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diff --git a/julia.html.markdown b/julia.html.markdown new file mode 100644 index 00000000..ff2f2339 --- /dev/null +++ b/julia.html.markdown @@ -0,0 +1,427 @@ +--- +language: julia +author: Leah Hanson +author_url: http://leahhanson.us +--- + +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. + +```julia +# 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 +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(2) #=> "0000000000000000000000000000000000000000000000000000000000000010" +bits(2.0) #=> "0100000000000000000000000000000000000000000000000000000000000000" + +# 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 +some_other_var #=> ERROR: some_other_var not defined + +# Variable Names: +Some!Other1Var! = 6 #=> 6 # You can use uppercase letters, digits, and exclamation points as well. +☃ = 8 #=> 8 # You can also use unicode characters + +# 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 sequences +li = Int64[] #=> 0-element Int64 Array +# 1-dimensional array literals can be written with comma-separated values. +other_li = [4, 5, 6] #=> 3-element Int64 Array: [4, 5, 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!(li,1) #=> [1] +push!(li,2) #=> [1,2] +push!(li,4) #=> [1,2,4] +push!(li,3) #=> [1,2,4,3] +append!(li,other_li) #=> [1,2,4,3,4,5,6] +# Remove from the end with pop +pop!(other_li) #=> 6 and other_li is now [4,5] +# Let's put it back +push!(other_li,6) # other_li is now [4,5,6] again. + +# Remember that Julia indexes from 1, not 0! +li[1] #=> 1 + +# 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 +li[0] # ERROR: BoundsError() in getindex at array.jl:270 +# 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 +li = [1:5] #=> 5-element Int64 Array: [1,2,3,4,5] + +# You can look at ranges with slice syntax. +li[1:3] #=> [1, 2, 3] +# Omit the beginning +li[2:] #=> [2, 3, 4, 5] + +# Remove arbitrary elements from a list with splice! +splice!(li,2) #=> 2 ; li is now [1, 3, 4, 5] + +# Concatenate lists with append! +other_li = [1,2,3] +append!(li,other_li) # Now li is [1, 3, 4, 5, 1, 2, 3] + +# Check for existence in a list with contains +contains(li,1) #=> true + +# Examine the length with length +length(li) #=> 7 + +# Tuples are like lists but are immutable. +tup = (1, 2, 3) +tup[0] #=> 1 +try: + tup[0] = 3 # Raises a TypeError +except TypeError: + print "Tuples cannot be mutated." + +# You can do all those list thingies on tuples too +len(tup) #=> 3 +tup + (4, 5, 6) #=> (1, 2, 3, 4, 5, 6) +tup[:2] #=> (1, 2) +2 in tup #=> True + +# You can unpack tuples into variables +a, b, c = (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 +# Now look how easy it is to swap two values +e, d = d, e # d is now 5 and e is now 4 + + +# Dictionaries store mappings +empty_dict = {} +# Here is a prefilled dictionary +filled_dict = {"one": 1, "two": 2, "three": 3} + +# Look up values with [] +filled_dict["one"] #=> 1 + +# Get all keys as a list +filled_dict.keys() #=> ["three", "two", "one"] +# Note - Dictionary key ordering is not guaranteed. +# Your results might not match this exactly. + +# Get all values as a list +filled_dict.values() #=> [3, 2, 1] +# Note - Same as above regarding key ordering. + +# Check for existence of keys in a dictionary with in +"one" in filled_dict #=> True +1 in filled_dict #=> False + +# Trying to look up a non-existing key will raise a KeyError +filled_dict["four"] #=> KeyError + +# Use get method to avoid the KeyError +filled_dict.get("one") #=> 1 +filled_dict.get("four") #=> None + +# The get method supports a default argument when the value is missing +filled_dict.get("one", 4) #=> 1 +filled_dict.get("four", 4) #=> 4 + +# Setdefault method is a safe way to add new key-value pair into dictionary +filled_dict.setdefault("five", 5) #filled_dict["five"] is set to 5 +filled_dict.setdefault("five", 6) #filled_dict["five"] is still 5 + + +# Sets store ... well sets +empty_set = set() +# Initialize a set with a bunch of values +some_set = set([1,2,2,3,4]) # filled_set is now set([1, 2, 3, 4]) + +# Since Python 2.7, {} can be used to declare a set +filled_set = {1, 2, 2, 3, 4} # => {1 2 3 4} + +# Add more items to a set +filled_set.add(5) # filled_set is now {1, 2, 3, 4, 5} + +# Do set intersection with & +other_set = set{3, 4, 5, 6} +filled_set & other_set #=> {3, 4, 5} + +# Do set union with | +filled_set | other_set #=> {1, 2, 3, 4, 5, 6} + +# Do set difference with - +{1,2,3,4} - {2,3,5} #=> {1, 4} + +# Check for existence in a set with in +2 