Wat is een list comprehension?

Oefenen met coding-interviewvragen in Python

Kirill Smirnov

Data Science Consultant, Altran

List comprehension

nums = [2, 4, 6, 8, 10]
print(nums)

[2, 4, 6, 8, 10]

nums_new = []
for i in range(1, 6):
    nums_new.append(2*i)

print(nums_new)

[2, 4, 6, 8, 10]

List comprehension

nums = [2, 4, 6, 8, 10]
print(nums)

[2, 4, 6, 8, 10]

                      for num in range(1, 6)

List comprehension

nums = [2, 4, 6, 8, 10]
print(nums)

[2, 4, 6, 8, 10]

           [          for num in range(1, 6)]

List comprehension

nums = [2, 4, 6, 8, 10]
print(nums)

[2, 4, 6, 8, 10]

           [(2 * num) for num in range(1, 6)]

List comprehension

nums = [2, 4, 6, 8, 10]
print(nums)

[2, 4, 6, 8, 10]

nums_new = [(2 * num) for num in range(1, 6)]

print(nums_new)

[2, 4, 6, 8, 10]

Samenvatting

Een list comprehension bestaat uit:

Samenvatting

Een list comprehension bestaat uit:

een iterabel object (bijv. list, tuple, set)

[          for num in range(1, 6)]

Samenvatting

Een list comprehension bestaat uit:

een iterabel object (bijv. list, tuple, set)
een bewerking op een element

[(2 * num) for num in range(1, 6)]

(optioneel) voorwaarden

List comprehension met voorwaarde

nums = [2, 4, 6, 8, 10]
print(nums)

[2, 4, 6, 8, 10]

1 2 3 4 5 6 7 8 9 10

List comprehension met voorwaarde

nums = [2, 4, 6, 8, 10]
print(nums)

[2, 4, 6, 8, 10]

1 2 3 4 5 6 7 8 9 10 $\rightarrow$ 2 4 6 8 10

Een voorwaarde toevoegen

nums_new = [num for num in range(1, 11)]

print(nums_new)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Een voorwaarde toevoegen

nums_new = [num for num in range(1, 11) if num % 2 == 0]

print(nums_new)

[2, 4, 6, 8, 10]

Meer voorbeelden

text = 'list COMPREHENSION is A way TO create LISTS'

Opdracht:
Maak een lijst met de lengte van elk woord in lowercase.

list, is, way, create $\rightarrow$ [4, 2, 3, 6]

Meer voorbeelden

text = 'list COMPREHENSION is A way TO create LISTS'

Opdracht:
Maak een lijst met de lengte van elk woord in lowercase.

list, is, way, create $\rightarrow$ [4, 2, 3, 6]

output = [          for word in text.split()                  ]

Meer voorbeelden

text = 'list COMPREHENSION is A way TO create LISTS'

Opdracht:
Maak een lijst met de lengte van elk woord in lowercase.

list, is, way, create $\rightarrow$ [4, 2, 3, 6]

output = [          for word in text.split() if word.islower()]

Meer voorbeelden

text = 'list COMPREHENSION is A way TO create LISTS'

Opdracht:
Maak een lijst met de lengte van elk woord in lowercase.

list, is, way, create $\rightarrow$ [4, 2, 3, 6]

output = [len(word) for word in text.split() if word.islower()]

print(output)

[4, 2, 3, 6]

Meerdere lussen

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

Maak alle mogelijke paren tussen numbers en letters:

[
    (1, 'a'), (1, 'b'), (1, 'c'),
    (2, 'a'), (2, 'b'), (2, 'c'),
    (3, 'a'), (3, 'b'), (3, 'c'),
]

Itereren over meerdere lussen

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

pairs = [       for i in numbers                 ]

Itereren over meerdere lussen

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

pairs = [       for i in numbers for j in letters]

Itereren over meerdere lussen

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

pairs = [(i, j) for i in numbers for j in letters]

print(pairs)

[
    (1, 'a'), (1, 'b'), (1, 'c'),
    (2, 'a'), (2, 'b'), (2, 'c'),
    (3, 'a'), (3, 'b'), (3, 'c'),
]

Dieper kijken

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

pairs = [(i, j) for i in numbers for j in letters]

pairs = []
for i in numbers:
    for j in letters:
        pairs.append((i, j))

Dieper kijken

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

pairs = [(i, j)                  for j in letters]

pairs = []

    for j in letters:
        pairs.append((i, j))

Dieper kijken

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

pairs = [(i, j) for i in numbers                 ]

pairs = []
for i in numbers:

        pairs.append((i, j))

Vierkante haken toevoegen

pairs = [ (i, j) for i in numbers  for j in letters]

Vierkante haken toevoegen

pairs = [[(i, j) for i in numbers] for j in letters]

print(pairs)

[
    [(1, 'a'), (2, 'a'), (3, 'a')],
    [(1, 'b'), (2, 'b'), (3, 'b')],
    [(1, 'c'), (2, 'c'), (3, 'c')]
]

Vierkante haken toevoegen

pairs = [[(i, j) for i in numbers] for j in letters]

pairs = []
for j in letters:
    temp = []
    for i in numbers:
        temp.append((i, j))
    pairs.append(temp)

Vierkante haken toevoegen

pairs = [[(i, j) for i in numbers]                 ]

pairs = []

    temp = []
    for i in numbers:
        temp.append((i, j))
    pairs.append(temp)

Vierkante haken toevoegen

pairs = [[(i, j)                 ] for j in letters]

pairs = []
for j in letters:
    temp = []

        temp.append((i, j))
    pairs.append(temp)

Numbers en letters omwisselen

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

pairs = [[(i, j) for i in numbers] for j in letters]
print(pairs)

[
    [(1, 'a'), (2, 'a'), (3, 'a')],
    [(1, 'b'), (2, 'b'), (3, 'b')],
    [(1, 'c'), (2, 'c'), (3, 'c')]
]

Numbers en letters omwisselen

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

pairs = [[(i, j) for i in letters] for j in numbers]
print(pairs)

[
    [('a', 1), ('b', 1), ('c', 1)],
    [('a', 2), ('b', 2), ('c', 2)],
    [('a', 3), ('b', 3), ('c', 3)]
]

Verschil tussen list comprehensions

numbers = [1, 2, 3]
letters = ['a', 'b', 'c']

pairs = [(i, j) for i in numbers for j in letters]

pairs = [[(i, j) for i in numbers] for j in letters]

pairs = [[(i, j) for i in letters] for j in numbers]

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Oefenen met coding-interviewvragen in Python