CODEWITHSUNDEEP
pythonsorting
prosseek
prosseek

Reputation: 190769

Introducing some randomness with sorting in Python

With this code, I get sorted list based on the length of element. 

>>> a = [[1,2,3],[3,4,5,6],[7,8],[9,10,11]]
>>> print sorted(a, key=lambda m: len(m))
[[7, 8], [1, 2, 3], [9, 10, 11], [3, 4, 5, 6]]

When sorting the list of list, I need to introduce some randomness when the length of the element is the same. For the previous example. I need to get sorted results

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

or 

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

in random.

How can I make that in Python?

Upvotes: 1

Views: 373

Answers (3)

Tim Peters
Tim Peters

Reputation: 70592

Here's an "obvious" way. In your example, it will produce the two possible outcomes with equal probability:

from random import random
a = [[1,2,3],[3,4,5,6],[7,8],[9,10,11]]
print(sorted(a, key=lambda m: (len(m), random())))

Because of the way tuple comparison works, when elements tie on the length the comparison goes on to compare the results from calling random().

Upvotes: 3

fjarri
fjarri

Reputation: 9726

import random    
print sorted(a, key=lambda m: (len(m), random.random()))

Upvotes: 4

Geoff Reedy
Geoff Reedy

Reputation: 36011

Python's sort functions are stable, meaning that for elements that compare equal the order is unchanged from the original list. Therefore, you can shuffle the list and then sort according to the length.

import random

a = [[1,2,3],[3,4,5,6],[7,8],[9,10,11]]
random.shuffle(a)
a.sort(key = len)

Note that this code shuffles and sorts the list a in-place.

Upvotes: 4

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