Python programming : Arrays

Single Dimensional Arrays
Creating an Array
Syntax array_name = array(type_code, [elements])
Example-1 a = array(‘i’, [4, 6, 2, 9])
Example-2 a = array(‘d’, [1.5, -2.2, 3, 5.75])

Creating an Array
Typecode C Type Sizes
‘b’ signed integer 1
‘B’ unsigned integer 1
‘i’ signed integer 2
‘I’ unsigned integer 2
‘l’ signed integer 4
‘L’ unsigned integer 4
‘f’ floating point 4
‘d’ double precision floating point
8
‘u’ unicode character 2

Importing an Array Module
import array a = array.array(‘i’, [4, 6, 2, 9])
import array as ar a = ar.array(‘i’, [4, 6, 2, 9])
from array import * a = array(‘i’, [4, 6, 2, 9])

Example-1
import array
#Create an array
a = array.array("i", [1, 2, 3, 4])
#print the items of an array
print("Items are: ")
for i in a:
print(i)

Example-2
from array import *
#Create an array
a = array("i", [1, 2, 3, 4])
for i in a:
print(i)

Example-3
from array import *
#Create an array
a = array('u', ['a', 'b', 'c', 'd'])
#Here, 'u' stands for unicode character
for ch in a:
print(ch)

Example-4
from array import *
#Create first array
a = array('i', [1, 2, 3, 4])
#From first array create second
b = array(a.typecode, (i for i in a))
#print the second array items
for i in b:
print(i)
#From first array create third
c = array(a.typecode, (i * 3 for i in a))
#print the second array items
for i in c:
print(i)

Indexing & Slicing on Array
Example-1: Indexing
#To retrieve the items of an array using array index
from array import *
#Create an array
a = array('i', [1, 2, 3, 4])
#Get the length of the array
n = len(a)
#print the Items
for i in range(n):
print(a[i], end=' ')

Example-2: Indexing
#To retrieve the items of an array using array index using while loop
from array import *
#Create an array
a = array('i', [1, 2, 3, 4])
#Get the length of the array
n = len(a)
#print the Items
i = 0
while i < n:
print(a[i], end=' ')
i += 1

Slicing
Syntax arrayname[start: stop: stride]
Example arr[1: 4: 1]
Prints items from index 1 to 3 with the step size of 1

Example-3: Slicing
#Create an array
x = array('i', [10, 20, 30, 40, 50, 60])
#Create array y with Items from 1st to 3rd from x
y = x[1: 4]
print(y)
#Create array y with Items from 0th till the last Item in x
y = x[0: ]
print(y)
#Create array y with Items from 0th till the 3rd Item in x
y = x[: 4]
print(y)
#Create array y with last 4 Items in x
y = x[-4: ]
print(y)
#Stride 2 means, after 0th Item, retrieve every 2nd Item from x
y = x[0: 7: 2]
print(y)
#To display range of items without storing in an array
for i in x[2: 5]:
print(i)

Example-4: Slicing
#To retrieve the items of an array using array index using for loop
from array import *
#Create an array
a = array('i', [1, 2, 3, 4])
#Display elements from 2nd
to 4th
only
for i in a[2: 5]:
print(i)

Processing the Array
Method Description
a.append(x) Adds an element x at the end of the existing array a
a.count(x) Returns the numbers of occurrences of x in the array a
a.extend(x) Appends x at the end of the array a. ‘x’ can be another array or an
iterable object
a.index(x) Returns the position number of the first occurrence of x in the
array. Raises ‘ValueError’ if not found
a.insert(i, x) Inserts x in the position i in the array

Method Description
a.pop(x) Removes the item x from the arry a and returns it
a.pop() Removes last item from the array a
a.remove(x) Removes the first occurrence of x in the array a. Raises
‘ValueError’ if not found
a.reverse() Reverse the order of elements in the array a
a.tolist() Converts the array ‘a’ into a list

