def order(food): food = food.upper() print( Could I have a big + food + please? ) return fresh + food

Transcription

1 CSCI 1101B Lists

2 Warm-up Exercise def order(food): food = food.upper() print( Could I have a big + food + please? ) return fresh + food food = order( pasta ) After this program runs 1. What is the global value of food? 2. What is printed out to the screen?

3 Warm-up Exercise def order(food): food = food.upper() print( Could I have a big + food + please? ) return fresh + food food = order( pasta ) After this program runs 1. Global value of food == fresh PASTA 2. Printed: Could I have a big PASTA please?

4 Today s Outline Lists: Definition, Built-in Functions Strings and Lists: Similarities Splitting Strings Mutable vs. Immutable, List Methods Iteration (Brief Intro)

5 Credit: Maggie Hoffman

6 Storing ingredients as values ingredient_1 = bread ingredient_2 = cheese ingredient_3 = butter

7 Credit: EncycloPetey

8 Storing ingredients as values ingredient_1 = bread ingredient_2 = cheese ingredient_3 = ham ingredient_4 = lettuce ingredient_5 = chicken salad ingredient_6 = tomato...

9 Part I Lists

10 Lists The general form of a list is: [<<expression_1>>, <<expression_2>>,, <<expression_n>>]

11 Lists The general form of a list is: [<<expression_1>>, <<expression_2>>,, <<expression_n>>] Examples: ingredients = [ bread, cheese, ham ]

12 Lists The general form of a list is: [<<expression_1>>, <<expression_2>>,, <<expression_n>>] Examples: ingredients = [ bread, cheese, ham ] test_scores = [97, 88, 95]

13 More Examples! Credit: Gary Peeples/USFWS; Sue Cameron

14 Lists can contain different data types The Cocoa Frog Credit: Stuart V Nielsen

15 Lists can contain different data types cocoa_frog = [ male, 2.5, resting ]

16 Nesting Lists cocoa_frog = [ male, [2.5, inches ], resting ]

17 List of Lists: Example 1 new_species = [ [ cocoa frog, 3], [ crayola katydid, 6], [ pac-man frog, 1], [ lilliputian beetle, 4] ]

18 List of Lists: Example 2 ingredients = [ [3.5, cup, flour ], [4, teaspoon, baking powder ], [5, tablespoon, butter ], [4, tablespoon, sugar ] ]

19 Lists: Built-in Functions len([ a, b, c ]) => 3

20 Lists: Built-in Functions len([ a, b, c ]) => 3 max([84, -2, 32]) => 84 min([84, -2, 32]) => -2

21 Lists: Built-in Functions len([ a, b, c ]) => 3 max([84, -2, 32]) => 84 min([84, -2, 32]) => -2 sum([1, 2, 0]) => 3

22 Lists: Built-in Functions len([ a, b, c ]) => 3 max([84, -2, 32]) => 84 min([84, -2, 32]) => -2 sum([1, 2, 0]) => 3 sorted([1, 2, 0]) => [0, 1, 2]

23 Part II Strings and Lists: Similarities

24 Strings and Lists: Similarities We can find the length of a list or a string with len() len([1, 3, 4, 4, 7]) => 5 len( some words ) => 10

25 The Empty List A list with nothing in it is called an empty list. no_elements = [] A string with no characters is called an empty string. no_characters =

26 The Empty List We can use conditionals to check if strings or lists are empty. if 0: print( This won t ever print. ) if : print( Neither will this. ) if []: print( Oh, and this won t, either. )

27 Strings and Lists: Similarities We can count the number of occurrences of an element in a list (just like we can count substrings) [1, 3, 4, 4, 7].count(4) => 2 We can find the index of the first occurrence of an element (again, just like substrings) [1, 3, 4, 4, 7].index(4) => 2

28 Strings and Lists: Similarities We can concatenate lists with the + operator [1, 2] + [3] => [1, 2, 3] We can also multiply a list by an int (not as common): [1, 2] * 3 = [1, 2, 1, 2, 1, 2]

29 Strings and Lists: Similarities We can use the in operator with lists! fruits = [ apple, lemon, kiwi ] user_fruit = input( Enter a fruit: ) if user_fruit.lower() in fruits: print( Oh! I know that one! )

30 Strings and Lists: Similarities We can also slice lists, just like strings! ingredients = [ bread, cheese, tomato ] ingredients[0] ingredients[-1] ingredients[:2] ingredients[:] => => => => bread tomato ['bread', 'cheese'] ['bread', 'cheese', tomato ]

