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+---
+language: Julia
+contributors:
+ - ["Leah Hanson", "http://leahhanson.us"]
+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 October 18th, 2013.
+
+```ruby
+
+# 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
+####################################################
+
+# Everything in Julia is a expression.
+
+# 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
+
+# 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
+
+# You can use the bits function to see the binary representation of a number.
+bits(12345)
+# => "0000000000000000000000000000000000000000000000000011000000111001"
+bits(12345.0)
+# => "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
+# Comparisons can be chained
+1 < 2 < 3 # => true
+2 < 3 < 2 # => false
+
+# Strings are created with "
+"This is a string."
+
+# Character literals are written with '
+'a'
+
+# 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.
+
+# 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
+####################################################
+
+# 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
+catch e
+ println(e)
+end
+
+# 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:
+#
+# * 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.
+#
+# * Names of functions and macros are in lower case, without underscores.
+#
+# * Functions that modify their inputs have names that end in !. These
+# 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
+
+# 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
+
+# 2-dimentional arrays use space-separated values and semicolon-separated rows.
+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]
+
+# Remove from the end with pop
+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!
+
+# end is a shorthand for the last index. It can be used in any
+# 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]
+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
+catch e
+ println(e)
+end
+
+# Errors list the line and file they came from, even if it's in the standard
+# library. If you built Julia from source, you can look in the folder base
+# 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]
+
+# You can look at ranges with slice syntax.
+a[1:3] # => [1, 2, 3]
+a[2:end] # => [2, 3, 4, 5]
+
+# 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, 2, 3, 4, 5, 1, 2, 3]
+
+# Check for existence in a list with in
+in(1, a) # => true
+
+# Examine the length with length
+length(a) # => 8
+
+# Tuples are immutable.
+tup = (1, 2, 3) # => (1,2,3) # an (Int64,Int64,Int64) tuple.
+tup[1] # => 1
+try:
+ 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)
+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
+
+# Tuples are created even 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
+
+# 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}()
+
+# 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
+
+# Get all keys
+keys(filled_dict)
+# => 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
+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 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-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 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
+
+# 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 values to a set
+push!(filled_set,5) # => Set{Int64}(5,4,2,3,1)
+
+# Check if the values are in the set
+in(2, filled_set) # => true
+in(10, filled_set) # => 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)
+
+
+####################################################
+## 3. Control Flow
+####################################################
+
+# Let's make a variable
+some_var = 5
+
+# 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.
+ println("some_var is smaller than 10.")
+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.
+# Iterable types include Range, Array, Set, Dict, and String.
+for animal=["dog", "cat", "mouse"]
+ 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 '='.
+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 $(a[2])")
+end
+# prints:
+# dog is a mammal
+# cat is a mammal
+# mouse is a mammal
+
+for (k,v) in ["dog"=>"mammal","cat"=>"mammal","mouse"=>"mammal"]
+ println("$k is a $v")
+end
+# prints:
+# 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/catch block
+try
+ error("help")
+catch e
+ println("caught it $e")
+end
+# => caught it ErrorException("help")
+
+
+####################################################
+## 4. Functions
+####################################################
+
+# The keyword 'function' creates new functions
+#function name(arglist)
+# body...
+#end
+function add(x, y)
+ println("x is $x and y is $y")
+
+ # Functions return the value of their last statement
+ x + y
+end
+
+add(5, 6) # => 11 after printing out "x is 5 and y is 6"
+
+# You can define functions that take a variable number of
+# positional arguments
+function varargs(args...)
+ return args
+ # use the keyword return to return anywhere in the function
+end
+# => varargs (generic function with 1 method)
+
+varargs(1,2,3) # => (1,2,3)
+
+# The ... is called a splat.
+# We just used it in a function definition.
+# It can also be used in a fuction call,
+# where it will splat an Array or Tuple's contents into the argument list.
+Set([1,2,3]) # => Set{Array{Int64,1}}([1,2,3]) # produces a Set of Arrays
+Set([1,2,3]...) # => Set{Int64}(1,2,3) # this is equivalent to Set(1,2,3)
+
+x = (1,2,3) # => (1,2,3)
+Set(x) # => Set{(Int64,Int64,Int64)}((1,2,3)) # a Set of Tuples
+Set(x...) # => Set{Int64}(2,3,1)
+
+
+# You can define functions with optional positional arguments
+function defaults(a,b,x=5,y=6)
+ return "$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"
+try
+ defaults('h') # => ERROR: no method defaults(Char,)
+ defaults() # => ERROR: no methods defaults()
+catch 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
+
+keyword_args(name2="ness") # => ["name2"=>"ness","k1"=>4]
+keyword_args(k1="mine") # => ["k1"=>"mine","name2"=>"hello"]
+keyword_args() # => ["name2"=>"hello","k1"=>4]
+
+# 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")
+ println("keyword arg: $keyword_arg")
+end
+
+all_the_args(1, 3, keyword_arg=4)
+# prints:
+# normal arg: 1
+# optional arg: 3
+# keyword arg: 4
+
+# Julia has first class functions
+function create_adder(x)
+ adder = function (y)
+ return x + y
+ end
+ return adder
+end
+
+# 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
+function create_adder(x)
+ function adder(y)
+ x + y
+ end
+ adder
+end
+
+add_10 = create_adder(10)
+add_10(3) # => 13
+
+
+# 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 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
+####################################################
+
+# 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 # not including a type annotation is the same as `::Any`
+end
+
+# 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")
+
+# The type doubles as the constructor function for values of that type
+sherekhan = typeof(tigger)(5.6,"fire") # => Tiger(5.6,"fire")
+
+# 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.
+
+####################################################
+## 6. Multiple-Dispatch
+####################################################
+
+# In Julia, all named functions are generic functions
+# This means that they are built up from many small methods
+# 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(animal::Panther)
+ "grrr"
+end
+
+function meow(animal::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
+
+# 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).