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Javascript Priority Queue

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Michael Zippo Michael Zippo 04.11.2021

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Python priority queue: a guide

A stores Python priority queue data in a particular order. There are two ways to implement a priority queue in Python:. Using the queue class and using the heapq module

You may want to sort the data based on the values ‚Äã‚Äãof each item in the list. For example, you might want the highest value to appear in the list first and the lowest value to appear in the last list.

is the queue of priority waiting come in. A queue of priority waiting is a data structure that stores data based on the value of its key in the ascending order. This gives you easy access to the smallest and the largest value in the queue .

This tutorial will explain why you should not use a list to create queues of priority waiting. We are going to show you two more efficient approaches that you can use to create a Python priority queue.

What is a Python priority queue?

Priority queues are a modified version of a queue that stores data in order to which item has the highest priority. The priority of each item in a priority queue is decided based on the item’s value.

In calculation, queues are data structures that elements of memory in FIFO (first in, first) order. -Out). There are a few scenarios where using this facility can be helpful.

For example, suppose you are building an order tracking app for a restaurant. The person ordering must first be served in front of the people by placing the order later. . To keep track of commands you need to use a queue

There are two ways to set priority queue in Python:

  • Using the PriorityQueue Queue class
  • Using the heapq module

You can set a priority queue using a list structure, but this strategy is less efficient than using the queue class or module PriorityQueue heapq

Priority queue plate:. Queue.PriorityQueue

queue.PriorityQueue creates a Python priority queue. This class is part of the Python code library. You must import the queue library to use this class. To retrieve an item from a PriorityQueue, you can use the get () method.

To access the PriorityQueue class, we need to import it into our code, which we can do using this import Python statement :

Suppose we want to create a queue d ’ priority waiting for ticket holders at a con rt. We could do it using this code:

Our code returns:

In our code, we have To first import the PriorityQueue class from the file library, we initialize a priority queue called ticket_holders. Next, we put three tuples in our priority queue, the store ticket numbers, and the names associated with a ticket.

We use a Python loop to examine each item in the ticket_holders priority queue . Then we get this element using get ().

queue.PriorityQueue method is effective and easy to use, making it an excellent choice when you need to create a queue of priority waiting.

Python heapq priority queue module

The heapq module allows you to define a Python priority queue. A heapq data structure removes items in order of priority. The lowest value has the lowest priority and the highest value has the highest priority in the heapq structure

Before can use the heapq module, we must first import it into our code using the following import instruction:

Either dos go to our previous example. Suppose we want to create a priority queue to store information about ticket holders at a concert. We could then do this using the heapq module and this program:

Our code returns:

First , we imported the heapq library, then we initialized a Python variable called ticket_holders . We used the heappush () method to put three tuples in our priority queue. This queue stores the ticket numbers for each ticket holder and the name of each ticket holder.

We then created a loop while going through each item in our priority queue. This loop removes the element top of the queue of waiting using heappop () . Then the deleted item is printed to the console. As you can see, all the items in our queue are printed in order of priority.

Why you shouldn’t keep a list

Technically you can create a priority queue using the python list data structure. To do this, you would have to create a list, then sort them in ascending order.

However, this is a relatively inefficient way to maintain a priority queue. While editing the items in the list, you need to rearrange the list, which takes time.

You can use a traditional list as a priority queue if you just need to store a few values. But if you are looking to create a larger queue, lists are not a good option

For reference, let’s look at an example of a priority queue using lists. Suppose we want to create a priority queue that stores the order of ticket holders who should be admitted to a first concert. We could use the following code to create this queue:

Our code returns:

we created a list called ticket_holders , we added three tuples to the list. Each tuple contained a ticket number of the ticket holder and their name. Next, we used the Python sort () function to sort the list of ticket holders in reverse order.

We’ve created a long time loop that goes through each item in the ticket_holders list and the item at the top of the list. Then our code prints the deleted item to the console.

Conclusion

The two most common to create queue priority are to use the heapq or queue module . PriorityQueue class . Although it is technically possible to use a list as a priority queue, this approach does not work well.

This tutorial has discussed, referring to examples, how to create a priority queue in Python. You are now equipped with the knowledge to start building your priority queues like a Python pro!

