9/26/2023 0 Comments Python priority queue heapGoing back to the output issue, if you specify what you want you want the output to look like, I can try to help you more. Perhaps someone can edit my answer to add this code.Īdd this to the end of your code block, the turn it into a list, which is what heapq wants as input. Therefore, if you can figure out how to convert s into a list, then this should work. Using this approach, I get the following error: TypeError: heap argument must be a list Then, once you are no longer receiving this kind of error, at the end of your code block try: s = heapq.heappop(heap) Thus, as you said in your question, "I want to update val of element I don't know which in turn it is", this method gets you the smallest value automatically (I assume from your question that you want to substitute the 1 since it's the smallest value, as it is coupled with the name First) However, when I try to run your code myself I received so you should try to fix your code so that you can print output, same thing goes for print heap, same error type. I tried heapq.heappop(heap) which should return the smallest value. Therefore, changing this value from 1 to 5 should theoretically be easy. In the heapq module, heap is always designated the smallest item. It is difficult to run your code because there is no output. Alternatively, if you know your update will update the value by +/- 1, you could potentially just swap the position of the element you're trying to update with the element above or below it.Įdit: Here's a similar question with more answers: How to update elements within a heap? (priority queue) Call heapq.heapify(heap) which runs in O(N).Check dictionary to get the index of the element you want to update (after checking that the element is in the dictionary + corresponding heap).The basic steps for updating elements in the heap following the above logic would be: I also don't want to remove other elements from the heap. Since a heap in Python is basically just a standard list with the heapq interface used on top, the docs recommend possibly having an additional dictionary which maps your heap values to the index in your heap (list). The new implementation of heapq for Python3 includes some helpful notes on how to update heap elements, essentially using it as a priority queue.Įssentially, you can make a heap of tuples, and Python will evaluate the priority based on sequential comparisons of the tuples. The base makes sure this shouldn't be called if `_qsize` is 0.This is an old question, but in case someone sees this in the future and is looking for answers. Here is another implementation follows the official guide: import heapq Basic operations of Priority Queue include Insertion and Deletion. In case you want to prioritise a task later. Although there are many ways to implement Priority Queue like using arrays, linked lists, or binary search tree, Binary Heap data structure is an efficient way to implement Priority Queue. I left some print statements uncommented, but commented out the ones I put in while debugging it. I basically started from how they defined PriorityQueue in Queue.py and added a set into it to keep track of unique keys: from Queue import PriorityQueueĬlass UniquePriorityQueue(PriorityQueue):ĭef _put(self, item, heappush=heapq.heappush):īoaz Yaniv beat me to the punch by a few minutes, but I figured I'd post mine too as it supports the full interface of PriorityQueue. Priority: 0.4, Value: value1 (shall not be added again, even though it has different priority).Priority: 0.1, Value: value3 (shall be retrieved first automatically). What is the best way of creating a atomic priority queue (=can be used from multiple threads) with unique values? at the first position of a tuple), so it seems not be great for comparison of already existing values. There are some other implementations on stackoverflow with heapq, but heapq also uses priority within the value (e.g. The que class on the other hand does offer a function for checking if a value already exists and is even more natural in usage (without locking, but still atomic), but it does not offer a way to sort by priority. You would use (0.5, myvalue) as value in PriorityQueue and then it would be sorted by the first element of the tuple. Moreover, PriorityQueue needs the priority to remain within the value, so I could not even search for my value, as I would also have to know the priority. You need a lt method (or to use a builtin type such as tuple that is already comparable). Dan Getz at 23:26 Your Element s are not comparable, so I'm not sure how you can be using them with heapq. Python has Queue.PriorityQueue, but I cannot see a way to make each value in it unique as there is no method for checking if a value already exists (like find(name) or similar). A possible solution for using heapq to make a priority queue with update is right there in the documentation.
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