knowledge-kitchen / course-notes

Lists - Basics (in Python)

• create a list of integers: a = [0, 1, 2]
• create a list of strings: b = ["one", "two", "buckle", "my", "shoe"]
• concatenate lists: c = a + b # -> [0, 1, 2, 'one', 'two', 'buckle', 'my', 'shoe']
• lists can hold any mixture of data types: x = [0, "shoe", True, 4.0, None]
• lists are sequences, like strings: x[1] -> # "shoe"
• lists have positive and negative indices: x[-1] # -> None
• reversing lists is like reversing strings: x[::-1] # -> [None, 4.0, True, “shoe”, 0]`
• unlike strings, lists are mutable: x[1] = "u" # is not an error!

List slicing

• creating a list: x = ['c', 'a', 't']
• forward slicing: x[0:2] # -> ['c', 'a']
• reverse slicing: x[-1: -3, -1] # -> ['t', 'a', 'c']
• custom step amount: x[0:3:2] # -> ['c', 't']
• automatic beginning index: x[:2] # -> ['c', 'a']
• autotomatic end index: x[1:] # -> ['a', 't']
• automatic beginning and end indices: x[:] # -> ['c', 'a', 't']
• shortcut for reversing a list: x[::-1] # -> ['t', 'a', 'c']

More list techniques

• looping through elements in a list: for i in x:
• checking for a needle in a haystack: if i in x:

Aliases vs. copies

Know the difference between the two:

• alias
• copy

x = [3, 4, "lock", "the", "door"]
y = x # y and x are aliases - two variables that refer to the same memory address
z = x.copy() # z points to a copy of what x and y point to
x[2] = "open" # x and y both refer to the updated list
print(z[2]) # prints from z's copy of the original list -> "lock"

More about lists

Click to read more about lists, including List functions and operations.

Examples

Basic list looping and slicing

favoriteFoods = [
    'pizza',
    'bagels',
    'poutine',
    'london broil',
    'cheeseburger',
    'ice cream',
    'spaghetti',
    'quail',
    'sushi',
    'chocolate'
]

#loop through the list using a for loop
print("Looping using for loop...")
for food in favoriteFoods:
    print(food)

#loop through the list using a while loop
print("\nLooping using while loop...")
i = 0
while i < len(favoriteFoods):
    print(favoriteFoods[i])
    i = i + 1

#slice the list
print("\nSlicing the list to get the first five elements: ")
firstFiveFavoriteFoods = favoriteFoods[0:5]
for food in firstFiveFavoriteFoods:
    print(food, end=", ")

#slice the list in reverse
print("\n\nSlicing the list in reverse: ")
lastFiveFavoriteFoods = favoriteFoods[9:4:-1]
for food in lastFiveFavoriteFoods:
    print(food, end=", ")

#slice to get the middle of the list
print("\n\nSlicing to get the middle of the list: ")
middleFavoriteFoods = favoriteFoods[3:8]
for food in middleFavoriteFoods:
    print(food, end=", ")

#copy a list
print("\n\nCopying the list using a slice: ")
copyOfMyFavoriteFoods = favoriteFoods[0:10]
print(copyOfMyFavoriteFoods)

print("\n\nCopying the list using a slice with no parameters: ")
copyOfMyFavoriteFoods = favoriteFoods[:]
print(copyOfMyFavoriteFoods)

#reverse a list
print("\n\n Reversing the list using a slice: ")
reversedListOfFavoriteFoods = favoriteFoods[::-1]
print(reversedListOfFavoriteFoods)

Creating an alias of a List

favoriteFoods = [
    'pizza',
    'bagels',
    'poutine',
    'london broil',
    'cheeseburger',
    'ice cream',
    'spaghetti',
    'quail',
    'sushi',
    'chocolate'
]

aliasOfFavoriteFoods = favoriteFoods

favoriteFoods[1] = "rolls"

for food in aliasFavoriteFoods:
    print(food, end=", ") #outputs the modified list including 'rolls' instead of bagels

Example showing the 'in' operator

favoriteFoods = [
    'pizza',
    'bagels',
    'poutine',
    'london broil',
    'cheeseburger',
    'ice cream',
    'spaghetti',
    'quail',
    'sushi',
    'chocolate'
]

#search the list for a certain element
if "poutine" in favoriteFoods and "bagels" in favoriteFoods:
    print("Ah, yes delicious cheese curds")

Strings behave like Lists in some ways

They are both Sequence data types, after all.

x = "this that and the other"

for c in x:
    print(c)

subString = x[0:10]

print(subString)