Python Data Structures (Lists, Dicts, Tuples, Slicing)

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Python Data Structures

Lists

A variable that stores more than one item, enclosed in square brackets.

Subscripting

list = ["a", "b", "c"]
list[0]   # starts counting from left
 
>> "a"
 
list[-1]  # starts counting from right
 
>> "c"

Editing items

list = ["a", "b", "c"]
list[0] = "b"
 
list = ["b", "b", "c"]

Merging lists

list_a = ["apple", "apricot"]
list_b = ["banana", "blue"]
 
list_c = list_a + list_b
print(list_c)
 
>> ["apple", "apricot", "banana", "blue"]

Nesting lists

list_a = ["apple", "apricot"]
list_b = ["banana", "blue"]
 
list_c = [list_a, list_b]
print(list_c)
 
>> [["apple", "apricot"], ["banana", "blue"]]

Adding single items

list = ["a", "b"]
list.append("c")
print(list)
 
>> ["a", "b", "c"]

Adding multiple items

list = ["a", "b"]
list.extend(["c", "d"])
# OR
list += ["c", "d"]
print(list)
 
>> ["a", "b", "c", "d"]

Removing items

list = ["a", "b"]
list.remove("a")
print(list)
 
>> ["b"]

Finding index

list.index(item, start_index)
 
list = ["a", "b", "c", "a"]
list.index("a")      # gives 0
list.index("a", 1)   # gives 3

Joining items of a list

"__".join()
 
list = ["a", "b", "c"]
"and".join(list)
 
>> "aandbandc"
# or use for loop and string concatenation

Adding dicts to a list

list = [{"key": "value"}]
dict = {"key2": "value2"}
list.append(dict)
# Can't use list += dict, this only adds the key of the dictionary

pop()

a = ['a', 'b', 'c', 'd']
a.pop(1)
 
>> ['a', 'c', 'd']
 
a.pop()   # removes the last item

Dictionaries

Key–Value pairs.

Made up of {Key: Value} pairs

dict = {"name": "elkan", "age": "19"}

Uses curly brackets. Can have multiple items, separated by ,. Each key can only have one value.

Formatting your code

dict = {
    "name": "elkan",
    "age": "19",
}

Retrieving items

# allowed
dict = {"name": "elkan"}
print(dict["name"])
 
# allowed
dict = {123: "elkan"}
print(dict[123])
 
# not allowed
dict = {name: "elkan"}
print(dict[name])
 
# using key and get()
dict["name"] = "elkan"
dict["abc"]                           # gives you a KeyError because no such key exists
dict.get("abc", "Does not exist")     # if abc does not exist, it prints "Does not exist"

Adding new items

dict = {"name": "elkan"}
dict["age"] = "19"

Editing entries

dict = {"name": "elkan"}
dict["name"] = "chloe"

Empty dict / wiping

dict = {}

Looping through a dict

dict = {
    "one":   "value one",
    "two":   "value two",
    "three": "value three",
}
 
for key in dict:
    print(key)
 
# if not specified keys or values, it takes the key
# output gives the keys: one, two, three
 
for key in dict.keys():
    print(key)
 
for value in dict.values():
    print(value)
 
for key, value in dict.items():
    print(key)
    print(value)

Replace nested ifs with a dict lookup

# instead of
def calculate(n1, n2):
    if a == this:
        hello
    if a == that:
        bye
    if a == there:
        fat
 
# do this instead
calculate = {
    "this":  "hello",
    "that":  "bye",
    "there": "fat",
}

Functions in dictionaries

def add(n1, n2):
    return n1 + n2
 
calculate = {
    "+": add
}
 
function = calculate["+"]
function(2, 3)   # gives 5
 
# function becomes add, and add(2, 3) returns 5.
# don't need the add() or add(n1, n2)

Deleting items

dict = {
    "a": "one",
    "b": "two",
}
 
del dict["a"]

Random choice from a dict

dict = {
    "a": "one",
    "b": "two",
}
 
# To get a / b
random.choice(list(dict.keys()))
>> "a"
 
# To get one / two
random.choice(list(dict.values()))
>> "one"
 
# To get key and value
random.choice(list(dict.items()))
>> ("a", "one")

Tuples

Can’t change its values (immutable).

Creating

my_tuple = (1, 3, 8)

Converting tuple to list

tuple = (1, 3, 8)
list(tuple)

Slicing

Splitting up a list / tuple.

Splitting

list = ["a", "b", "c", "d", "e", "f", "g"]
 
print(list[2:5])
>>> ["c", "d", "e"]
 
print(list[2:])
>>> ["c", "d", "e", "f", "g"]
 
# View the numbers as lines between each item.
# | "a" | "b" | "c" | "d" | "e" | "f" | "g" |
  0     1     2     3     4     5     6     7

Splitting with increments

list = ["a", "b", "c", "d", "e", "f", "g"]
 
print(list[2:5:2])
>>> ["c", "e"]
 
# The third number is increment. It gives every 2nd item.
 
print(list[::2])
>>> ["a", "c", "e", "g"]

Negative increments

list = ["a", "b", "c", "d", "e", "f", "g"]
 
print(list[::-1])
>>> ["g", "f", "e", "d", "c", "b", "a"]
 
# Negative increments reverse the list

Slicing inside a for loop

list = ["a", "b", "c", "d", "e", "f", "g"]
 
for item in list[1:]:
    ...

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