For more tips on learning Python, see our Python Learning Guide .

👻 Read also: what is the best laptop for engineering students?

Javascript Priority Queue __del__: Questions

__del__

How can I make a time delay in Python?

5 answers

I would like to know how to put a time delay in a Python script.

2973

Answer #1

import time
time.sleep(5)   # Delays for 5 seconds. You can also use a float value.

Here is another example where something is run approximately once a minute:

import time
while True:
    print("This prints once a minute.")
    time.sleep(60) # Delay for 1 minute (60 seconds).

2973

Answer #2

You can use the sleep() function in the time module. It can take a float argument for sub-second resolution.

from time import sleep
sleep(0.1) # Time in seconds

Javascript Priority Queue __del__: Questions

__del__

How to delete a file or folder in Python?

5 answers

How do I delete a file or folder in Python?

2639

Answer #1

Path objects from the Python 3.4+ pathlib module also expose these instance methods:

2639

Answer #2

Path objects from the Python 3.4+ pathlib module also expose these instance methods:

2639

Answer #3

Python syntax to delete a file

import os
os.remove("/tmp/<file_name>.txt")

Or

import os
os.unlink("/tmp/<file_name>.txt")

Or

pathlib Library for Python version >= 3.4

file_to_rem = pathlib.Path("/tmp/<file_name>.txt")
file_to_rem.unlink()

Path.unlink(missing_ok=False)

Unlink method used to remove the file or the symbolik link.

If missing_ok is false (the default), FileNotFoundError is raised if the path does not exist.
If missing_ok is true, FileNotFoundError exceptions will be ignored (same behavior as the POSIX rm -f command).
Changed in version 3.8: The missing_ok parameter was added.

Best practice

  1. First, check whether the file or folder exists or not then only delete that file. This can be achieved in two ways :
    a. os.path.isfile("/path/to/file")
    b. Use exception handling.

EXAMPLE for os.path.isfile

#!/usr/bin/python
import os
myfile="/tmp/foo.txt"

## If file exists, delete it ##
if os.path.isfile(myfile):
    os.remove(myfile)
else:    ## Show an error ##
    print("Error: %s file not found" % myfile)

Exception Handling

#!/usr/bin/python
import os

## Get input ##
myfile= raw_input("Enter file name to delete: ")

## Try to delete the file ##
try:
    os.remove(myfile)
except OSError as e:  ## if failed, report it back to the user ##
    print ("Error: %s - %s." % (e.filename, e.strerror))

RESPECTIVE OUTPUT

Enter file name to delete : demo.txt
Error: demo.txt - No such file or directory.

Enter file name to delete : rrr.txt
Error: rrr.txt - Operation not permitted.

Enter file name to delete : foo.txt

Python syntax to delete a folder

shutil.rmtree()

Example for shutil.rmtree()

#!/usr/bin/python
import os
import sys
import shutil

# Get directory name
mydir= raw_input("Enter directory name: ")

## Try to remove tree; if failed show an error using try...except on screen
try:
    shutil.rmtree(mydir)
except OSError as e:
    print ("Error: %s - %s." % (e.filename, e.strerror))

__delete__

Is there a simple way to delete a list element by value?

5 answers

I want to remove a value from a list if it exists in the list (which it may not).

a = [1, 2, 3, 4]
b = a.index(6)

del a[b]
print(a)

The above case (in which it does not exist) shows the following error:

Traceback (most recent call last):
  File "D:zjm_codea.py", line 6, in <module>
    b = a.index(6)
ValueError: list.index(x): x not in list

So I have to do this:

a = [1, 2, 3, 4]

try:
    b = a.index(6)
    del a[b]
except:
    pass

print(a)

But is there not a simpler way to do this?

1055

Answer #1

To remove an element"s first occurrence in a list, simply use list.remove:

>>> a = ["a", "b", "c", "d"]
>>> a.remove("b")
>>> print(a)
["a", "c", "d"]

Mind that it does not remove all occurrences of your element. Use a list comprehension for that.

>>> a = [10, 20, 30, 40, 20, 30, 40, 20, 70, 20]
>>> a = [x for x in a if x != 20]
>>> print(a)
[10, 30, 40, 30, 40, 70]

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