DS4UX (Spring 2016)/Day 3 lecture

Lists

 * Use lists to store data where order matters.
 * Lists are indexed starting with 0.

List initialization
>>> my_list = [] >>> my_list [] >>> your_list = ["a", "b", "c", 1, 2, 3] >>> your_list ['a', 'b', 'c', 1, 2, 3]

Access and adding elements to a list
>>> len(my_list) 0 >>> my_list[0] Traceback (most recent call last): File " ", line 1, in IndexError: list index out of range >>> my_list.append("Alice") >>> my_list ['Alice'] >>> len(my_list) 1 >>> my_list[0] 'Alice' >>> my_list.insert(0, "Amy") >>> my_list ['Amy', 'Alice']

>>> my_list = ['Amy', 'Alice'] >>> 'Amy' in my_list True >>> 'Bob' in my_list False

Changing elements in a list
>>> your_list = [] >>> your_list.append("apples") >>> your_list[0] 'apples' >>> your_list[0] = "bananas" >>> your_list ['bananas']

Slicing lists
>>> her_list = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'] >>> her_list[0] 'a' >>> her_list[0:3] ['a', 'b', 'c'] >>> her_list[:3] ['a', 'b', 'c'] >>> her_list[-1] 'h' >>> her_list[5:] ['f', 'g', 'h'] >>> her_list[:] ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']

sorting lists
Use  to sort a list:

>>> names = ["Eliza", "Joe", "Henry", "Harriet", "Wanda", "Pat"] >>> names.sort >>> names ['Eliza', 'Harriet', 'Henry', 'Joe', 'Pat', 'Wanda'] >>> names.sort(reverse=True) ['Wanda', 'Pat', 'Joe', 'Henry', 'Harriet', 'Eliza']

Getting the maximum and minimum values from a list
>>> numbers = [0, 3, 10, -1] >>> max(numbers) 10 >>> min(numbers) -1

Formatting strings
Formatting strings makes it much easier to combine alphanumeric characters and other types of object (like ints, floats, and bools) and do things with them—like print!

>>> x = 1 >>> y = 1.234 >>> z = True >>> w = "elevator" >>> all_together_now = "You can put ints like %d, floating point numbers like %f, boolean values like %s, and other strings like %s into a string without changing them to strings first!" % (x,y,z,w) >>> print(all_together_now)

Dealing with whitespace
>>> text = " this is   a text  string   with lots    of extra      spaces     " >>> text.strip "this is   a text  string   with lots    of extra      spaces" >>> text.split ['this', 'is', 'a', 'text', 'string', 'with', 'lots', 'of', 'extra', 'spaces'] >>> " ".join(text.split) 'this is a text string with lots of extra spaces'

Tuples
Tuples are similar to lists, but unlike lists, once they're created ("assigned") they can't be changed. Since most of our work involves reading and writing files and building and manipulating sets of data, we might not have too much cause to use tuples. But Python uses them a lot "behind the scenes", and they're useful for other types of programming, so we'll go over them briefly here.

You can create a tuple just like a list... >>> my_tuple = ("John", "Terry", "Terry", "Graham", "Eric")

You can find items by index... >>> my_tuple[1] 'Terry'

BUT you can't edit them... >>> my_tuple[1] = "John" --- TypeError                                Traceback (most recent call last)  in > 1 my_tuple[1] = "Michael"

TypeError: 'tuple' object does not support item assignment

Generating a list of numbers easily with
>>> range(5) [0, 1, 2, 3, 4] >>> for i in range(5): ...    print("Hi" * i) ...

Hi HiHi HiHiHi HiHiHiHi

The  function returns a list of numbers. This is handy for when you want to generate a list of numbers on the fly instead of creating the list yourself.

>>> range(5) [0, 1, 2, 3, 4]

Use  when you want to loop over a bunch of numbers in a list, or perform an operation a certain number of times:

>>> numbers = range(5) >>> for number in numbers: ...    print(number * number) ... 0 1 4 9 16

We could rewrite the above example like this:

>>> for number in range(5): ...    print(number * number) ... 0 1 4 9 16

You can also set the start, end, and increment value (called "step") for a range. >>> for i in range(2,20,2): ...        print(i) 2 4 6 8 10 12 14 16 18

Using break statements to halt execution
word_list = ["the", "quick", "brown", "fox", "jumped", "over", "the", "lazy", "dog"] letter = "z" seen_letter = False for word in word_list: if letter in word: seen_letter = True print("%s contains the letter %s" % (word, letter)) else: print("no %s in %s" % (letter, word))