31 Strings and Lists: Differences? my_list = [1, 2, 3] my_list[0] = 7 => Now, my_list is [7, 2, 3]

32 Strings and Lists: Similarities my_list = [1, 2, 3] my_list[0] = 7 => Now, my_list is [7, 2, 3] my_string = hello my_string[0] = y => This results in a TypeError

33 Part III Mutable vs. Immutable

34 Strings are Immutable x = one y = x z = y

35 Strings are Immutable x y z z = one = x = y += two

36 Strings are Immutable x y z z = one = x = y += two What are the values assigned to x, y, and z? x == y == one z == onetwo

37 Lists are Mutable x = [1, 2, 3] y = x z = y

38 Lists are Mutable x y z z = [1, 2, 3] = x = y += [4]

39 Lists are Mutable x y z z = [1, 2, 3] = x = y += [4] What are the values assigned to x, y, and z? x == y == z == [1, 2, 3, 4]

40 Lists are Mutable If you want to copy a list (so that the original list doesn t mutate), use slicing notation: x = [1, 2, 3] y = x[:] y += [8] Now x = [1, 2, 3] and y = [1, 2, 3, 8]

41 More List Methods insert(index, object): inserts object into list before index pop(index): remove and return object at index (defaults to last if no argument given) remove(value): removes first occurrence of value reverse(): reverses the contents of the list sort(): like built-in sorted function Be careful - all of these except for pop() return None

42 Assigning to a non-existent index my_list = [ apple, lemon, kiwi ]

43 Assigning to a non-existent index my_list = [ apple, lemon, kiwi ] my_list[0] = strawberry

44 Assigning to a non-existent index my_list = [ apple, lemon, kiwi ] my_list[0] = strawberry my_list[6] = blueberry

45 Assigning to a non-existent index my_list = [ apple, lemon, kiwi ] my_list[0] = strawberry my_list[6] = blueberry This will cause an IndexError - we are trying to access an index that is out of the range of indices available.

46 append() is different from extend() (or +) append() adds exactly one element my_list = [1, 2, 3] my_list.append(4) # => my_list = [1, 2, 3, 4]

47 append() is different from extend() (or +) append() adds exactly one element my_list = [1, 2, 3] my_list.append(4) # => my_list = [1, 2, 3, 4] extend() (or +) will merge two lists my_list = [1, 2, 3] my_list.extend([4, 5]) # => my_list = [1, 2, 3, 4, 5]

48 split() and join() We can make strings into lists: sent = once upon a time sent.split( ) => [ once, upon, a, time ]...and lists into strings: words = [ once, upon, a, time ].join(words) => once upon a time

49 Question What if we wanted to perform an operation on every element of a list (or every character of a string)? Example: multiply each number in a list by three. multiply_by_three([1, 2, 3, 4]) => [3, 6, 9, 12]

50 Part IV Iteration

51 Another way to use the in operator Loops let us repeat code! Here is the general form for a for each loop over a list: for <<variable>> in <<list>>: <<block>>

52 Example: Singing a Song Really Loudly Credit:sunnyskyz.com

53 For Each Loops help us repeat code phrases = [ the real life, just fantasy ]

54 For Each Loops help us repeat code phrases = [ the real life, just fantasy ] for phrase in phrases:

55 For Each Loops help us repeat code phrases = [ the real life, just fantasy ] for phrase in phrases: print( IS THIS + phrase.upper() +? )

56 For Each Loops help us repeat code phrases = [ the real life, just fantasy ] for phrase in phrases: print( IS THIS + phrase.upper() +? ) IS THIS THE REAL LIFE? IS THIS JUST FANTASY?

57 For Each Loop - Examples Strings: for character in a little string : print(character) Lists: for element in [1, 4, 7, 11]: print(element * element)

58 Summary (1) Lists are one way to store collections. We can use the in operator to not only determine whether elements of a list exist, but to iterate through a list (or string) using a for each loop. Lists have a lot in common with strings. However, there are some key differences. For example...

59 Summary (2) Lists are mutable: We can change elements without creating an entirely new object. Applying methods, like pop(), will change the list object. To keep a list from changing, use a copy (or assign a value). Strings (and ints, floats, and bools) are immutable. We can t change individual elements. All we can do is assign a new value to the same name. Applying methods will not change the original object without a reassignment statement.