Step by Step solution about How to resolve the algorithm Queue/Definition step by step in the Python programming language

FIFO queue class (class-based)

This Python code defines a class called FIFO that implements a first-in, first-out (FIFO) queue. A FIFO queue is a data structure that maintains the order of elements added to it, where the first element added is the first one to be removed.

Class Definition

class FIFO(object):
   # ...

The FIFO class is defined as a new-style class (indicated by the object base class in Python 2) with the following methods and attributes:

Constructor

def __init__(self, *args):
   self.contents = list(args)

The constructor takes any number of arguments and initializes the contents attribute as a list containing these arguments.

Callable

def __call__(self):
   return self.pop()

The class can be called like a function to retrieve the first element in the queue. This is equivalent to calling the pop method.

Length

def __len__(self):
   return len(self.contents)

The __len__ method returns the number of elements in the queue.

Pop

def pop(self):
   return self.contents.pop(0)

The pop method removes and returns the first element in the queue.

Push

def push(self, item):
   self.contents.append(item)

The push method adds an item to the end of the queue.

Extend

def extend(self,*itemlist):
   self.contents += itemlist

The extend method adds multiple items to the end of the queue.

Empty

def empty(self):
   return bool(self.contents)

The empty method returns True if the queue is empty (has no elements) and False otherwise.

Iterator

def __iter__(self):
   return self

def next(self):
   if self.empty():
       raise StopIteration
   return self.pop()

The __iter__ and next methods implement the Python iterator protocol, allowing the FIFO object to be used in a for loop. It iterates over the elements in the queue, returning each element in FIFO order until the queue is empty.

Example Usage

The example usage at the bottom of the code demonstrates how to use the FIFO class:

• A FIFO object is created and elements are pushed onto it.
• The elements are then popped off the queue and printed, showing that they are retrieved in FIFO order.

FIFO queue class (old-style class)

This code defines a non-new-style class (old-style class in Python 2) called FIFO that implements a FIFO queue.

Class Definition

class FIFO:  ## NOT a new-style class, must not derive from "object"
   # ...

The FIFO class does not derive from the object base class, indicating that it is an old-style class.

getattr

def __getattr__(self, attr):
   return getattr(self.contents,attr)

In the absence of a __getattr__ method, accessing attributes of the FIFO object would raise an AttributeError. The overriding implementation of __getattr__ allows attributes of the underlying contents list to be accessed directly on the FIFO object.

Example Usage

The example usage at the bottom of the code demonstrates how to use the FIFO class:

• A FIFO object is created and elements are added to it.
• The elements are then retrieved and printed, showing that they are retrieved in FIFO order.

FIFO queue using collections.deque

The collections.deque class is a built-in Python data structure that provides deque (double-ended queue) functionality, which includes FIFO behavior.

Example Usage

The example usage at the bottom of the code demonstrates how to use the deque class:

• A deque object is created and elements are appended to it.
• The elements are then popped from the left side of the deque (FIFO order).
• The isempty attribute is used to check if the deque is empty.

   class FIFO(object):
       def __init__(self, *args):
           self.contents = list(args)
       def __call__(self):
           return self.pop()
       def __len__(self):
           return len(self.contents)
       def pop(self):
           return self.contents.pop(0)
       def push(self, item):
           self.contents.append(item)
       def extend(self,*itemlist):
           self.contents += itemlist
       def empty(self):
           return bool(self.contents)
       def __iter__(self):
           return self
       def next(self):
           if self.empty():
               raise StopIteration
           return self.pop()

if __name__ == "__main__":
    # Sample usage:
    f = FIFO()
    f.push(3)
    f.push(2)
    f.push(1)
    while not f.empty():
        print f.pop(),
    # >>> 3 2 1
    # Another simple example gives the same results:
    f = FIFO(3,2,1)
    while not f.empty():
        print f(),
    # Another using the default "truth" value of the object
    # (implicitly calls on the length() of the object after
    # checking for a __nonzero__ method
    f = FIFO(3,2,1)
    while f:
        print f(),
    # Yet another, using more Pythonic iteration:
    f = FIFO(3,2,1)
    for i in f:
        print i,


class FIFO:  ## NOT a new-style class, must not derive from "object"
   def __init__(self,*args):
       self.contents = list(args)
   def __call__(self):
       return self.pop()
   def empty(self):
       return bool(self.contents)
   def pop(self):
       return self.contents.pop(0)
   def __getattr__(self, attr):
       return getattr(self.contents,attr)
   def next(self):
       if not self:
           raise StopIteration
       return self.pop()


from collections import deque
fifo = deque()
fifo. appendleft(value) # push
value = fifo.pop()
not fifo # empty
fifo.pop() # raises IndexError when empty