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Diffstat (limited to 'julia.html.markdown')
| -rw-r--r-- | julia.html.markdown | 299 |
1 files changed, 220 insertions, 79 deletions
diff --git a/julia.html.markdown b/julia.html.markdown index cf3a464b..4ebd50ff 100644 --- a/julia.html.markdown +++ b/julia.html.markdown @@ -8,7 +8,7 @@ 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 the current development version of Julia, as of October 18th, 2013. ```ruby @@ -20,20 +20,20 @@ This is based on the current development version of Julia, as of June 29th, 2013 # Everything in Julia is a expression. -# You have numbers +# 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}) -# Math is what you would expect +# 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 -5 / 2 #=> 2.5 -div(5, 2) #=> 2 2 ^ 2 #=> 4 # power, not bitwise xor 12 % 10 #=> 2 @@ -77,11 +77,13 @@ 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 +# 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 reccommended (map, for loops, etc). # $ can be used for string interpolation: "2 + 2 = $(2 + 2)" #=> "2 + 2 = 4" @@ -94,10 +96,10 @@ false ## 2. Variables and Collections #################################################### -# Printing is pretty easy +# Printing is easy println("I'm Julia. Nice to meet you!") -# No need to declare variables before assigning to them. +# You don't declare variables before assigning to them. some_var = 5 #=> 5 some_var #=> 5 @@ -108,12 +110,14 @@ 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. +# 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 +# These are especially handy for mathematical notation +2 * π #=> 6.283185307179586 # A note on naming conventions in Julia: # @@ -158,6 +162,10 @@ a[1] #=> 1 # remember that Julia indexes from 1, not 0! # indexing expression 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] @@ -182,23 +190,24 @@ 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[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] # 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(a,1) #=> 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, 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) @@ -209,22 +218,26 @@ end # Many list functions also work on tuples length(tup) #=> 3 tup[1:2] #=> (1,2) -contains(tup,2) #=> true +in(tup,2) #=> 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 -# Tuples are created by default if you leave out the parentheses +# Tuples are created even if you leave out the parentheses d, e, f = 4, 5, 6 #=> (4,5,6) -# Now look how easy it is to swap two values +# A 1-element tuple is distinct from the value it contains +(1,) == 1 #=> false +(1) == 1 #=> true + +# 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}() -# Here is a prefilled dictionary +# You can create a dictionary using a literal filled_dict = ["one"=> 1, "two"=> 2, "three"=> 3] # => Dict{ASCIIString,Int64} @@ -241,31 +254,35 @@ values(filled_dict) #=> 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 +# Check for existence of keys in a dictionary with in, haskey +in(filled_dict, ("one", 1)) #=> true +in(filled_dict, ("two", 3)) #=> 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-existant key will raise an error try 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 -# Sets store sets +# Use Sets to represent collections of unordered, unique values empty_set = Set() #=> Set{Any}() -# Initialize a set with a bunch of values +# 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) + +# Check if the values are in the set +in(filled_set,2) #=> true +in(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) @@ -273,10 +290,6 @@ 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 for existence in a set with contains -contains(filled_set,2) #=> true -contains(filled_set,10) #=> false - #################################################### ## 3. Control Flow @@ -285,8 +298,7 @@ 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. @@ -294,44 +306,56 @@ 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 try @@ -346,11 +370,14 @@ end ## 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 @@ -360,13 +387,16 @@ add(5, 6) #=> 11 after printing out "x is 5 and y is 6" # 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) # 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 +# 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) @@ -399,7 +429,7 @@ keyword_args(name2="ness") #=> ["name2"=>"ness","k1"=>4] keyword_args(k1="mine") #=> ["k1"=>"mine","name2"=>"hello"] keyword_args() #=> ["name2"=>"hello","k2"=>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 +450,15 @@ 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 @@ -436,61 +469,114 @@ end add_10 = create_adder(10) 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] -# We can use list comprehensions for nice maps and filters +# 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 used 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 tyep, in order the order they are listed in the definition +tigger = Tiger(3.5,"orange") #=> Tiger(3.5,"orange") + +# The type doubles as the constructor function for values of that type +sherekhan = typeof(tigger)(5.6,"fire") #=> Tiger(5.6,"fire") -# Abtract Types -abstract Cat # just a name and point in the type hierarchy +# These struct-style types are called concrete types +# They can be instantiated, but cannot have subtypes. +# The other kind of types is abstract types. -# * 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. +# 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 Panter 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: +# 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(cat::Lion) - cat.roar # access properties using dot notation + cat.roar # access type properties using dot notation end function meow(cat::Panther) @@ -501,21 +587,75 @@ function meow(cat::Tiger) "rawwwr" end +# 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,) 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 ``` @@ -523,3 +663,4 @@ pet_cat(Lion(Panther(),"42")) #=> prints "The cat says 42" 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). |