A list can also be empty: empty_list = [] Lists are similar to strings, but there are a few key differences.

Transcription

1 Chap 7 Python Lists and Dictionaries Introduction to Lists Lists are a data type you can use to store a collection of different pieces of information as a sequence under a single variable name. (Data type: strings, numbers, and Booleans) Assign items to a list with an expression of the form with the items in between brackets: list_name = [item_1, item_2] A list can also be empty: empty_list = [] Lists are similar to strings, but there are a few key differences. zoo_animals = ["pangolin", "cassowary", "sloth", "panda"]; Access by Index Access an individual item on the list by its index. An index is like an address that identifies the item s place in the list. The index (starts from 0) appears directly after the list name, in between brackets, like: list_name[index] numbers = [1,9,9,2,0,3,1,8] print numbers[2] + numbers[6] # = 10 New Neighbors A list index behaves like any other variable name. It can be used to access as well as assign values. 01. Access a list value: zoo_animals[0] 02. Assign a value to an item in a list: zoo_animals[2] = hyena # changes the original value to a new value Late Arrivals & List Length A list doesn t have to have a fixed length. You can add items to the end of a list any time by using append(). letters = [ a, b, c, d ] letters.append( e ) print len(letters)

2 print letters suitcase = [] suitcase.append("sunglasses") suitcase.append('flops') suitcase.append('towel') suitcase.append('swimming suit') list_length = len(suitcase) # Set this to the length of the suitcase list print "There are %d items in the suitcase." % (list_length) print suitcase List Slicing Sometimes, only want to access a portion of a list. Start at the index before the colon and continue up to but not including the index after the colon. letters = ['a', 'b', 'c', 'd', 'e'] slice = letters[1:3] # take a subsection and store it in the slice list # starts at the 1 index item ( b ) but not includes the 3 index item ( d ) print slice print letters # a portion items of the list [ b, c ] # ['a', 'b', 'c', 'd', 'e'] suitcase = ["sunglasses", "hat", "passport", "laptop", "suit", "shoes"] first = suitcase[0:2] middle = suitcase[2:4] last = suitcase[4:6] # The first and second items (index zero and one) # Third and fourth items (index two and three) # The last two items (index four and five)

3 Slicing Lists and Strings You can slice a string exactly like a list. Think of strings as lists of characters: each character is a sequential item in the list, string from index 0. my_list[:2] my_list[3:] # grabs the first two items # grabs the forth through last items If your list slice includes the very first or last item in a list (or a string), the index for that item doesn t have to be included. animals = "catdogfrog" cat = animals[:3] dog = animals[3:6] frog = animals[6:] # The first three characters of animals # The fourth through sixth characters # From the seventh character to the end Maintaining Order Sometimes you need to search for an item in a list: animals = [ ant, bat, cat ] print animals.index( bat ) # prints the index that contains the string Insert items into a list: animals.insert(1, dog ) print animals # [ ant, dog, bat, cat ] animals = ["aardvark", "badger", "duck", "emu", "fennec fox"] duck_index = animals.index("duck") # use a variable to store the index of the string animals.insert(duck_index, 'cobra') # use the above variable as the index argument

4 print animals # ['aardvark', 'badger', 'cobra', 'duck', 'emu', 'fennec fox'] For One and All If want to do sth. with every item in the list, use a for loop. The for loops in Python are different from loops in JavaScript. Syntax: for variable in list_name: # Do stuff! A variable name follows the for keyword; it will be assigned the value of each list item in turn. The line ends with a colon (:) and the indented code that follows it will be executed once per item in the list. my_list = [1,9,3,8,5,7] for number in my_list: print 2 * number # 2, 18, 6, 16, 10, 14 More with for If the list is a jumbled mess, you may need to sort() it. animals = ["cat", "ant", "bat"] animals.sort() # sort the list in alphabetical order for animal in animals: print animal # ["ant"] ["bat"] ["cat"] on separated lines start_list = [5, 3, 1, 2, 4] square_list = [] for x in start_list: # x is just a holdplace and it will be replaced by the items in the list square_list.append(number ** 2)

