How to reverse a priority queue in Python without using classes?

Question

I am just learning the priority queues in Python, and I have made the following code:

def main():
    q=Queue.PriorityQueue()
    while True:
        n=input("numbre?")
        if n==0:
            break
        else:
            q.put(n)
            print n

while not q.empty():
        print q.get()

when I input data like: 9, 1, 4, 5

it prints 1,4,5,9 which it seems correct, but I would like to know how can I do to deque in reverse order, I mean: 9,5,4,1

I know how to do that with a class, but in this case it seems the following extra code:

def __cmp__():
        -cmp(q.get(),q.get())

does not work, any help?

thefourtheye · Accepted Answer · 2014-10-18 15:24:36Z

22

The common pattern is to insert the data, as a tuple, along with the priority. So, you can simply change the put like this

q.put((-n ,n))

So, when the tuples are compared, if the numbers are 9, 1, 4 and 5, they will be compared like this (-9, 9), (-1, 1), (-4, 4) and (-5, 5). Since, -9 is the smallest of all, it will be retrieved first and then -5 and then -4 and then -1.

Example:

from Queue import PriorityQueue
numbers, Q = [9, 1, 4, 5], PriorityQueue()
for number in numbers:
    Q.put((-number, number))

while not Q.empty():
    print Q.get()

Output

(-9, 9)
(-5, 5)
(-4, 4)
(-1, 1)

To get only the actual value, just print only the second element, like this

while not Q.empty():
    print Q.get()[1]

edited Oct 18, 2014 at 15:24

answered Oct 18, 2014 at 15:17

thefourtheye

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2 Comments

Layla Over a year ago

thanks @thefourtheye, but how to do it to print only 9,5,4,1 and not in a tuple form?

thefourtheye Over a year ago

@Layla We just have to get the second element, like I have shown in the updated example

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