every type has a super type; use the `super` function to get it. +typeof(5) # => Int64 +super(Int64) # => Signed +super(Signed) # => Real +super(Real) # => Number +super(Number) # => Any +super(super(Signed)) # => Number +super(Any) # => Any +# All of these type, except for Int64, are abstract. + +# <: is the subtyping operator type Lion <: Cat # Lion is a subtype of Cat mane_color roar::String end +# You can define more constructors for your type +# Just define a function of the same name as the type +# and call an existing constructor to get a value of the correct type +Lion(roar::String) = Lion("green",roar) +# This is an outer constructor because it's outside the type definition + type Panther <: Cat # Panther is also a subtype of Cat eye_color Panther() = new("green") # Panthers will only have this constructor, and no default constructor. end +# Using inner constructors, like Panther does, gives you control +# over how values of the type can be created. +# When possible, you should use outer constructors rather than inner ones. -# Multiple Dispatch +#################################################### +## 6. Multiple-Dispatch +#################################################### # In Julia, all named functions are generic functions # This means that they are built up from many small methods -# For example, let's make a function meow: -function meow(cat::Lion) - cat.roar # access properties using dot notation +# Each constructor for Lion is a method of the generic function Lion. + +# For a non-constructor example, let's make a function meow: + +# Definitions for Lion, Panther, Tiger +function meow(animal::Lion) + animal.roar # access type properties using dot notation end -function meow(cat::Panther) +function meow(animal::Panther) "grrr" end -function meow(cat::Tiger) +function meow(animal::Tiger) "rawwwr" end -meow(tigger) #=> "rawwr" -meow(Lion("brown","ROAAR")) #=> "ROAAR" -meow(Panther()) #=> "grrr" +# Testing the meow function +meow(tigger) # => "rawwr" +meow(Lion("brown","ROAAR")) # => "ROAAR" +meow(Panther()) # => "grrr" +# Review the local type hierarchy +issubtype(Tiger,Cat) # => false +issubtype(Lion,Cat) # => true +issubtype(Panther,Cat) # => true + +# Defining a function that takes Cats function pet_cat(cat::Cat) println("The cat says $(meow(cat))") end +pet_cat(Lion("42")) # => prints "The cat says 42" try - pet_cat(tigger) #=> ERROR: no method pet_cat(Tiger,) + pet_cat(tigger) # => ERROR: no method pet_cat(Tiger,) catch e println(e) end -pet_cat(Lion(Panther(),"42")) #=> prints "The cat says 42" +# In OO languages, single dispatch is common; +# this means that the method is picked based on the type of the first argument. +# In Julia, all of the argument types contribute to selecting the best method. + +# Let's define a function with more arguments, so we can see the difference +function fight(t::Tiger,c::Cat) + println("The $(t.coatcolor) tiger wins!") +end +# => fight (generic function with 1 method) + +fight(tigger,Panther()) # => prints The orange tiger wins! +fight(tigger,Lion("ROAR")) # => prints The orange tiger wins! + +# Let's change the behavior when the Cat is specifically a Lion +fight(t::Tiger,l::Lion) = println("The $(l.mane_color)-maned lion wins!") +# => fight (generic function with 2 methods) + +fight(tigger,Panther()) # => prints The orange tiger wins! +fight(tigger,Lion("ROAR")) # => prints The green-maned lion wins! + +# We don't need a Tiger in order to fight +fight(l::Lion,c::Cat) = println("The victorious cat says $(meow(c))") +# => fight (generic function with 3 methods) + +fight(Lion("balooga!"),Panther()) # => prints The victorious cat says grrr +try + fight(Panther(),Lion("RAWR")) # => ERROR: no method fight(Panther,Lion) +catch +end +# Also let the cat go first +fight(c::Cat,l::Lion) = println("The cat beats the Lion") +# => Warning: New definition +# fight(Cat,Lion) at none:1 +# is ambiguous with +# fight(Lion,Cat) at none:2. +# Make sure +# fight(Lion,Lion) +# is defined first. +#fight (generic function with 4 methods) + +# This warning is because it's unclear which fight will be called in: +fight(Lion("RAR"),Lion("brown","rarrr")) # => prints The victorious cat says rarrr +# The result may be different in other versions of Julia + +fight(l::Lion,l2::Lion) = println("The lions come to a tie") +fight(Lion("RAR"),Lion("brown","rarrr")) # => prints The lions come to a tie + + +# Under the hood +# You can take a look at the llvm and the assembly code generated. + +square_area(l) = l * l # square_area (generic function with 1 method) + +square_area(5) #25 + +# What happens when we feed square_area an integer? +code_native(square_area, (Int32,)) + # .section __TEXT,__text,regular,pure_instructions + # Filename: none + # Source line: 1 # Prologue + # push RBP + # mov RBP, RSP + # Source line: 1 + # movsxd RAX, EDI # Fetch l from memory? + # imul RAX, RAX # Square l and store the result in RAX + # pop RBP # Restore old base pointer + # ret # Result will still be in RAX + +code_native(square_area, (Float32,)) + # .section __TEXT,__text,regular,pure_instructions + # Filename: none + # Source line: 1 + # push RBP + # mov RBP, RSP + # Source line: 1 + # vmulss XMM0, XMM0, XMM0 # Scalar single precision multiply (AVX) + # pop RBP + # ret + +code_native(square_area, (Float64,)) + # .section __TEXT,__text,regular,pure_instructions + # Filename: none + # Source line: 1 + # push RBP + # mov RBP, RSP + # Source line: 1 + # vmulsd XMM0, XMM0, XMM0 # Scalar double precision multiply (AVX) + # pop RBP + # ret + # +# Note that julia will use floating point instructions if any of the +# arguements are floats. +# Let's calculate the area of a circle +circle_area(r) = pi * r * r # circle_area (generic function with 1 method) +circle_area(5) # 78.53981633974483 + +code_native(circle_area, (Int32,)) + # .section __TEXT,__text,regular,pure_instructions + # Filename: none + # Source line: 1 + # push RBP + # mov RBP, RSP + # Source line: 1 + # vcvtsi2sd XMM0, XMM0, EDI # Load integer (r) from memory + # movabs RAX, 4593140240 # Load pi + # vmulsd XMM1, XMM0, QWORD PTR [RAX] # pi * r + # vmulsd XMM0, XMM0, XMM1 # (pi * r) * r + # pop RBP + # ret + # + +code_native(circle_area, (Float64,)) + # .section __TEXT,__text,regular,pure_instructions + # Filename: none + # Source line: 1 + # push RBP + # mov RBP, RSP + # movabs RAX, 4593140496 + # Source line: 1 + # vmulsd XMM1, XMM0, QWORD PTR [RAX] + # vmulsd XMM0, XMM1, XMM0 + # pop RBP + # ret + # ``` ## Further Reading You can get a lot more detail from [The Julia Manual](http://docs.julialang.org/en/latest/manual/) +The best place to get help with Julia is the (very friendly) [mailing list](https://groups.google.com/forum/#!forum/julia-users). |