--- 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/)