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diff --git a/julia.html.markdown b/julia.html.markdown new file mode 100644 index 00000000..1023e303 --- /dev/null +++ b/julia.html.markdown @@ -0,0 +1,525 @@ +--- +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 June 29th, 2013. + +```ruby + +# Single line comments start with a hash. + +#################################################### +## 1. Primitive Datatypes and Operators +#################################################### + +# 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 + +# 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 written with ' +'a' + +# A string can be treated like a list of characters +"This is a string"[1] #=> 'T' # Julia indexes from 1 + +# $ 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 + +#################################################### +## 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 + +# Accessing a previously unassigned variable is an error +try + 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 + +# You can also use unicode characters +☃ = 8 #=> 8 + +# A note on naming conventions in Julia: +# +# * Names of variables are in lower case, with word separation indicated by +# underscores ('\_'). +# +# * 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!(a) #=> 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 + +# 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:] #=> [2, 3, 4, 5] + +# Remove arbitrary elements from a list 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] + +# Check for existence in a list with contains +contains(a,1) #=> true + +# Examine the length with length +length(a) #=> 7 + +# Tuples are immutable. +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) +catch e + println(e) +end + +# Many list functions also work on tuples +length(tup) #=> 3 +tup[1:2] #=> (1,2) +contains(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 +d, e, f = 4, 5, 6 #=> (4,5,6) + +# 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 + + +# Dictionaries store mappings +empty_dict = Dict() #=> Dict{Any,Any}() + +# Here is a prefilled dictionary +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 contains, haskey +contains(filled_dict, ("one", 1)) #=> true +contains(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 +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 +# get(dictionary,key,default_value) +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) + +# Add more items to a set +add!(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 for existence in a set with contains +contains(filled_set,2) #=> true +contains(filled_set,10) #=> false + + +#################################################### +## 3. Control Flow +#################################################### + +# 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" +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 + + +# For loops iterate over iterables, such as ranges, lists, sets, dicts, strings. + +for animal=["dog", "cat", "mouse"] + # You can use $ to interpolate into strings + println("$animal is a mammal") +end +# prints: +# dog is a mammal +# cat is a mammal +# mouse is a mammal + +# You can use in instead of =, if you want. +for animal in ["dog", "cat", "mouse"] + println("$animal is a mammal") +end + +for a in ["dog"=>"mammal","cat"=>"mammal","mouse"=>"mammal"] + println("$(a[1]) is $(a[2])") +end + +for (k,v) in ["dog"=>"mammal","cat"=>"mammal","mouse"=>"mammal"] + println("$k is $v") +end + + +# While loops go until a condition is no longer met. +# prints: +# 0 +# 1 +# 2 +# 3 +x = 0 +while x < 4 + println(x) + x += 1 # Shorthand for x = x + 1 +end + +# Handle exceptions with a try/except block +try + error("help") +catch e + println("caught it $e") +end +#=> caught it ErrorException("help") + + +#################################################### +## 4. Functions +#################################################### + +# Use the keyword function to create new functions +function add(x, y) + println("x is $x and y is $y") + + # Functions implicitly return the value of their last statement + x + y +end + +add(5, 6) #=> 11 after printing out "x is 5 and y is 6" + +# You can define functions that take a variable number of +# positional arguments +function varargs(args...) + return args +end + +varargs(1,2,3) #=> (1,2,3) + +# The ... is called a splat. +# It can also be used in a fuction call +# to splat a list or tuple out to be the arguments +Set([1,2,3]) #=> Set{Array{Int64,1}}([1,2,3]) # produces a Set of Arrays +Set([1,2,3]...) #=> Set{Int64}(1,2,3) # this is equivalent to Set(1,2,3) + +x = (1,2,3) #=> (1,2,3) +Set(x) #=> Set{(Int64,Int64,Int64)}((1,2,3)) # a Set of Tuples +Set(x...) #=> Set{Int64}(2,3,1) + + +# You can define functions with optional positional arguments +function defaults(a,b,x=5,y=6) + return "$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","k2"=>4] + +# You can also do both at once +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 + +# or equivalently +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 + +# 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 +[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 +#################################################### + +# Type definition +type Tiger + taillength::Float64 + coatcolor # no type annotation is implicitly 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 + +# Abtract Types +abstract Cat # just a name and point in the type hierarchy + +# * 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. + +type Lion <: Cat # Lion is a subtype of Cat + mane_color + roar::String +end + +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 + +# 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 +end + +function meow(cat::Panther) + "grrr" +end + +function meow(cat::Tiger) + "rawwwr" +end + +meow(tigger) #=> "rawwr" +meow(Lion("brown","ROAAR")) #=> "ROAAR" +meow(Panther()) #=> "grrr" + +function pet_cat(cat::Cat) + println("The cat says $(meow(cat))") +end + +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" + +``` + +## Further Reading + +You can get a lot more detail from [The Julia Manual](http://docs.julialang.org/en/latest/manual/) + |