[Python-Dev] Re: lists v. tuples (original) (raw)

Christian Tismer tismer@tismer.com
Sat, 15 Mar 2003 16:59:19 +0100

Alex Martelli wrote:

On Friday 14 March 2003 04:42 pm, Christian Tismer wrote: ...

And the key assumption for sorting things is that the things are sortable, which means there exists and order on the basic set. Which again suggests that list elements usually have something in common. If a list contains ONE complex number and no other number, then the list can be sorted.

By a similar argument, tuples of one element can be sorted and reversed, just by doing nothing :-)

If the list contains elements that having something in common, by both being complex numbers, then it cannot be sorted.

Sure it can, by supplying a compare function, which implements the particular sorting operation that you want. Perhaps you want to sort them by their abs value or something. (And then you probably will want a stable sort, which is meanwhile a nice fact thanks to Tim:

a=[1, 2, 2+2j, 3+1j, 1+3j, 3-3j, 3+1j, 1+3j] a.sort(lambda x, y:cmp(abs(x), abs(y))) a [1, 2, (2+2j), (3+1j), (1+3j), (3+1j), (1+3j), (3-3j)]

)

Complex just has no total order, which makes it impossible to provide a meaningful default ordering.

So, lists whose elements have LESS in common (by being of widely different types) are more likely to be sortable than lists some of whose elements have in common the fact of being numbers (if one or more of those numbers are complex).

I agree that my statement does not apply when putting non-sortable things into a list. But I don't believe that people are putting widely different types into a list in order to sort them. (Although there is an arbitrary order between strings and numbers, which I would drop in Python 2.4, too).

chris

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