Examples
from array import *
#Create an array
a = array('i', [1, 2, 3, 4, 5])
print(a)
#Append 6 to an array
a.append(6)
print(a)
#Insert 11 at position 1
a.insert(1, 11)
print(a)
#Remove 11 from the array
a.remove(11)
print(a)
#Remove last item using pop()
item = a.pop()
print(a)
print("Item pop: ", item)

Exercises
1. To store student’s marks into an array and find total marks and percentage of marks
2. Implement Bubble sort
3. To search for the position of an item in an array using sequential search
4. To search for the position of an element in an array using index() method

Numpy

Importing an numpy
import numpy a = numpy.array([4, 6, 2, 9])
import numpy as np a = np.array([4, 6, 2, 9])
from numpy import * a = array([4, 6, 2, 9])

Creating an Array: numpy-array()
Example-1: To create an array of int datatype
a = array([10, 20, 30, 40, 50], int)
Example-2: To create an array of float datatype
a = array([10.1, 20.2, 30.3, 40.4, 50.5], float)
a = array([10.1, 20.2, 30.3, 40.4, 50.5], float)
Example-3: To create an array of float datatype without specifying the float datatype
a = array([10, 20, 30.3, 40, 50])
Note: If one item in the array is of float type, then Python interpreter converts
remaining items into the float datatype
Example-4: To create an array of char datatype
a = array([‘a’, ‘b’, ‘c’, ‘d’])
Note: No need to specify explicitly the char datatype

Program-1: To create an array of char datatype
from numpy import *
a = array(['a', 'b', 'c', 'd'])
print(a)
Program-2: To create an array of str datatype
from numpy import *
a = array(['abc', 'bcd', 'cde', 'def'], dtype=str)
print(a)

Program-3: To create an array from another array using numpy
from numpy import *
a = array([1, 2, 3, 4, 5])
print(a)
#Create another array using array() method
b = array(a)
print(a)
#Create another array by just copy
c = a
print(a)

Creating an Array: numpy-linspace()
Syntax linspace(start, stop, n)
Example a = linspace(0, 10, 5)
Description Create an array ‘a’ with starting element 0 and ending 10.
This range is divide into 5 equal parts
Hence, items are 0, 2.5, 5, 7.5, 10
Program-1: To create an array with 5 equal points using linspace
from numpy import *
#Divide 0 to 10 into 5 parts and take those points in the array
a = linspace(0, 10, 5)
print(a)

Creating an Array: numpy-logspace()
Syntax logspace(start, stop, n)
Example a = logspace(1, 4, 5)
Description Create an array ‘a’ with starting element 10^1 and ending 10^4.
This range is divide into 5 equal parts
Hence, items are 10. 56.23413252 316.22776602 1778.27941004 10000.
Program-1: To create an array with 5 equal points using logspace
from numpy import *
#Divide the range 10^1 to 10^4 into 5 equal parts
a = logspace(1, 4, 5)
print(a)

Creating an Array: numpy-arange()
Syntax arange(start, stop, stepsize)
Example-1 arange(10) Produces items from 0 - 9
Example-2 arange(5, 10) Produces items from 5 - 9
Example-3 arange(1, 10, 3) Produces items from 1, 4, 7
Example-4 arange(10, 1, -1) Produces items from [10 9 8 7 6 5 4 3 2]
Example-5 arange(0, 10, 1.5) Produces [0. 1.5 3. 4.5 6. 7.5 9.]
Program-1: To create an array with even number upto 10
from numpy import *
a = arange(2, 11, 2)
print(a)

Creating Array: numpy-zeros() & ones()
Syntax zeros(n, datatype)
ones(n, datatype)
Example-1 zeros(5) Produces items [0. 0. 0. 0. 0.]
Default datatype is float
Example-2 zeros(5, int) Produces items [0 0 0 0 0]
Example-3 ones(5, float) Produces items [1. 1. 1. 1. 1.]
Program-1: To create an array using zeros() and ones()
from numpy import *
a = zeros(5, int)
print(a)
b = ones(5) #Default datatype is float
print(b)

