CODEWITHSUNDEEP
pythonpython-2.7heap
Janmajay
Janmajay

Reputation: 53

Unusual result from heappop?

I have a simple heap defined as a list of lists. I was using heapop from the heapq module to extract the list with the smallest key (which I learnt is implicitly the first element of the inner list). But in the following case, the pop operation seems to be giving unusual results.

Can someone explain why?

heap=[[0, 0, 0], [inf, 1, 1], [inf, 2, 2], [5, 3, 3], [inf, 4, 4]]

heapq.heappop(heap)

[0, 0, 0]

heapq.heappop(heap)

[inf, 1, 1]

heapq.heappop(heap)

[5, 3, 3]

heapq.heappop(heap)

[inf, 2, 2]

heapq.heappop(heap)

[inf, 4, 4]

Upvotes: 1

Views: 571

Answers (1)

Dr Xorile
Dr Xorile

Reputation: 1009

The problem is that you are using heapq on a list that is not a heap. The documentation discusses using the heapify command, and that indeed works:

>>> import heapq
>>> from numpy import inf
>>> heap=[[0, 0, 0], [inf, 1, 1], [inf, 2, 2], [5, 3, 3], [inf, 4, 4]]
>>> heapq.heapify(heap)
>>> heap
[[0, 0, 0], [5, 3, 3], [inf, 2, 2], [inf, 1, 1], [inf, 4, 4]]
>>> heapq.heappop(heap)
[0, 0, 0]
>>> heapq.heappop(heap)
[5, 3, 3]
>>> heapq.heappop(heap)
[inf, 1, 1]
>>> heapq.heappop(heap)
[inf, 2, 2]
>>> heapq.heappop(heap)
[inf, 4, 4]

Upvotes: 5

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