Sorting: x values increase, y values decrease

Question

How do you sort a list of lists such that the x-values increase, and if 2 x-values are the same, the y-value that is lower takes priority?

For example: [[0, 1], [1, 0], [1, 2], [1, 3]] should be sorted into [[0, 1], [1, 3], [1, 2], [1, 1]].

A way would be to first apply the .sort() function on the list, then compare y values for equal x, but is there a simpler way (using lambda, etc.)

Answer 1

It looks like all the items you're sorting are two-element lists of numeric values, so the following should work. You can specify a key argument to the built-in sorted function, to create sort keys for each item. In this case, the key you want is just the same as the item itself, but with negative y values to flip their ordering.

>>> data = [[0, 1], [1, 0], [1, 2], [1, 3]]
>>> print(sorted(data, key=lambda x: (x[0], -x[1])))
[[0, 1], [1, 3], [1, 2], [1, 0]]

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If this doesn't work for some reason (perhaps you've got non-numerics in there somewhere), you can use the same approach, but construct the key function differently to provide appropriate comparators for whatever your data actually looks like.