Vectorized Operations
Example-1 a = array([10, 20 30.5, -40])
a = a + 5 #Adds 5 to each item of an array
Example-2 a1 = array([10, 20 30.5, -40])
a2 = array([1, 2, 3, 4])
a3 = a1 + a2 #Adds each item of a1 and a2
Importance of vectorized operations
1. Operations are faster
- Adding two arrays in the form a + b is faster than taking corresponding items of both
arrays and then adding them.
2. Syntactically clearer
- Writing a + b is clearer than using the loops
3. Provides compact code

Mathematical Operations
sin(a) Calculates sine value of each item in the array a
arcsin(a) Calculates sine inverse value of each item in the array a
log(a) Calculates natural log value of each item in the array a
abs(a) Calculates absolute value of each item in the array a
sqrt(a) Calculates square root value of each item in the array a
power(a, n) Calculates a ^ n
exp(a) Calculates exponential value of each item in the array a
sum(a) Calculates sum of each item in the array a
prod(a) Calculates product of each item in the array a
min(a) Returns min value in the array a
max(a) Returns max value in the array a

Comparing Arrays

Relational operators are used to compare arrays of same size

These operators compares corresponding items of the arrays and return another array with
Boolean values
Program-1: To compare two arrays and display the resultant Boolean type array
from numpy import *
a = array([1, 2, 3])
b = array([3, 2, 3])
c = a == b
print(c)
c = a > b
print(c)
c = a <= b
print(c)

Comparing Arrays

any(): Used to determine if any one item of the array is True

all(): Used to determine if all items of the array are True
Program-2: To know the effects of any() and all()
from numpy import *
a = array([1, 2, 3])
b = array([3, 2, 3])
c = a > b
print(c)
print("any(): ", any(c))
print("all(): ", all(c))
if (any(a > b)):
print("a contains one item greater than those of b")

Comparing Arrays

logical_and(), logical_or() and logical_not() are useful to get the Boolean array as a

result of comparing the compound condition
Program-3: To understand the usage of logical functions
from numpy import *
a = array([1, 2, 3])
b = array([3, 2, 3])
c = logical_and(a > 0, a < 4)
print(c)

Comparing Arrays

where(): used to create a new array based on whether a given condition is True or False

Syntax: a = where(condition, exp1, exp2)

If condition is True, the exp1 is evaluated, the result is stored in array

a, else exp2 will be evaluated
Program-4: To understand the usage of where function
from numpy import *
a = array([1, 2, 3], int)
c = where(a % 2 == 0, a, 0)
print(c)

Comparing Arrays

where(): used to create a new array based on whether a given condition is True or False

Syntax: a = where(condition, exp1, exp2)

If condition is True, the exp1 is evaluated, the result is stored in array

a, else exp2 will be evaluated
Exercise-1: To retrieve the biggest item after comparing two arrays using where()

Comparing Arrays

nonzero(): used to know the positions of items which are non-zero

Returns an array that contains the indices of the items of the array which
are non-zero

Syntax: a = nonzero(array)
Program-5: To retrieve non zero items from an array
from numpy import *
a = array([1, 2, 0, -1, 0, 6], int)
c = nonzero(a)
#Display the indices
for i in c:
print(i)
#Display the items
print(a[c])

Aliasing Arrays

‘Aliasing means not copying’. Means another name to the existing object
Program-1: To understand the effect of aliasing
from numpy import *
a = arange(1, 6)
b = a
print(a)
print(b)
#Modify 0th Item
b[0] = 99
print(a)
print(b)

Viewing & Copying

view(): To create the duplicate array

Also called as ‘shallow copying’
Program-1: To understand the view()
from numpy import *
a = arange(1, 6)
b = a.view() #Creates new array
print(a)
print(b)
#Modify 0th Item
b[0] = 99
print(a)
print(b)

Viewing & Copying

copy(): To create the copy the original array

Also called as ‘deep copying’
Program-1: To understand the view()
from numpy import *
a = arange(1, 6)
b = a.copy() #Creates new array
print(a)
print(b)
#Modify 0th Item
b[0] = 99
print(a)
print(b)

