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How To Use Python List as Stack vs Queue vs Comprehension vs Nested Comprehension ?


In this post , we will try to understand the Difference of Using Python List as Stack vs Queue vs List Comprehension vs Nested Comprehension. or How To Use a Python List as Stack vs Queue vs Comprehension vs Nested Comprehension. Both the Video content as well as the Notes are given below.  

List as a Stack:

  • Easy to use
  • Last-in, First-out Approach - The last element added is the first element retrieved.
  • Use append() - To add an item to the top of the stack, .
  • Use pop() - To retrieve an item from the top of the stack (without an explicit index)

\>>> stack = \[3, 4, 5\]

>>> stack.append(6)
>>> stack.append(7)

>>> stack
\[3, 4, 5, 6, 7\]

>>> stack.pop()
7
>>> stack
\[3, 4, 5, 6\]

>>> stack.pop()
6
>>> stack.pop()
5
>>> stack
\[3, 4\]

List as a Queue :

  • First-in, First-out Approach - first element added is the first element retrieved
  • Appends and pops from the end of list are fast.
  • Performing inserts or pops from the beginning of a list is slow (because all elements have to be shifted to make room for new element).
  • Better to use collections.deque  (designed for fast appends &  pops from either both ends)

\>>> from collections import deque
>>> queue = deque(\["A", "B", "C"\])  


>>> queue.append("D") 
>>> queue.append("E")    

   
>>> queue.popleft()                 
'A'
>>> queue.popleft()                 
'B'


>>> queue      # Remaining queue in order

of append first-in-first-served
deque(\['C', 'D', 'E'\])

List Comprehension:

  • Concise way to create lists.
  • Used to create new lists - after applying some operation on each element of some other sequence\iterable
  • Used to create a Sub-sequence by selecting specific elements only (based on conditions)

\>>> listexample = \[\]

#Apply some operation(Square each element)

>>> for x in range(10):
...     listexample.append(x\*\*2)
…

>>> listexample
\[0, 1, 4, 9, 16, 25, 36, 49, 64, 81\]

#Alternatively

listexample = list(map(lambda x: x\*\*2, range(10)))

>>> listexample
\[0, 1, 4, 9, 16, 25, 36, 49, 64, 81\]

Nested List Comprehension:

  • A List consisting of "Nested" List comprehensions i.e. It is a list which contains another list(s).
  • zip() - Built-in function useful for dealing with Nested List comprehensions.

\>>> matrix = \[
...     \[1, 2, 3\],
...     \[5, 6, 7\],
...     \[9, 10, 11\],
... \]

>>> list(zip(\*matrix))
\[(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)\]

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