Python Queue: FIFO, LIFO Example
Steve Campbell
What is Python Queue?
A queue is a container that holds data. The data that is entered first will be removed first, and hence a queue is also called “First in First Out” (FIFO). The queue has two ends front and rear. The items are entered from the rear and removed from the front side.
How does Python Queue Work?
The queue can be easily compared with the real-world example the line of people waiting in a queue at the ticket counter, the person standing first will get the ticket first, followed by the next person and so on. The same logic goes for the queue data structure too.
Here is a diagrammatic representation of queue:
The Rear represents the point where the items are inserted inside the queue. In this example, 7 is value for that.
The Front represents the point where the items from the queue will be removed. If you remove an item from the queue, the first element you will get is 1, as shown in the figure.
Item 1 was the first one to be inserted in the queue, and while removing it is the first one to come out. Hence the queue is called FIRST IN FIRST OUT (FIFO)
In a queue, the items are removed in order and cannot be removed from in between. You just cannot remove the item 5 randomly from the queue, to do that you will have to remove all the items before 5. The items in queue will be removed in the order they are inserted.
Types of Queue in Python
There are mainly two types of queue in Python:
- First in First out Queue: For this, the element that goes first will be the first to come out.To work with FIFO, you have to call Queue() class from queue module.
- Last in First out Queue: Over here, the element that is entered last will be the first to come out.To work with LIFO, you have to call LifoQueue() class from the queue module.
Python queue Installation
It is very easy to work with queue in python. Here are the steps to follow to make use of queue in your code.
Step 1) You just have to import the queue module, as shown below:
import queue
The module is available by default with python, and you don’t need any additional installation to start working with the queue. There are 2 types of queue FIFO (first in first out) and LIFO (last in first out).
Step 2) To work with FIFO queue , call the Queue class using the queue module imported as shown below:
import queue q1 = queue.Queue()
Step 3) To work with LIFO queue call the LifoQueue() class as shown below:
import queue q1 = queue.LifoQueue()
Methods available inside Queue and LifoQueue class
Following are the important methods available inside Queue and LifoQueue class:
- put(item): This will put the item inside the queue.
- get(): This will return you an item from the queue.
- empty(): It will return true if the queue is empty and false if items are present.
- qsize(): returns the size of the queue.
- full(): returns true if the queue is full, otherwise false.
First In First Out Queue Example
In the case of first in first out, the element that goes first will be the first to come out.
Add and item in a queue
Let us work on an example to add an item in a queue. To start working with the queue, first import the module queue, as shown in the example below.
To add an item , you can make use of put() method as shown in the example:
import queue q1 = queue.Queue() q1.put(10) #this will additem 10 to the queue.
By default, the size of the queue is infinite and you can add any number of items to it. In case you want to define the size of the queue the same can be done as follows
import queue q1 = queue.Queue(5) #The max size is 5. q1.put(1) q1.put(2) q1.put(3) q1.put(4) q1.put(5) print(q1.full()) # will return true.
Output:
True
Now the size of the queue is 5, and it will not take more than 5 items, and the method q1.full() will return true. Adding any more items will not execute the code any further.
Remove an item from the queue
To remove an item from the queue, you can use the method called get(). This method allows items from the queue when called.
The following example shows how to remove an item from the queue.
import queue
q1 = queue.Queue()
q1.put(10)
item1 = q1.get()
print('The item removed from the queue is ', item1)
Output:
The item removed from the queue is 10
Last In First Out queue Example
In the case of last in the first out queue, the element that is entered last will be the first to come out.
To work with LIFO, i.e., last in the first out queue, we need to import the queue module and make use of the LifoQueue() method.
Add and item in a queue
Here we will understand how to add an item to the LIFO queue.
import queue q1 = queue.LifoQueue() q1.put(10)
You have to use the put() method on LifoQueue, as shown in the above example.
Remove an item from the queue
To remove an item from the LIFOqueue you can make use of get() method .
import queue
q1 = queue.LifoQueue()
q1.put(10)
item1 = q1.get()
print('The item removed from the LIFO queue is ', item1)
Output:
The item removed from the LIFO queue is 10
Add more than 1 item in a Queue
In the above examples, we have seen how to add a single item and remove the item for FIFO and LIFOqueue. Now we will see how to add more than one item and also remove it.
Add and item in a FIFOqueue
import queue
q1 = queue.Queue()
for i in range(20):
q1.put(i) # this will additem from 0 to 20 to the queue
Remove an item from the FIFOqueue
import queue
q1 = queue.Queue()
for i in range(20):
q1.put(i) # this will additem from 0 to 20 to the queue
while not q1.empty():
print("The value is ", q1.get()) # get() will remove the item from the queue.
Output:
The value is 0 The value is 1 The value is 2 The value is 3 The value is 4 The value is 5 The value is 6 The value is 7 The value is 8 The value is 9 The value is 10 The value is 11 The value is 12 The value is 13 The value is 14 The value is 15 The value is 16 The value is 17 The value is 18 The value is 19
Add and item in a LIFOqueue
import queue
q1 = queue.LifoQueue()
for i in range(20):
q1.put(i) # this will additem from 0 to 20 to the queue
Remove an item from the LIFOqueue
import queue
q1 = queue.LifoQueue()
for i in range(20):
q1.put(i) # this will additem from 0 to 20 to the queue
while not q1.empty():
print("The value is ", q1.get()) # get() will remove the item from the queue.
Output:
The value is 19 The value is 18 The value is 17 The value is 16 The value is 15 The value is 14 The value is 13 The value is 12 The value is 11 The value is 10 The value is 9 The value is 8 The value is 7 The value is 6 The value is 5 The value is 4 The value is 3 The value is 2 The value is 1 The value is 0
Sorting Queue
Following example shows the queue sorting.The algorithm used for sorting is bubble sort.
import queue
q1 = queue.Queue()
#Addingitems to the queue
q1.put(11)
q1.put(5)
q1.put(4)
q1.put(21)
q1.put(3)
q1.put(10)
#using bubble sort on the queue
n = q1.qsize()
for i in range(n):
x = q1.get() # the element is removed
for j in range(n-1):
y = q1.get() # the element is removed
if x > y :
q1.put(y) #the smaller one is put at the start of the queue
else:
q1.put(x) # the smaller one is put at the start of the queue
x = y # the greater one is replaced with x and compared again with nextelement
q1.put(x)
while (q1.empty() == False):
print(q1.queue[0], end = " ")
q1.get()
Output:
3 4 5 10 11 21
Reversing Queue
To reverse the queue, you can make use of another queue and recursion.
The following example shows how to get the queue reversed.
Example:
import queue
q1 = queue.Queue()
q1.put(11)
q1.put(5)
q1.put(4)
q1.put(21)
q1.put(3)
q1.put(10)
def reverseQueue (q1src, q2dest) :
buffer = q1src.get()
if (q1src.empty() == False) :
reverseQueue(q1src, q2dest) #using recursion
q2dest.put(buffer)
return q2dest
q2dest = queue.Queue()
qReversed = reverseQueue(q1,q2dest)
while (qReversed.empty() == False):
print(qReversed.queue[0], end = " ")
qReversed.get()
Output:
10 3 21 4 5 11
Summary
- A queue is a container that holds data. There are two types of Queue, FIFO, and LIFO.
- For a FIFO (First in First out Queue), the element that goes first will be the first to come out.
- For a LIFO (Last in First out Queue), the element that is entered last will be the first to come out.
- An item in a queue is added using the put(item) method.
- To remove an item, get() method is used.