Multi Dimensional Arrays
Numpy

Creating an Array
Example-1: To create an 2D array with 2 rows and 3 cols
a = array([[1, 2, 3],
[4, 5, 6]]
a = array([10.1, 20.2, 30.3, 40.4, 50.5], float)
Example-2: To create an 3D array with 2-2D arrays with each 2 rows and 3 cols
a = array([[[1, 2, 3],[4, 5, 6]]
[[1, 1, 1], [1, 0, 1]]]

Attributes of an Array: The ndim
Example-2: To understand the usage of the ndim attribute
a = array([[[1, 2, 3],[4, 5, 6]]
[[1, 1, 1], [1, 0, 1]]]
print(a.ndim)
● The ‘ndim’ attribute represents the number of dimensions or axes of an array
● The number of dimensions are also called as ‘rank’
Example-1: To understand the usage of the ndim attribute
a = array([1, 2, 3])
print(a.ndim)

Attributes of an Array: The shape
Example-2: To understand the usage of the ‘shape’ attribute
a = array([[1, 2, 3],[4, 5, 6]])
print(a.shape)
Outputs: (2, 3)
● The ‘shape’ attribute gives the shape of an array
● The shape is a tuple listing the number of elements along each dimensions
Example-1: To understand the usage of the ‘shape’ attribute
a = array([1, 2, 3])
print(a.shape)
Outputs: (5, )
Example-3: To ‘shape’ attribute also changes the rows and cols
a = array([[1, 2, 3],[4, 5, 6]])
a.shape = (3, 2)
print(a)
Outputs:
[[1 2]
[3 4]
[5 6]]

Attributes of an Array: The size
Example-2: To understand the usage of the ‘size’ attribute
a = array([[1, 2, 3],[4, 5, 6]])
print(a.size)
Outputs: 6
● The ‘size’ attribute gives the total number of items in an array
a = array([1, 2, 3])
print(a.size)
Outputs: 5

Attributes of an Array: The itemsize
a = array([1.1, 2.3])
print(a.itemsize)
Outputs: 8
● The ‘itemsize’ attribute gives the memory size of an array element in bytes
Example-1: To understand the usage of the ‘itemsize’ attribute
a = array([1, 2, 3, 4, 5])
print(a.itemsize)
Outputs: 4

Attributes of an Array: The dtype
Example-2: To understand the usage of the ‘dtype’ attribute
a = array([1.1, 2.3])
print(a.dtype)
Outputs: float64
● The ‘dtype’ attribute gives the datatype of the elements in the array
Example-1: To understand the usage of the ‘dtype’ attribute
a = array([1, 2, 3, 4, 5])
print(a.dtype)
Outputs: int32

Attributes of an Array: The nbytes
Example-2: To understand the usage of the ‘nbytes’ attribute
a = array([1.1, 2.3])
print(a.nbytes)
Outputs: 16
● The ‘nbytes’ attribute gives the total number of bytes occupied by an array
Example-1: To understand the usage of the ‘nbytes’ attribute
a = array([1, 2, 3, 4, 5])
print(a.nbytes)
Outputs: 20

Methods of an Array: The reshape()
Example-2: To understand the usage of the ‘reshape’ method
#Change the shape to 5 rows, 2 cols
a = a.reshape(5, 2)
print(a)
Outputs:
[[0 1]
[2 3]
[4 5]
[6 7]
[8 9]]
● The ‘reshape’ method is useful to change the shape of an array
Example-1: To understand the usage of the ‘reshape’ method
a = arange(10)
#Change the shape as 2 Rows, 5 Cols
a = a.reshape(2, 5)
print(a)
Outputs:
[[0 1 2 3 4]
[5 6 7 8 9]]

Methods of an Array: The flatten()
● The ‘flatten’ method is useful to return copy of an array collapsed into ine
dimension
Example-1: To understand the usage of the ‘flatten’ method
#flatten() method
a = array([[1, 2], [3, 4]])
print(a)
#Change to 1D array
a = a.flatten()
print(a)
Outputs:
[1 2 3 4]

