How to use lists, tuples, dictionaries, and sets

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list

It can store data of any type (integer, floating point number, string, etc.). The elements are in order and you can specify them using indexes. List elements can be changed . Use it like an "array" in other programming languages.

Example of use

Determining if a person can get on a taxi seat

>>> seat = []
>>> min = 0
>>> max = 4
>>> min <= len(seat) < max
True
>>> seat.append("person")
>>> len(seat)
1
>>> seat.append("person")
>>> seat.append("person")
>>> seat.append("person")
>>> min <= len(seat) < max
False
>>> seat.pop(0)
"person"

Tuple

It can store data of any type (integer, floating point number, string, etc.). The elements are in order and you can specify them using indexes. Unlike lists, tuple elements are immutable

Example of use

quiz

>>> chose_from_two = ("A", "B", "C")
>>> answer = []
>>> answer.append("A")
>>> answer.append("C")

Dictionary type

Stores data represented by key / value pairs. Elements have no order (elements cannot be specified by index). Corresponds to "associative array" in other programming languages

Example of use

Fruit EC site

>>> fruits = {"apple": 100, "banana": 200, "orange": 300}
>>> fruits.items()
dict_items([('apple', 100), ('banana', 200), ('orange', 300)])
>>> fruits.keys()
dict_keys(['apple', 'banana', 'orange'])
>>> fruits.values()
dict_values([100, 200, 300])
>>> fruits["apple"]
100
>>> fruits.get("apple")
100
>>> fruits.pop("apple")
100
>>> "apple" in fruits
False

set

A type for handling "sets" in mathematics. Each element does not overlap and has no order (elements cannot be specified by index)

Example of use

Find "common friends" on SNS

>>> my_friends = {"A", "C", "D"}
>>> A_friends = {"B", "D", "E", "F"}
>>> my_friends & A_friends
{'D'}
>>> fruits = ["apple", "banana", "apple", "banana"]
>>> kind = set(fruits)
>>> kind
{'apple', 'banana'}

Other

Count the number of arbitrary character strings

>>> s = "fdjsafiewafjdsaeiwfdafke"
>>> d = {}
>>> for c in s:
...     if c not in d:
...             d[c] = 0
...     d[c] += 1
>>> print(d)
{'f': 5, 'd': 3, 'j': 2, 's': 2, 'a': 4, 'i': 2, 'e': 3, 'w': 2, 'k': 1}

You can write the following using the setdefault function

>>> s = "fdjsafiewafjdsaeiwfdafke"
>>> d = {}
>>> for c in s:
...     d.setdefault(c, 0)
...     d[c] += 1
>>> print(d)
{'f': 5, 'd': 3, 'j': 2, 's': 2, 'a': 4, 'i': 2, 'e': 3, 'w': 2, 'k': 1}

You don't have to call setdefault every time you use the python standard library defaultdict

>>> from collections import defaultdict
>>> d = defaultdict(int)
>>> for c in s:
...     d[c] += 1
...
>>> print(d)
defaultdict(<class 'int'>, {'f': 5, 'd': 3, 'j': 2, 's': 2, 'a': 4, 'i': 2, 'e': 3, 'w': 2, 'k': 1})