python sorting two lists

Question

I am trying to sort two lists together:

list1 = [1, 2, 5, 4, 4, 3, 6]
list2 = [3, 2, 1, 2, 1, 7, 8]

list1, list2 = (list(x) for x in zip(*sorted(zip(list1,          


        

Answer 1

The trick here is that when Python does tuple comparisons, it compares the elements in order from left to right (for example, (4, 1) < (4, 2), which is the reason that you don't get the ordering you want in your particular case). That means you need to pass in a key argument to the sorted function that tells it to only use the first element of the pair tuple as its sort expression, rather than the entire tuple.

This is guaranteed to retain the ordering you want because:

sorts are guaranteed to be stable. That means that when multiple records have the same key, their original order is preserved.

(source)

>>> list1 = [1, 2, 5, 4, 4, 3, 6]
>>> list2 = [3, 2, 1, 2, 1, 7, 8]
>>> 
>>> list1, list2 = (list(x) for x in zip(*sorted(zip(list1, list2), key=lambda pair: pair[0])))
>>> 
>>> print list1
[1, 2, 3, 4, 4, 5, 6]
>>> print list2
[3, 2, 7, 2, 1, 1, 8]