5 square_list.sort() print start_list # [5, 3, 1, 2, 4] print square_list # [1, 4, 9, 16, 25] This Next Part is Key You access values by looking up a key instead of an index. Dictionaries are enclosed in curly braces, like so: d = {'key1' : 1, 'key2' : 2, 'key3' : 3} This is a dictionary called d with 3 key-value pairs. The key1 points to the value 1. Dictionaries are great for things like phone books (pairing a name with a phone number), login pages (pairing an address with a username), and more. Accessing the dictionary values by key is just like accessing list values by index: residents = {'Puffin' : 104, 'Sloth' : 105, 'Burmese Python' : 106} print residents['puffin'] # gets the value 104 New Entries Like Lists, Dictionaries are mutable. This means they can be changed after they are created. One advantage of this is that we can add new key/value pairs to the dictionary after it is created like so: dict_name[new key] = new_value An empty pair of curly braces {} is an empty dictionary, just like an empty pair of [] is an empty list. dict_name = {} The len() of a dictionary is the number of key-value pairs it has. Each pair counts only once, even if the value is a list. (You can put lists inside dictionaries!) len(dic_name) menu = {} menu['chicken Alfredo'] = # Empty dictionary # Adding new key-value pair

6 print menu['chicken Alfredo'] #14.50 menu['broccoli Beef'] = menu['fired Rice'] = menu['dumplings'] = menu['pastry'] = 6.50 print "There are " + str(len(menu)) + " items on the menu." print menu # There are 5 items on the menu. # {'Dumplings': 11.5, 'Chicken Alfredo': 14.5, 'Broccoli Beef': 12.5, 'Fired Rice': 10.0, 'Pastry': 6.5} Changing Your Mind Items can be removed from a dictionary with the del command: (just can remove items one by one, cannot remove several items at a time) del dict_name[key_name] # removes the key and its associated value from the dictionary A new value can be associated with a key by assigning a value to the key, like so: dict_name[key] = new_value del zoo_animals['sloth'] del zoo_animals['bengal Tiger'] zoo_animals['rockhopper Penguin']='panda' Remove a Few Things Sometimes need to remove sth. from a list:.remove(item) beatles = ["john","paul","george","ringo","stuart"]

7 beatles.remove("stuart") print beatles # ["john","paul","george","ringo"] More Notes about Dictionaries Go over a few last notes about dictionaries: 01. Get the value from a list under a key in libraries dict_name[ key_name ][ index ] my_dict = { # create a dictionary that holds many types of values "fish": ["c", "a", "r", "p"], "cash": -4483, "luck": "good" } # a list # an int # a string print my_dict["fish"][0] # prints out the first item (0 index) in the fish list c Use list functions with a list stored in a dictionary as follows: dict_name[ list_key ].list_function() inventory = { 'gold' : 500, # An int variable 'pouch' : ['flint', 'twine', 'gemstone'], # A list 'backpack' : ['xylophone','dagger', 'bedroll','bread loaf'] } # A list # Adding a key 'burlap bag' and assigning a list to it inventory['burlap bag'] = ['apple', 'small ruby', 'three-toed sloth'] # Sorting the list found under the key 'pouch' inventory['pouch'].sort() # Adding a key pocket and assigning a list to it

8 inventory['pocket']=['seashell', 'strange berry', 'lint'] # Sorting the list found under the key backpack inventory['backpack'].sort() # Removing the dagger item from the backpack list inventory['backpack'].remove('dagger') # Adding 50 to the number stored under the gold key inventory['gold']+= 50 print inventory # {'pocket': ['seashell', 'strange berry', 'lint'], 'backpack': ['bedroll', 'bread loaf', 'xylophone'], 'pouch': ['flint', 'gemstone', 'twine'], 'burlap bag': ['apple', 'small ruby', 'three-toed sloth'], 'gold': 550} Chap 7 Exercise: A Day at the Supermarket BeFOR We Begin Go over the for loop in terms of how it relates to lists and dictionaries. for loops allow us to iterate through all the elements in a list from the left-most (or zeroth element) to the right-most element. A sample loop would be structured as follows: a = [ List of some sort ] for x in a: # Do sth. for every x The loop will run all of the code in the indented block. The item in the list that is currently being evaluated will be x. The variable between for and in can be set to any variable name (currently item), but should be careful to avoid using list as a

