Sorted function using compare function

Question

I'd like to sort a 2D list where each "row" is of size 2, like that for example

[[2,5],[2,3],[10,11]]

These rows represent ranges in fact, so Its always [a,b] where a <= b

I want to sort it exactly this way, each element of the list being a 2-list, I'd have (by order of priority): [a1, b1] compared to [a2, b2]

 1. If a1 < a2 do not permute
 2. If a1 > a2 permute
 3. If a1 == a2 then permute if (b1 - a1) > (b2 - a2)

What I find kind of stupid is that python doesnt allow anymore for comparison functions. Instead it uses a key function. Theres no way I can make a valid key with that as I base my comparison on two parameters, the numeric value of 'a' (which prevails) and then length of the range (b - a).

How can I sort this? I mean, without calling two times sorted() or something, which in my opinion is plain ugly.

Is it even possible? Or is there something I don't see?

Thanks!

When invoking sorted(elements, cmp=func) on python 3.3.3 it gives me "'cmp' is an invalid keyword argument for this function" — Yannick
– Yannick, Commented Jul 15, 2015 at 5:20
The docs offer a cmp to key conversion function but it is not all that pretty. — Paul Rooney
– Paul Rooney, Commented Jul 15, 2015 at 5:24
You can also create a class for inner list and override __cmp__ function — Ashwani
– Ashwani, Commented Jul 15, 2015 at 5:32

John La Rooy · Accepted Answer · 2015-07-15 05:27:02Z

5

While there are cases that can't be handled by a key. This is not one of them. The solution is to make the key function return a tuple

>>> L = [[2, 5], [2, 3], [10, 11]]
>>> sorted(L, key=lambda x:(x[0], x[1] - x[0]))
[[2, 3], [2, 5], [10, 11]]

answered Jul 15, 2015 at 5:27

John La Rooy

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4 Comments

Delgan Over a year ago

I am curious about cases that can not be solved using key. Do you have some example, please?

Yannick Over a year ago

Much thanks! I second @Delgan, would be nice to know which cases cannot be solved (I wasnt aware tuples was a working solution)

John La Rooy Over a year ago

@Delgan, Suppose the sublists list were [firstname, surname] I need to sort by surname. Where the surnames are equal I need to sort by firstname - but in reverse.

Delgan Over a year ago

@JohnLaRooy key=lambda x:(x[1], [-ord(i) for i in x[0]]) should work, am I wrong? However, this is probably not great, I wonder what is the proper way for solving this issue.

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