CODEWITHSUNDEEP
pythonsortingpriority-queueheapq
jupcan
jupcan

Reputation: 466

heapq custom compareTo

I'm trying to define a custome method to make an ordered insert into a priority queue in python but not getting the expected results. Once defined the insert method into the queue like the following:

def insert(self, node):
    if isinstance(node, treeNode):
        heapq.heappush(self._frontier, (node._f, node))
    else:
        print("Error. It is not a node.")

And implementing the following lt in the 'node' class:

def __lt__(self, other):
    return self._f < other._f

The insertion is not done by the 'f' attribute value which is what I wanna do, insert in an ascendant order determined by that value. Any help will be much appreciated.

Ex of failure:

[(141.09530289033592, <treeNode.treeNode object at 0x7f08bb840fd0>), (484.8315227978057, <treeNode.treeNode object at 0x7f08bb849da0>), (390.0514031446352, <treeNode.treeNode object at 0x7f08bb840e48>)]

It only puts in the first position de lowest value, which does make sense since using a priority queue but then the following ones are not sorted by the custom method I wanna declare.

Upvotes: 0

Views: 774

Answers (1)

Dani Mesejo
Dani Mesejo

Reputation: 61910

As @Bakuriu mentioned the heapq only mantains the invariant of the heap, if you want to obtain the elements in order use nsmallest, for instance:

import heapq


class TreeNode:
    def __init__(self, f):
        self.f = f

    def __repr__(self):
        return 'f: {}'.format(self.f)

    def __lt__(self, other):
        return self.f < other.f


class Frontier:
    def __init__(self):
        self.frontier = []

    def insert(self, node):
        if isinstance(node, TreeNode):
            heapq.heappush(self.frontier, (node.f, node))
        else:
            print("Error. It is not a node.")

    def __len__(self):
        return len(self.frontier)


t = Frontier()

for n in [TreeNode(141.09530289033592), TreeNode(484.8315227978057), TreeNode(390.0514031446352)]:
    t.insert(n)

print(heapq.nsmallest(len(t), t.frontier))

Output

[(141.09530289033592, f: 141.09530289033592), (390.0514031446352, f: 390.0514031446352), (484.8315227978057, f: 484.8315227978057)]

Upvotes: 1

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