9 variable, since that s a reserved word (that is, it means sth. special) in the Python language. names = ["Adam","Alex","Mariah","Martine","Columbus"] for name in names: print name This is KEY! Use a for loop on a dictionary to loop through its keys with the following: # A simple dictionary d = { foo : bar } for key in d: print d[key] # prints bar Dictionaries are unordered, meaning that any time you loop through a dictionary, you will go through every key, but you are not guaranteed to get them in any particular order. Control Flow and Looping The blocks of code in a for loop can be as big or as small as they need to be. While looping, you may want to perform different actions depending on the particular item in the list. numbers = [1,3,4,7] for number in numbers: if number > 6: # add a condition print number # prints 7 a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] for number in a: if number % 2 == 0:

10 print number # prints out the even number in a list Lists + Functions Functions can also take lists as inputs and perform various operations on those lists. def count_small(numbers): # declare a function count_small() with a numbers argument total = 0 for n in numbers: if n < 10: total +=1 # total = total + 1 return total lost = [4,8,15,16,23,42] small = count_small(lost) # declare a lost list/array # call the count_small function with a list argument print small # 4+8 = 12 def fizz_count(x): # declare a function fizz_count with one argument x count = 0 for item in x: if item == "fizz": count += 1 return count print fizz_count(["fizz", "cat", "fizz"]) # call the function with a list argument

11 String Looping Strings are like lists with characters as elements. You can loop through strings the same way you loop through lists. for letter in "Codecademy": # think of the string as a list of character elements print letter print # Empty lines to make the output pretty print word = "Programming is fun!" for letter in word: if letter == "i": # Only print out the letter i print letter Your Own Store! Create a dictionary called animal_counts with 3 entries with keys and values. Syntax: DictionaryName = { Key1 : Value1, Key2 : Value2 } animal_counts = { ant : 3, bear : 6, crow : 2 } Investing in Stock

12 prices = { "banana": 4, "apple": 2, "orange": 1.5, "pear": 3 } stock = { "banana": 6, "apple": 0, "orange": 32, "pear": 15 } Keeping Track of the Produce Print out all of your inventory information. once = {'a': 1, 'b': 2} # create two dictionaries with the same keys twice = {'a': 2, 'b': 4} for key in once: # with the same keys, just loop through one dictionary and prints values from both two dictionaries print "Once: %s" % once[key] print "Twice: %s" % twice[key]

13 for key in prices: print key print "price: %s" % prices[key] # banana # price: 4 (the value under the key) print "stock: %s" % stock[key] # stock: 6 Something of Value Record the total value of the inventory. total = 0 # declares a global variable total and initializes it to 0 for key in prices: print key print "price: %s" % prices[key] print "stock: %s" % stock[key] print prices[key] * stock[key] sum = prices[key] * stock[key] total +=sum # print sum for each category # declares a local variable sum # total = total + sum print total # Shopping at the Market In order for customers to order online, we are going to have to make a consumer interface. 01. Make a list groceries = ["banana", "apple", "orange"] 02. Make a purchase <example> def sum(numbers): total = 0

14 for number in numbers: # for each number in the list, add that number to the running sum total total += number return total n = [1, 2, 5, 10, 13] print sum(n) shopping_list = ["banana", "orange", "apple"] def compute_bill(food): # declare a function with a food argument total = 0 for item in food: # loop through the food arugment total +=prices[item] # prices is a list of food price, with the same keys as the food argument return total Stocking Out Take the stock/inventory of a particular item into account when computing the cost. If an item isn t in stock, then it shouldn t be included in the total. def compute_bill(food): total = 0 for item in food: if stock[item] > 0: # check the inventory for each item total +=prices[item]

15 stock[item] -= 1 # subtract 1 from the inventory record return total Let s Check Out! Recap: 01.Using for loops with lists and dictionaries 02.Writing functions with loops, lists, and dictionaries 03.Updating data in response to changes in the environment (for instance, decreasing the number of amount in stock by 1 when you sell an item)