In response to this post, Ben Bitdiddle inquires:
I understand the concept of using a companion set to remove duplicates from a list while preserving the order of its elements. But what should I do if these elements are composed of smaller pieces? For instance, say I am generating combinations of numbers in which order is unimportant. How do I make a set recognize that
[1,2,3]is the same as[3,2,1]in this case?
There are a couple points that should help here.
While lists are unhashable and therefore cannot be put into sets, tuples are perfectly capable of this. Therefore I cannot do this.
s = set()
s.add([1,2,3])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
But this works just fine (extra space added for emphasis of tuple parentheses).
s.add( (1,2,3) )
(3,2,1) and (1,2,3) may not hash to the same thing, but tuples are easily sortable. If I sort them before adding them to a set, they look the same.
tuple(sorted( (3,2,1) ))
(1, 2, 3)
If I want to be a little fancier, I can user itertools.combinations. The following generates all unique 3-digit combinations of integers from 1 to 4:
from itertools import combinations
list(combinations(range(1,5), 3))
[(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)]
Now say I want to only find those that match some condition. I can add a filter to return, say, only those 3-digit combinations of integers from 1 to 6 that multiply to a number divisible by 10:
list(filter(
lambda x: not (x[0]*x[1]*x[2]) % 10,
combinations(range(1, 7), 3)
))
[(1, 2, 5),
(1, 4, 5),
(1, 5, 6),
(2, 3, 5),
(2, 4, 5),
(2, 5, 6),
(3, 4, 5),
(3, 5, 6),
(4, 5, 6)]