Python - List comprehension - SolutionHacker.com

List


>>> a = [66.25, 333, 333, 1, 1234.5]
>>> print(a.count(333), a.count(66.25), a.count('x'))
2 1 0
>>> a.insert(2, -1)
>>> a.append(333)
>>> a
[66.25, 333, -1, 333, 1, 1234.5, 333]
>>> a.index(333)
1
>>> a.remove(333)
>>> a
[66.25, -1, 333, 1, 1234.5, 333]
>>> a.reverse()
>>> a
[333, 1234.5, 1, 333, -1, 66.25]
>>> a.sort()
>>> a
[-1, 1, 66.25, 333, 333, 1234.5]
>>> a.pop()
1234.5
>>> a
[-1, 1, 66.25, 333, 333]

>>> a = [66.25, 333, 333, 1, 1234.5]

>>> print(a.count(333), a.count(66.25), a.count('x'))

2 1 0

>>> a.insert(2, -1)

>>> a.append(333)

>>> a

[66.25, 333, -1, 333, 1, 1234.5, 333]

>>> a.index(333)

>>> a.remove(333)

>>> a

[66.25, -1, 333, 1, 1234.5, 333]

>>> a.reverse()

>>> a

[333, 1234.5, 1, 333, -1, 66.25]

>>> a.sort()

>>> a

[-1, 1, 66.25, 333, 333, 1234.5]

>>> a.pop()

1234.5

>>> a

[-1, 1, 66.25, 333, 333]

You can use list to implement stack (ie. LIFO). For that, we can use:

append to add to the end of the list
pop to remove from the front of the list.

List comprehension (LC)

It is syntactic sugar that transforms one list (any iterable actually) into another list. During this process, elements can be filtered and transformed if needed. In term of performance, LC actually runs faster.

General form:

[ expression for item in list if conditional ]

expression can use the variable item
there can be nested for loop sequencing in order
if-condition is the filter

List comprehension examples


"""
S = {x² : x in {0 ... 9}}
V = (1, 2, 4, 8, ..., 2¹²)
M = {x | x in S and x even}
"""
# example 1: Simple basic usage referencing what we want to do above
S = [x**2 for x in range(10)]
V = [2**i for i in range(13)]
M = [x for x in S if x % 2 == 0]


# example 2: Flatten matrix LC vs expanded form
flattened = [n for row in matrix for n in row]

flattened = []
for row in matrix:
    for n in row:
        flattened.append(n)


# example 3: create list within list
words = 'The quick brown fox jumps over the lazy dog'.split()
stuff = [[w.upper(), w.lower(), len(w)] for w in words]


# example 4: expression on 2 lists
newList = [x+y for x in [10,30,50] for y in [20,40,60]]
print(newList)
>> [30, 50, 70, 50, 70, 90, 70, 90, 110]


# example 5: cross product of 2 sets
colours = [ "red", "green", "yellow", "blue" ]
things = [ "house", "car", "tree" ]
coloured_things = [ (x,y) for x in colours for y in things ]
>>> print coloured_things
[('red', 'house'), ('red', 'car'), ('red', 'tree'), ('green', 'house'), ('green', 'car'), ('green', 'tree'), ('yellow', 'house'), ('yellow', 'car'), ('yellow', 'tree'), ('blue', 'house'), ('blue', 'car'), ('blue', 'tree')]


## example 6: transposition
matrix = [[1, 2, 3, 4],
          [5, 6, 7, 8],
          [9, 10, 11, 12]]

transposeMatrix = [[row[i] for row in matrix] for i in range(4)]
print(transposeMatrix)
>>> [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]

"""

S = {x² : x in {0 ... 9}}

V = (1, 2, 4, 8, ..., 2¹²)

M = {x | x in S and x even}

"""

# example 1: Simple basic usage referencing what we want to do above

S = [x**2 for x in range(10)]

V = [2**i for i in range(13)]

M = [x for x in S if x % 2 == 0]

# example 2: Flatten matrix LC vs expanded form

flattened = [n for row in matrix for n in row]

flattened = []

for row in matrix:

for n in row:

flattened.append(n)

# example 3: create list within list

words = 'The quick brown fox jumps over the lazy dog'.split()

stuff = [[w.upper(), w.lower(), len(w)] for w in words]

# example 4: expression on 2 lists

newList = [x+y for x in [10,30,50] for y in [20,40,60]]

print(newList)

>> [30, 50, 70, 50, 70, 90, 70, 90, 110]

# example 5: cross product of 2 sets

colours = [ "red", "green", "yellow", "blue" ]

things = [ "house", "car", "tree" ]

coloured_things = [ (x,y) for x in colours for y in things ]

>>> print coloured_things

[('red', 'house'), ('red', 'car'), ('red', 'tree'), ('green', 'house'), ('green', 'car'), ('green', 'tree'), ('yellow', 'house'), ('yellow', 'car'), ('yellow', 'tree'), ('blue', 'house'), ('blue', 'car'), ('blue', 'tree')]

## example 6: transposition

matrix = [[1, 2, 3, 4],

[5, 6, 7, 8],

[9, 10, 11, 12]]

transposeMatrix = [[row[i] for row in matrix] for i in range(4)]

print(transposeMatrix)

>>> [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]

Set and dictionary comprehension


basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
print(basket)

basketSet = {fruit for fruit in basket}
print(basketSet)

basketCounterDict = {fruit: basket.count(fruit) for fruit in basket}
print(basketCounterDict)

Python – List comprehension

List

List comprehension (LC)

General form:

List comprehension examples

Set and dictionary comprehension

Python – List comprehension

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