Get user input with
>>> for i in range(100): ...    my_input = input("Please type something> ") ...    if input == "Quit": ...        print("Goodbye!") ...        break ...    else: ...        print("You said: " + my_input) ... Please type something> Hello You said: Hello Please type something> How are you? You said: How are you? Please type something> Quit Goodbye! >>>

Iterating an indeterminate number of times with loops
grocery_list = [] testAnswer = input('Press y if you want to enter more groceries: ') while testAnswer == 'y': food = input('Next item:') grocery_list.append(food) testAnswer = input('Press y if you want to enter more groceries: ')

print('Your grocery list:') for food in grocery_list: print(food)

Dictionaries

 * Use dictionaries to store key/value pairs.
 * Dictionaries do not guarantee ordering.
 * A given key can only have one value, but multiple keys can have the same value.

Initialization
>>> my_dict = {} >>> my_dict {} >>> your_dict = {"Alice" : "chocolate", "Bob" : "strawberry", "Cara" : "mint chip"} >>> your_dict {'Bob': 'strawberry', 'Cara': 'mint chip', 'Alice': 'chocolate'}

Types
>>> type(my_dict) 

Adding and removing elements
>>> your_dict["Dora"] = "vanilla" >>> your_dict {'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'chocolate'}

>>> del your_dict["Dora"] >>> your_dict {'Bob': 'strawberry', 'Cara': 'mint chip', 'Alice': 'chocolate'}

Accessing elements of a dictionary
>>> your_dict["Alice"] 'chocolate' >>> your_dict.get("Alice") 'chocolate'

>>> your_dict["Eve"] Traceback (most recent call last): File " ", line 1, in KeyError: 'Eve' >>> "Eve" in your_dict False >>> "Alice" in your_dict True >>> your_dict.get("Eve") >>> person = your_dict.get("Eve") >>> print(person) None >>> print(type(person))  >>> your_dict.get("Alice") 'chocolate'

Dictionary keys can be integers, and their values can be any data type
>>> mixed_dict = {1:3, 2:'two', 3:False, 'four':['john','terry','graham']} >>> print(mixed_dict[1]) 3 >>> print(mixed_dict[2]) two >>> print(mixed_dict[3]) False >>> print(mixed_dict['four'][2]) graham

Changing elements of a dictionary
>>> your_dict["Alice"] = "coconut" >>> your_dict {'Bob': 'strawberry', 'Cara': 'mint chip', 'Dora': 'vanilla', 'Alice': 'coconut'}

Looping through a dictionary
The builtin functions  and   provide you with a lot of flexibility when iterating through dictionaries.

>>>for i in your_dict.items: >>>   print(i) ('Bob', 'strawberry') ('Cara', 'mint chip') ('Dora', 'vanilla') ('Alice', 'chocolate')

>>>for i_key in your_dict.keys: >>>   print(i_key + " is a key in this dictionary") Bob is a key in this dictionary Cara is a key in this dictionary Dora is a key in this dictionary Alice is a key in this dictionary

>>>for i_val in your_dict.values: >>>   print(i_val + " is a value in this dictionary") strawberry is a value in this dictionary mint chip is a value in this dictionary vanilla is a value in this dictionary chocolate is a value in this dictionary

>>> for i_key, i_val in your_dict.items: >>>    print(i_key + " is the key for " + i_val) >>>    print(i_val + " is the value for " + i_key) >>>    print("\n") Bob is the key for strawberry strawberry is the value for Bob ... Cara is the key for mint chip mint chip is the value for Cara ... Dora is the key for vanilla vanilla is the value for Dora ... Alice is the key for chocolate chocolate is the value for Alice

Sorting dictionaries with and
We've already learned how you can use  to create a sorted version of a list. accepts an optional  argument to tell it what to sort on. You can use  with   builtin dictionary function and the   function of the   module to create sorted versions of dictionaries!

>>> import operator >>> family = {'ozy':2, 'jonathan':34, 'portia':10, 'eva':6, 'dana':28} >>> sorted(family.items, key=operator.itemgetter(1), reverse=True) [('jonathan', 34), ('dana', 28), ('portia', 10), ('eva', 6), ('ozy', 2)]

You can also use this approach to sort other complex data structures:

>>> family = 'ozy',2], ['portia',10], ['jonathan',34], ['dana', 28], ['eva', 6 >>> sorted(family, key=operator.itemgetter(1)) 'ozy', 2], ['eva', 6], ['portia', 10], ['dana', 28], ['jonathan', 34 >>> sorted(family, key=operator.itemgetter(0), reverse=True) 'portia', 10], ['ozy', 2], ['jonathan', 34], ['eva', 6], ['dana', 28

Exercise
Click here to download the scripts for this week's in-class exercise