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Priority Queue Wrong Order


Priority Queue Wrong Order

By : Norma Vega Shek
Date : October 17 2020, 11:12 AM
this one helps. Elements in your priority queue are pointers. Since you don't provide a function that takes 2 pointers to Node objects , default compare function compares 2 pointers.
code :


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How to preserve the order of elements of the same priority in a priority queue implemented as binary heap?

How to preserve the order of elements of the same priority in a priority queue implemented as binary heap?


By : axaxaxax
Date : March 29 2020, 07:55 AM
I hope this helps you . One solution is to add time of insertion attribute to the inserted element. That may be just a simple counter incremented each time a new element is inserted into the heap. Then when two elements are equal by priority, compare the time of insertion.
Java: strange order of queue made from priority queue

Java: strange order of queue made from priority queue


By : th6
Date : March 29 2020, 07:55 AM
Hope this helps The way you've implemented compare is wrong, and would only work if it's called only in a very specific way that you're assuming. However, you have no idea in what context the PriorityQueue actually calls compare. The compare function might well be called on an existing element inside the data structure, instead of the new one, or vice versa.
(Even if you did read the source code and traced it and found that this particular implementation works in a certain way, you shouldn't depend on that if you want your code to be maintainable. At the least, you'd be making yourself more work by having to explain why it works.)
code :
int compare(Object x, Object y){
    return x.getSomeProperty() - y.getSomeProperty();
}
Priority concurrent queue which removes lowest priority element when size reaches queue's capacity

Priority concurrent queue which removes lowest priority element when size reaches queue's capacity


By : Syme0n
Date : March 29 2020, 07:55 AM
this one helps. There is no Collection that offers ordering, concurrency and bounded capacity at the same time, so you have to pick two of those features and add a hand-written solution for the third.
However checking the size is trivial and equally thread-safe with the PriorityBlockingQueue:
code :
while (myQueue.size() >= THRESHOLD) {
  Element e = myQueue.poll();
  if (e != null) {
    process(e);
  }
}
myQueue.add(newElement);
Why populating priority queue in java using Offer() populates it in alphabetical order while it also gives priority to c

Why populating priority queue in java using Offer() populates it in alphabetical order while it also gives priority to c


By : rosemary
Date : March 29 2020, 07:55 AM
I wish this help you I think what you're questioning is the order of items in the array. That is, when you execute the line System.out.printf("%s ",q);, you get the output [Zhird, first, aecond], and you're wondering why the second item in the array is first, when it is lexicographically larger than aecond.
The answer lies in how the priority queue is structured. It's structured as a binary heap, and is subject to the way that a binary heap is constructed.
code :
q.offer("first");
q.offer("aecond");
q.offer("Zhird");
       "first"
      /
  "aecond"
       "aecond"
      /
  "first"
       "aecond"
      /       \
  "first"   "Zhird"
       "Zhird"
      /       \
  "first"   "aecond"
q.offer("Zhird");
q.offer("first");
q.offer("aecond");
q.offer("Zhird");
q.offer("aecond");
q.offer("first");
Python: Priority queue does'nt keep order on same priority elements

Python: Priority queue does'nt keep order on same priority elements


By : CharlesN
Date : March 29 2020, 07:55 AM
Does that help Priority queues "are often implemented with heaps" and Python is no exception. As the documentation says, it's "using the heapq module". And heaps don't naturally offer stability. That's also why heapsort "is not a stable sort". If you want stability, you'll need to enforce it yourself. Fortunately it's as simple as storing entries "as 3-element list including the priority, an entry count, and the task".
Note that you give Python's priority queue pairs of priority and task, but the queue doesn't care. It doesn't think of the two values as priority and task. It just thinks of the pair as one "item" and it never even looks into it. Only we the users think of the pair as priority and task. So you could also give it task strings alone, without extra priorities. The queue wouldn't even notice. It doesn't try to extract some priority. For its prioritization it just asks the whole item whether it's smaller than another. That's why, when you want to prioritize tasks not just by their natural order (e.g., the string 'job1' being smaller than the string 'job2'), you use a tuple of priority and task. Tuples are ordered lexicographically, so (a, b) is smaller than (c, d) if a is smaller than c or if they're equal and b is smaller than d. So when the queue asks such a tuple whether it's smaller than another, it's the tuple that looks into itself and considers the priority first and then potentially the task second.
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