in filled_set #=> True +10 in filled_set #=> False + + +#################################################### +## 3. Control Flow +#################################################### + +# Let's just make a variable +some_var = 5 + +# Here is an if statement. INDENTATION IS SIGNIFICANT IN PYTHON! +# prints "some var is smaller than 10" +if some_var > 10: + print "some_var is totally bigger than 10." +elif some_var < 10: # This elif clause is optional. + print "some_var is smaller than 10." +else: # This is optional too. + print "some_var is indeed 10." + + +""" +For loops iterate over lists +prints: + dog is a mammal + cat is a mammal + mouse is a mammal +""" +for animal in ["dog", "cat", "mouse"]: + # You can use % to interpolate formatted strings + print "%s is a mammal" % animal + +""" +While loops go until a condition is no longer met. +prints: + 0 + 1 + 2 + 3 +""" +x = 0 +while x < 4: + print x + x += 1 # Shorthand for x = x + 1 + +# Handle exceptions with a try/except block + +# Works on Python 2.6 and up: +try: + # Use raise to raise an error + raise IndexError("This is an index error") +except IndexError as e: + pass # Pass is just a no-op. Usually you would do recovery here. + +# Works for Python 2.7 and down: +try: + raise IndexError("This is an index error") +except IndexError, e: # No "as", comma instead + pass + + +#################################################### +## 4. Functions +#################################################### + +# Use def to create new functions +def add(x, y): + print "x is %s and y is %s" % (x, y) + return x + y # Return values with a return statement + +# Calling functions with parameters +add(5, 6) #=> 11 and prints out "x is 5 and y is 6" +# Another way to call functions is with keyword arguments +add(y=6, x=5) # Keyword arguments can arrive in any order. + +# You can define functions that take a variable number of +# positional arguments +def varargs(*args): + return args + +varargs(1, 2, 3) #=> (1,2,3) + + +# You can define functions that take a variable number of +# keyword arguments, as well +def keyword_args(**kwargs): + return kwargs + +# Let's call it to see what happens +keyword_args(big="foot", loch="ness") #=> {"big": "foot", "loch": "ness"} + +# You can do both at once, if you like +def all_the_args(*args, **kwargs): + print args + print kwargs +""" +all_the_args(1, 2, a=3, b=4) prints: + [1, 2] + {"a": 3, "b": 4} +""" + +# You can also use * and ** when calling a function +args = (1, 2, 3, 4) +kwargs = {"a": 3, "b": 4} +foo(*args) # equivalent to foo(1, 2, 3, 4) +foo(**kwargs) # equivalent to foo(a=3, b=4) +foo(*args, **kwargs) # equivalent to foo(1, 2, 3, 4, a=3, b=4) + +# Python has first class functions +def create_adder(x): + def adder(y): + return x + y + return adder + +add_10 = create_adder(10) +add_10(3) #=> 13 + +# There are also anonymous functions +(lambda x: x > 2)(3) #=> True + +# There are built-in higher order functions +map(add_10, [1,2,3]) #=> [11, 12, 13] +filter(lambda 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 in [1, 2, 3]] #=> [11, 12, 13] +[x for x in [3, 4, 5, 6, 7] if x > 5] #=> [6, 7] + +#################################################### +## 5. Classes +#################################################### + +# We subclass from object to get a class. +class Human(object): + + # A class attribute. It is shared by all instances of this class + species = "H. sapiens" + + # Basic initializer + def __init__(self, name): + # Assign the argument to the instance's name attribute + self.name = name + + # An instance method. All methods take self as the first argument + def say(self, msg): + return "%s: %s" % (self.name, msg) + + # A class method is shared among all instances + # They are called with the calling class as the first argument + @classmethod + def get_species(cls): + return cls.species + + # A static method is called without a class or instance reference + @staticmethod + def grunt(): + return "*grunt*" + + +# Instantiate a class +i = Human(name="Ian") +print i.say("hi") # prints out "Ian: hi" + +j = Human("Joel") +print j.say("hello") #prints out "Joel: hello" + +# Call our class method +i.get_species() #=> "H. sapiens" + +# Change the shared attribute +Human.species = "H. neanderthalensis" +i.get_species() #=> "H. neanderthalensis" +j.get_species() #=> "H. neanderthalensis" + +# Call the static method +Human.grunt() #=> "*grunt*" +``` + +## Further Reading + +Still up for more? Try [Learn Python The Hard Way](http://learnpythonthehardway.org/book/) + |