Monday, June 29, 2015

Python on the go - List Comprehensions

Back to what I have been doing. Here is a way of simplifying some of the things that are done with lists


5.1.4. List Comprehensions

List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.
For example, assume we want to create a list of squares, like:
>>> squares = []
>>> for x in range(10):
...     squares.append(x**2)
>>> squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
We can obtain the same result with:
squares = [x**2 for x in range(10)]
This is also equivalent to squares = map(lambda x: x**2, range(10)), but it’s more concise and readable.
A list comprehension consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses. The result will be a new list resulting from evaluating the expression in the context of the for and ifclauses which follow it. For example, this listcomp combines the elements of two lists if they are not equal:
>>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]

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