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Lists
CSCI 1101B
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?
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?
Today’s Outline
● Lists: Definition, Built-in Functions
● Strings and Lists: Similarities
● Splitting Strings
● Mutable vs. Immutable, List Methods
● Iteration (Brief Intro)
Storing ingredients as values
ingredient_1 = “bread”
ingredient_2 = “cheese”
ingredient_3 = “butter”
Credit: EncycloPetey
Storing ingredients as values
ingredient_1 = “bread”
ingredient_2 = “cheese”
ingredient_3 = “ham”
ingredient_4 = “lettuce”
ingredient_5 = “chicken salad”
ingredient_6 = “tomato”
...
Part I
Lists
Lists
The general form of a list is:
[<<expression_1>>, <<expression_2>>, …, <<expression_n>>]
Lists
The general form of a list is:
[<<expression_1>>, <<expression_2>>, …, <<expression_n>>]
Examples:
ingredients = [“bread”, “cheese”, “ham”]
Lists
The general form of a list is:
[<<expression_1>>, <<expression_2>>, …, <<expression_n>>]
Examples:
ingredients = [“bread”, “cheese”, “ham”]
test_scores = [97, 88, 95]
More Examples!
Credit: Gary Peeples/USFWS; Sue
Cameron
Lists can contain different data types
The “Cocoa Frog”
Credit: Stuart V Nielsen
Lists can contain different data types
cocoa_frog = [“male”, 2.5, “resting”]
Nesting Lists
cocoa_frog = [“male”, [2.5, “inches”], “resting”]
List of Lists: Example 1
new_species = [
[“cocoa frog”, 3],
[“crayola katydid”, 6],
[“pac-man frog”, 1],
[“lilliputian beetle”, 4]
]
List of Lists: Example 2
ingredients = [
[3.5, “cup”, “flour”],
[4, “teaspoon”, “baking powder”],
[5, “tablespoon”, “butter”],
[4, “tablespoon”, “sugar”]
]
Lists: Built-in Functions
len([“a”, “b”, “c”]) => 3
Lists: Built-in Functions
len([“a”, “b”, “c”]) => 3
max([84, -2, 32]) => 84min([84, -2, 32]) => -2
Lists: Built-in Functions
len([“a”, “b”, “c”]) => 3
max([84, -2, 32]) => 84min([84, -2, 32]) => -2
sum([1, 2, 0]) => 3
Lists: Built-in Functions
len([“a”, “b”, “c”]) => 3
max([84, -2, 32]) => 84min([84, -2, 32]) => -2
sum([1, 2, 0]) => 3
sorted([1, 2, 0]) => [0, 1, 2]
Part II
Strings and Lists:Similarities
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
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 = “”
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.”)
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
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]
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!”)
Strings and Lists: Similarities
We can also slice lists, just like strings!
ingredients = [“bread”, “cheese”, “tomato”]
ingredients[0] => ‘bread’ingredients[-1] => ‘tomato’ingredients[:2] => ['bread', 'cheese']ingredients[:] => ['bread', 'cheese', ‘tomato’]
0 1 2
Strings and Lists: Differences?
my_list = [1, 2, 3]
my_list[0] = 7
=> Now, my_list is [7, 2, 3]
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
Part III
Mutable vs. Immutable
Strings are Immutable
x = “one”y = xz = y
Strings are Immutable
x = “one”y = xz = yz += “two”
Strings are Immutable
x = “one”y = xz = yz += “two”
What are the values assigned to x, y, and z?
x == y == “one”z == “onetwo”
Lists are Mutable
x = [1, 2, 3]y = xz = y
Lists are Mutable
x = [1, 2, 3]y = xz = yz += [4]
Lists are Mutable
x = [1, 2, 3]y = xz = yz += [4]
What are the values assigned to x, y, and z?
x == y == z == [1, 2, 3, 4]
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]
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
Assigning to a non-existent index
my_list = [“apple”, “lemon”, “kiwi”]
Assigning to a non-existent index
my_list = [“apple”, “lemon”, “kiwi”]
my_list[0] = “strawberry”
Assigning to a non-existent index
my_list = [“apple”, “lemon”, “kiwi”]
my_list[0] = “strawberry”
my_list[6] = “blueberry”
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.
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]
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]
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”
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]
Part IV
Iteration
Another way to use the “in” operator
for <<variable>> in <<list>>:
<<block>>
Loops let us repeat code!
Here is the general form for a “for each” loop over a list:
Example: Singing a Song Really Loudly
Credit:sunnyskyz.com
For Each Loops help us repeat code
phrases = [“the real life”, “just fantasy”]
For Each Loops help us repeat code
phrases = [“the real life”, “just fantasy”]
for phrase in phrases:
For Each Loops help us repeat code
phrases = [“the real life”, “just fantasy”]
for phrase in phrases: print(“IS THIS ” + phrase.upper() + “?”)
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?
For Each Loop - Examples
Strings:
for character in “a little string”: print(character)
Lists:
for element in [1, 4, 7, 11]: print(element * element)
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...
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.
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