3.6. Sequence Get Item

3.6.1. Rationale

• Index must int (positive, negative or zero)

• Index must be less or equal to length of object

• Negative index starts from the end and go right to left

3.6.2. Positive Index

data = ['a', 'b', 'c', 'd']

data[0]             # 'a'
data[1]             # 'b'
data[2]             # 'c'

3.6.3. Negative Index

data = ['a', 'b', 'c', 'd']

data[-0]            # 'a'
data[-1]            # 'd'
data[-2]            # 'c'

3.6.4. Out of Range

Listing 3.19. Accessing not existing element

data = ['a', 'b', 'c', 'd']

data[100]           # IndexError: string index out of range
data[-100]          # IndexError: string index out of range

3.6.5. Ordered Sequence

Listing 3.20. Get item from str

data = 'abcd'

data[0]             # 'a'
data[1]             # 'b'
data[2]             # 'c'

data[-0]            # 'a'
data[-1]            # 'd'
data[-2]            # 'c'

Listing 3.21. Get item from list

data = ['a', 'b', 'c', 'd']

data[0]             # 'a'
data[1]             # 'b'
data[2]             # 'c'

data[-0]            # 'a'
data[-1]            # 'd'
data[-2]            # 'c'

Listing 3.22. Get item from tuple

data = ('a', 'b', 'c', 'd')

data[0]             # 'a'
data[1]             # 'b'
data[2]             # 'c'

data[-0]            # 'a'
data[-1]            # 'd'
data[-2]            # 'c'

3.6.6. Unordered Sequence

Listing 3.23. Get item from set is impossible. set is unordered data structure.

data = {'a', 'b', 'c', 'd'}

data[0]             # TypeError: 'set' object is not subscriptable
data[1]             # TypeError: 'set' object is not subscriptable
data[2]             # TypeError: 'set' object is not subscriptable

data[-0]            # TypeError: 'set' object is not subscriptable
data[-1]            # TypeError: 'set' object is not subscriptable
data[-2]            # TypeError: 'set' object is not subscriptable

Listing 3.24. Get item from frozenset is impossible. frozenset is unordered data structure.

data = frozenset({'a', 'b', 'c', 'd'})

data[0]             # TypeError: 'frozenset' object is not subscriptable
data[1]             # TypeError: 'frozenset' object is not subscriptable
data[2]             # TypeError: 'frozenset' object is not subscriptable

data[-0]            # TypeError: 'frozenset' object is not subscriptable
data[-1]            # TypeError: 'frozenset' object is not subscriptable
data[-2]            # TypeError: 'frozenset' object is not subscriptable

3.6.7. Assignments

3.6.7.1. Sequence Getitem Select

• Complexity level: easy

• Lines of code to write: 6 lines

• Estimated time of completion: 8 min

• Solution: solution/sequence_getitem_select.py

English

1. Use data from "Input" section (see below)

2. Separate header from data (row with index 0)

3. Write header (first line) to header: tuple variable

4. Create result: list

5. Select row at index 5, convert it to list and add to result

6. Select row at index 10, convert it to tuple and add to result

7. Select row at index -10, convert it to set and add to result

8. Select row at index -5, convert it to frozenset and add to result

9. Append to result: empty list, empty tuple, empty set and empty frozenset

10. Use only indexes and do not use for, while or slice()

11. Compare result with "Output" section (see below)

Polish

1. Użyj danych z sekcji "Input" (patrz poniżej)

2. Odseparuj nagłówek od danych (wiersz o indeksie 0)

3. Zapisz nagłówek (pierwsza linia) do zmiennej header: tuple

4. Stwórz result: list

5. Wybierz wiersz o indeksie 5, przekonwertuj go do listy i dodaj do result

6. Wybierz wiersz o indeksie 10, przekonwertuj go do tuple i dodaj do result

7. Wybierz wiersz o indeksie -10, przekonwertuj go do set i dodaj do result

8. Wybierz wiersz o indeksie -5, przekonwertuj go do frozenset i dodaj do result

9. Dodaj na koniec result: pustą list, pustą tuple, pusty set, pusty frozenset

10. Korzystaj tylko z indeksów i nie używaj for, while lub slice()

11. Porównaj wyniki z sekcją "Output" (patrz poniżej)

Input

DATA = [
    ('Sepal length', 'Sepal width', 'Petal length', 'Petal width', 'Species'),
    (5.8, 2.7, 5.1, 1.9, 'virginica'),
    (5.1, 3.5, 1.4, 0.2, 'setosa'),
    (5.7, 2.8, 4.1, 1.3, 'versicolor'),
    (6.3, 2.9, 5.6, 1.8, 'virginica'),
    (6.4, 3.2, 4.5, 1.5, 'versicolor'),
    (4.7, 3.2, 1.3, 0.2, 'setosa'),
    (7.0, 3.2, 4.7, 1.4, 'versicolor'),
    (7.6, 3.0, 6.6, 2.1, 'virginica'),
    (4.9, 3.0, 1.4, 0.2, 'setosa'),
    (4.9, 2.5, 4.5, 1.7, 'virginica'),
    (7.1, 3.0, 5.9, 2.1, 'virginica'),
    (4.6, 3.4, 1.4, 0.3, 'setosa'),
    (5.4, 3.9, 1.7, 0.4, 'setosa'),
    (5.7, 2.8, 4.5, 1.3, 'versicolor'),
    (5.0, 3.6, 1.4, 0.3, 'setosa'),
    (5.5, 2.3, 4.0, 1.3, 'versicolor'),
    (6.5, 3.0, 5.8, 2.2, 'virginica'),
    (6.5, 2.8, 4.6, 1.5, 'versicolor'),
    (6.3, 3.3, 6.0, 2.5, 'virginica'),
    (6.9, 3.1, 4.9, 1.5, 'versicolor'),
    (4.6, 3.1, 1.5, 0.2, 'setosa'),
]

Output

header: tuple
# ('Sepal length', 'Sepal width', 'Petal length', 'Petal width', 'Species')

result: list
# [[6.4, 3.2, 4.5, 1.5, 'versicolor'],
#  (4.9, 2.5, 4.5, 1.7, 'virginica'),
#  {0.3, 1.4, 3.4, 4.6, 'setosa'},
#  frozenset({2.2, 3.0, 5.8, 6.5, 'virginica'}),
#  [],
#  (),
#  set(),
#  frozenset()]

The whys and wherefores

• Using nested data structures

• Using indexes

• Type casting

Hint

• from pprint import pprint

• pprint(result)