Sorting python list of lists with custom comparator

Question

I'm trying to sort the below list of list:

points = [[1, 4, 'start'], [1, 6, 'start'], [2, 8, 'end'], [2, 5, 'end'], [3, 4, 'end'], [3, 9, 'start']]

Expected output:

[[1, 6, 'start'], [1, 4, 'start'], [2, 5, 'end'], [2, 8, 'end'], [3, 9, 'start'], [3, 4, 'end']]

Conditions for sorting are:

If p1[0] == p2[0] and p1[2] == 'start' and p2[2] == 'start', then p1[1] or p2[1] with greater value should come first.
If p1[0] == p2[0] and p1[2] == 'end' and p2[2] == 'end', then p1[1] or p2[1] with lesser value should come first.
If p1[0] == p2[0] and p1[2] == 'start' and p2[2] == 'end', then point with 'start' should come first.

I tried to write a custom comparator (getting correct answer), just wondering is this the right approach? Can it be more simpler?

def mycmp(p1, p2):
    if p1[0] < p2[0]:
        return -1

    if p1[0] == p2[0]:
        if p1[2] == 'start' and p2[2] == 'start' and p1[1] > p2[1]:
            return -1
        elif p1[2] == 'end' and p2[2] == 'end' and p1[1] < p2[1]:
            return -1
        elif p1[2] == 'start' and p2[2] == 'end':
            return -1
    return 0

points.sort(key=cmp_to_key(mycmp))

This function would never return 1? That doesn't seem quite right for a sort callback. — Ja͢ck
– Ja͢ck, Commented Mar 29, 2020 at 8:33

Suraj · Accepted Answer · 2020-03-29 09:13:47Z

2

points.sort(key = lambda x: (x[0], x[2] == 'end', -x[1] if x[2] == 'start' else x[1]))

Can you use lambda, the key is to specify the priorities in order, thanks to @Jack.

Used totally 3 keys inside the lambda function.

You first sort on x[0] in ascending order,

In case there is a tie there, you give high priority to x[2] = end

And finally you sort x[1] in descending order if x[2]=start and ascending if x[2]=end

Logic is to specify your priorities in order inside the lambda function

edited Mar 29, 2020 at 9:13

answered Mar 29, 2020 at 8:36

Suraj

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7 Comments

Ja͢ck Over a year ago

start should come before end; that doesn't seem to be considered here?

Suraj Over a year ago

What is the priority order, is it 0>1>2, the numbers denoting the indices.

Ja͢ck Over a year ago

also, for [2] == 'start', then [1] is descending, otherwise it's ascending.

Ja͢ck Over a year ago

Probably this will fix it: (x[0], x[2] == 'end', -x[1] if x[2] == 'start' else x[1])

Suraj Over a year ago

points.sort(key = lambda x: (x[0], x[2] == 'end', -x[1] if x[2] == 'start' else x[1])) Yes this seems to work, I'll update my answer, thanks.

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