Python Language Tutorial Part- 3 (Data Structure in List)

Neeraj Kumar
Oct 03, 2015
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Today, I took a new part of python. In this Part we will see the usage of list and how can use multiple ways for arranging data in list. All the information and points are interesting in this part of the tutorial.

I have already told you.

Download Python LanguageTutorials and give star on GitHub if that files are helpful for you.

In this article you will understand the following:

List
Stack
Queue

The concepts you may have known theoretically, but today I’ll explain how we can implement these concepts in python with simple code implementation.

List

List is an important data type in Python. It stores many data. In Python, List has many function that make easy and simple for handling list in python.

list.append(X) : Add an item to the end of the list.
list.extend(L): Extend the list by appending all the items in the given list.
list.insert(I, X) : Insert an item at a given position.
list.remove(X) :Remove the first item from the list.
list.pop([I]) : Remove the item at the given position in the list.
list.clear() : Remove all items from the list.
list.index(X) : The index in the list of the first item whose value is X.
list.count(X) : The number of times X appears in the list.
list.sort() : Sort the items of the list in place.
list.reverse() : Reverse the elements of the list in place.
list.copy() : A copy of the list.

Now see the example below:

Example 1:

#!/usr/bin/python
# -*- coding: utf-8 -*-
a = [
1,
4,
3,
6,
3,
5,
6,
74,
6,
4,
3,
6,
2,
8,
]
# number of times values appears in the list.
print (
'a.count(3)=',
a.count(3),
'a.count(6)=',
a.count(6),
"a.count('x')=",
a.count('x'),
)
# Insert an item at a given position
a.insert(2, -1)
print ('a.insert(2, -1)=', a)
# Add an item to the end of the list
a.append(3)
print ('a.append(3)=', a)
# the index in the list of the first item . It is an error if there is no such item
print ('a.index(3)=', a.index(3))
# Remove the first item from the list.. It is an error if there is no such item
a.remove(3)
print ('a.remove(3)=', a)
# Reverse the elements of the list
a.reverse()
print ('a.reverse()=', a)
# Sort the items of the list
a.sort()
print ('a.sort()=', a)
# removes and returns the last item in the list.
print ('a.pop()=', a.pop())

Output:

a.count(3)= 3 a.count(6)= 4 a.count('x')= 0
a.insert(2, -1)= [1, 4, -1, 3, 6, 3, 5, 6, 74, 6, 4, 3, 6, 2, 8]
a.append(3)= [1, 4, -1, 3, 6, 3, 5, 6, 74, 6, 4, 3, 6, 2, 8, 3]
a.index(3)= 3
a.remove(3)= [1, 4, -1, 6, 3, 5, 6, 74, 6, 4, 3, 6, 2, 8, 3]
a.reverse()= [3, 8, 2, 6, 3, 4, 6, 74, 6, 5, 3, 6, -1, 4, 1]
a.sort()= [-1, 1, 2, 3, 3, 3, 4, 4, 5, 6, 6, 6, 6, 8, 74]
a.pop()= 74

Example 2:

In this example you will see how to calculate Square of each list value:

square=[(x, x**2) for x in range(6)]
#print square of each value
print(square)
square_list = [x**2 for x in range(15)]
#print square of each value
print(square_list)

Output:

[(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196]

Example 3:

In this example you will see how to combine two lists.

comp_list=[(x, y) for x in [1,2,3] for y in [4,5,6] if x != y]
print(comp_list)
combs = []
for x in [1,2,3,]:
for y in [4,5,6]:
if x != y:
a. combs.append((x, y))
print(combs)

Output:

[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]
[(1, 4), (1, 5), (1, 6)]
[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6)]
[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]

Example 4:

In this example you will see how to get element in List.

vec = [[1,2,3], [4,5,6], [7,8,9]]
elem=[num for elem in vec for num in elem]
print(elem)

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Example 5:

In this example you will see use of ‘pi’:

#!/usr/bin/python
# -*- coding: utf-8 -*-
from math import pi
PI = [str(round(pi, i)) for i in range(1, 10)]
print PI

Output:

['3.1', '3.14', '3.142', '3.1416', '3.14159', '3.141593', '3.1415927', '3.14159265', '3.141592654']

Example 6:

In this example you will see matrix transpose.

matrix = [
[1, 2, 3, 4],
[5, 6, 7, 8],
[9, 10, 11, 12],
]
print(matrix)
transpose_mat1=[[row[i] for row in matrix] for i in range(4)]
print(transpose_mat1)
transposed_mat2 = []
for i in range(4):
# the following 3 lines implement the nested listcomp
rows = []
for row in matrix:
rows.append(row[i])
transposed_mat2.append(rows)
print(transposed_mat2)

Output:

[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]

Stack

Stack is work on (“last-in, first-out”) concept. In List, the methods can be easily used for making stack concept on List. Add an item to the top of the stack, use append(). To retrieve an item from the top of the stack, use pop() without an explicit index.

In Stack we’ll use the following two methods:

append()
pop()

Example:

#!/usr/bin/python
# -*- coding: utf-8 -*-
stack = [12, 34, 45]
# Add an item to the top of the stack
stack.append(23)
print ('stack.append(23)=', stack)
# Add an item to the top of the stack
stack.append(73)
print ('stack.append(73)=', stack)
# To retrieve an item from the top of the stack
stack.pop()
print ('stack.pop()=', stack)
# To retrieve an item from the index 'X' of the stack
stack.pop(1)
print ('stack.pop(1)=', stack)
# To retrieve an item from the top of the stack
stack.pop()
print ('stack.pop()=', stack)

Output:

stack.append(23)= [12, 34, 45, 23]
stack.append(73)= [12, 34, 45, 23, 73]
stack.pop()= [12, 34, 45, 23]
stack.pop(1)= [12, 45, 23]
stack.pop()= [12, 45]

Queue

Queue is work on (“first-in, first-out”) concept. List are not efficient for this purpose.Becuase pop() method retrieve the top of the stack value.In Queue we always retrieve data from beginning of the list. To implement a queue in list we use collections.deque which are used for pop data from the beginning of the list.

Example:

#!/usr/bin/python
#- * -coding: utf - 8 - * -
from collections
import deque
queue = deque(['Gmail', 'Yahoo', 'Outlook'])
print queue
# Add an item in the queue
queue.append('Rediff')
print('queue.append("Rediff") =', queue)
# To retrieve an item from Beginning in the queue
queue.popleft()
print('queue.popleft() =', queue)
# To retrieve an item from Beginning in the queue
queue.popleft()
print('queue.popleft() =', queue)

Output:

deque(['Gmail', 'Yahoo', 'Outlook'])
queue.append("Rediff") = deque(['Gmail', 'Yahoo', 'Outlook', 'Rediff'])
queue.popleft() = deque(['Yahoo', 'Outlook', 'Rediff'])
queue.popleft() = deque(['Outlook', 'Rediff'])