What is comprehension?
Comprehensions are a concise notation for performing some operation for a collection of elements, and/or selecting a subset of them that meet some condition.
Types of Comprehensions:
List Comprehensions.
Dict Comprehensions.
Set Comprehensions.
Simply Put, you can use comprehensions to define new Sequences(List, Dict, Set) from existing once in an elegant way.
Let's see comprehensions in Action.
List Comprehensions
- Suppose, you want to create a list with the squares of the first 10 natural numbers.
You can do this using a for loop like this:
squares = []
for n in range(10):
squares.append(n * n)
print(squares) # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
You can perform the same thing using list comprehensions in a much more elegant and concise way:
squares = [n * n for n in range(10)]
print(squares) # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
Same result!!
Basically we have put a for loop within square brackets.
- You can also use an
ifcondition inside of comprehension.
Let's select the even numbers from the square list we just created:
squares = [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
even_nums = [num for num in squares if not num % 2]
print(even_nums) # [0, 4, 16, 36, 64]
Quite neat! isn't it?
- Nested Comprehensions: You can use multiple for loop inside a comprehension.
Suppose you want to create a matrix from two lists:
Using for loop:
matrix = []
for a in range(3):
for b in range(3):
matrix.append([a, b])
print(matrix)
""" Output:
[[0, 0], [0, 1], [0, 2],
[1, 0], [1, 1], [1, 2],
[2, 0], [2, 1], [2, 2]]
"""
Using list comprehension:
matrix = [[a, b] for a in range(3) for b in range(3)]
print(matrix)
""" Output:
[[0, 0], [0, 1], [0, 2],
[1, 0], [1, 1], [1, 2],
[2, 0], [2, 1], [2, 2]]
"""
From 4 lines to 1 line!!
Note: for loop over b has a dependency on a, it must follow the for loop over a in the comprehension.
Set Comprehensions
Set comprehension work exactly like list comprehension, only there is a little difference in the syntax.
name = "Harry"
letters1 = set(c for c in name) # First way using set()
letters2 = {c for c in name} # Second way using {}
print(letters1) # {'H', 'y', 'a', 'r'}
print(letters2) # {'H', 'y', 'a', 'r'}
Dict Comprehensions
Dict comprehension is similar to list and set comprehensions but it requires a key: value pair instead of single values.
d1 = dict((k, v) for k, v in enumerate("Hello", 1))
print(d1) # {1: 'H', 2: 'e', 3: 'l', 4: 'l', 5: 'o'}
d2 = {k: v for k, v in enumerate("Bye", 1)}
print(d2) # {1: 'B', 2: 'y', 3: 'e'}
Note:
- enumerate function returns a key: value pair using the iterable passed to it and the start value.
- Dictionaries do not allow duplication in the keys.
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