# How to create a List, Array and a Matrix in Python

Hello everyone, today I would like to explain how to use a list, arrays and matrices. As you know manage very well those concepts are a key to succeed in you career in you IT profession.

Moreover when you are doing several coding Interviews, usually you have few minutes to solve the problems and if you don’t remember the main concepts, will be difficult finish the test at time.

# Creation of a List

Let’s start by constructing the simplest list, that is the empty list by

mylist=[]


The empty list is very powerful because you can do several things with list, you can allocate things in memory, the empty list is not empty, indeed contains a list without elements . You can think that the empty list is like a empty bag that you will put objects inside.

Now let us add something to this empty list.

For example the string

'video_id'


There three methods for adding elements:

• append() - appends a single element to the list.
• extend() - appends elements of an iterable to the list.
• insert() - inserts a single item at a given position of the list.

If we want to add a single element in a list we use append

mylist.append('video_id')p


so you will get

['video_id']


if you want to add two elements together,

'video_id' and 'language'

you should think that two things are iterable, so you should write it them in a proper way, that is adding list to a list

mylist=[]
mylist.extend(['video_id','language'])


you will get

['video_id', 'language']


moreover also you can use tuples because is iterable,

mylist=[]
mytuple=('video_id','language')
mylist.extend(mytuple)

['video_id', 'language']


and finally if you want add 'language_code' to the first position

mylist=[]
mylist.extend(['video_id','language'])
mylist.insert(0, 'language_code')


you will get

['language_code', 'video_id', 'language']


remember that in Python the first index begin with 0 from left to right, but also you can say

that an index start from -1 right to left,

mylist=[]
mylist.extend(['video_id','language'])
mylist.insert(-3, 'language_code')


you will get the same results

['language_code', 'video_id', 'language']


# List comprehension

One of the amazing things that Python has that I have never seen in other programming languages is the list comprehension.

Is something like perform several loops for appending elements to the empty list.

For that reason, I have started to discuss first the empty list. This allow us understand better the list comprehension.

List comprehension is a useful way to construct new lists based on the values in other lists. The structure of list comprehension generally looks like the following:

mylist = [expression for element in iterable]


An expression can simply be the element in the iterable itself or some transformation of the element, such as checking the truth value of a condition or even more complex like perform an evaluation of your element,

for example :

Given a function f(x) defined as

def f(x):
y=2*x
return y


in the domain

lista=[1,2,3,4]


the list comprehension Z

[f(x) for x in lista ]


gives

[2, 4, 6, 8]


In ordering to understand how to manage a list compression, let us discuss the following example:

### Use case : Odd numbers in a list

Write a function that returns the elements on odd positions in a list

Let us assume that you have the following input :

input = [0,1,2,3,4,5]


First, we create a list of all positions of the input,

a=list(range(0,len(input)))


Second, we need take all elements of a list a, here is were can use a list compression,

num for num in a


Third, we need to identify if a number is odd, this is possible to know if the residue of 2 is not 0 otherwise the number should be even,

num % 2 != 0


Fourth, we select the elements of the input by the index,

input[i] for i in b


and again, we have used list compression.

Now we can put together into a new ffunction

def solution(input):
a=list(range(0,len(input)))
b=[num for num in a if num % 2 != 0 ]
c = [ input[i] for i in b]
return c


You can check that is valid this solution

assert solution([0,1,2,3,4,5]) == [1,3,5]


that is something that we expected.

The sort() method accepts a reverse parameter as an optional argument.

Setting reverse = True sorts the list in the descending order.

list.sort(reverse=True)


Alternatively for sorted(), you can use the following code.

sorted(list, reverse=True)


## Summary:

Let summarize what we have cover in few words:

l = [] “Define an empty list”

l[i] “Return value at index i in list”

len(l) “Return length of list”

l.append(x) “Add value x to the end of list”

l.sort() “Sort values in list - in place sort, returns None”

sorted(l) “Return sorted copy of list”

x in l: “Evaluate True if x is contained in the list”

# Matrices from a list

The next important element that we have to know is the construction of matrices.

If you want to create an empty matrix with a list you can define a matrix as,

matrix = [[]]


which is a list of lists, that can be obtained also

matrix = []
matrix.append([])
matrix.append([])


## Define a two-dimensional matrix

Let assume that you want to build a matrix with 0

# Creates a list containing 2 lists, each of 3 items, all set to 0
w, h = 3, 2
matrix = [[0 for x in range(w)] for y in range(h)]


[[0, 0, 0], [0, 0, 0]]


As you see again, the list compression appears again, I know that maybe is you are coming from Fortran or C like me, tis a bit non common,, where you we have used Procedural Programming .

### Use case : Add two matrices using list comprehension

Write a function that returns a matrix from the operation addition of two matrices

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

B = [[5,8,1],
[6,7,3],
[4,5,9]]

result = [[A[i][j] + B[i][j] for j in range(len(A[0]))] for i in range(len(A))]

for r in result:
print(r)


# Arrays

When you are dealing with matrices it s conveniently use a dictionary instead lists.

You can define an empty matrix as follows

matrix = {}


then you can populate it

Matrix[0,0] = 13


This works because 0,0 is a tuple, and you’re using it as a key to index the dictionary.

## NumPy Array

NumPy is a Python package useful for generating arrays, which have many distinctions from Python lists.

The biggest difference is NumPy arrays use fewer resources than Python lists, which becomes important when storing a large amount of data.

To proceed, let’s import the NumPy package:

import numpy as np


then let us create a matrix from list inputs into

lista=[1,2,3,4,5,6]

matrix = np.array(lista)


if you print the previous array, you got

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


now if you wants to give a shape of matrix you can do the follow, define the shape of the matrix, you want, for example

2X3, so you says

shape = (2,3)
matrix=matrix.reshape(shape)


and the output

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


This is matrix in a numpy array. with the form matrix[m,n]

The first row is corresponds to matrix[0] a the second row matrix[1]

matrix[1]
#array([4, 5, 6])


you can choose one element like

matrix[0,0]


which is 1.

## Numpy matrix to List

Moreover we can return back to our matrix in terms of list

matrix_list=matrix.tolist()

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


## Convert Matrix to dictionary

Having the previous matrix in terms of list we can do

# using dictionary comprehension for iteration
matrix_dic = {idx + 1 : matrix_list[idx] for idx in range(len(matrix_list))}


then

matrix_dic


gives

{1: [1, 2, 3], 2: [4, 5, 6]}


by taking

matrix_dic[1]


you will obtain

[1, 2, 3]


Another possibility to get the dictionary is by use enumerate and again use comprehension list

# enumerate used to perform assigning row number
matrix_dic = {idx: val for idx, val in enumerate(matrix_dic, start = 1)}


It is possible get your keys by using

matrix_dic.keys()


you get

dict_keys([1, 2])


and values

matrix_dic.values()

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


## Summary

d = {} “Define an empty Dictionary”

d[x] “Return value for key x”

d[x] = 1 “Set value for key x to 1”

d.keys() “Return list of keys”

d.values() “Return list of values”

Congratulation! We have created Lists, Arrays and a Matrices in Python

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