Methods of creating an 2D-Array
● Using array() function
● Using ones() and zeroes() functions
● Uisng eye() function
● Using reshape() function

Creation of an 2D-Array: array()
Example-1:
a = array([[1, 2], [3, 4]])
print(a)
Outputs:
[[1, 2],
[3, 4]]

Creation of an 2D-Array: ones() & zeros()
Syntax zeros((r, c), dtype)
ones((r, c), dtype)
Example-1 a = ones((3, 4), float) Produces items
[[1. 1. 1. 1.]
[1. 1. 1. 1.]
[1. 1. 1. 1.]]
Example-2 b = zeros((3, 4), int) Produces items
[[0 0 0 0]
[0 0 0 0]
[0 0 0 0]]

Creation of an 2D-Array: The eye()
Syntax eye(n, dtype=datatype)
Description - Creates ‘n’ rows & ‘n’ cols
- Default datatype is float
Example-1 a = eye(3) - Creates 3 rows and 3 cols
[[1. 0. 0.]
[0. 1. 0.]
[0. 0. 1.]]
● The eye() function creates 2D array and fills the items in the diagonal with 1’s

Creation of an 2D-Array: The reshape()
Syntax reshape(arrayname, (n, r, c))
Description arrayname – Represents the name of the array whose elements to be
converted
n – Numbers of arrays in the resultant array
r, c – Number of rows & cols respectively
Example-1 a = array([1, 2, 3, 4, 5, 6])
b = reshape(a, (2, 3))
print(b)
Outputs:
[[1 2 3]
[4 5 6]]
● Used to convert 1D into 2D or nD arrays

Creation of an 2D-Array: The reshape()
Syntax reshape(arrayname, (n, r, c))
Description arrayname – Represents the name of the array whose elements to be
converted
n – Numbers of arrays in the resultant array
r, c – Number of rows & cols respectively
Example-2 a = arange(12)
b = reshape(a, (2, 3, 2))
print(b)
Outputs:
[[0 1]
[2 3]
[4 5]]
[[6 7]
[8 9]
[10 11]]
● Used to convert 1D into 2D or nD arrays

Indexing of an 2D-Array
Program-1: To understand indexing of 2D arrays
from numpy import *
#Create an 2D array with 3 rows, 3 cols
a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
#Display only rows
for i in range(len(a)):
print(a[i])
#display item by item
for i in range(len(a)):
for j in range(len(a[i])):
print(a[i][j], end=' ')

Slicing of an 2D-Array
#Create an array
a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
a = reshape(a, (3, 3))
print(a)
Produces:
[[1 2 3]
[4 5 6]
[7 8 9]]
a[:, :]
a[:]
a[: :]
Produces:
[[1 2 3]
[4 5 6]
[7 8 9]]
#Display 0th
row
a[0, :]
#Display 0th
col
a[:, 0]
#To get 0th
row, 0th
col item
a[0:1, 0:1]

Matrices in Numpy
Syntax matrix-name = matrix(2D Array or String)
Example-1 a = [[1, 2, 3], [4, 5, 6]]
a = matrix(a)
print(a)
Outputs:
[[1 2 3]
[4 5 6]]
Example-2 a = matrix([[1, 2, 3], [4, 5, 6]]) Outputs:
[[1 2 3]
[4 5 6]]
Example-3 a = ‘1 2; 3 4; 5 6’
b = matrix(a)
[[1 2]
[3 4]
[5 6]]

Matrices in Numpy
Getting Diagonal Items
Function diagonal(matrix)
Example-1 #Create 3 x 3 matrix
a = matrix("1 2 3; 4 5 6; 7 8 9")
#Find the diagonal items
d = diagonal(a)
print(d)
Outputs:
[1 5 9]

Matrices in Numpy
Finding Max and Min Items
Function max()
min()
Example-1 #Create 3 x 3 matrix
a = matrix("1 2 3; 4 5 6; 7 8 9")
#Print Max + Min Items
big = a.max()
small = a.min()
print(big, small)
Outputs:
9 1

Matrices in Numpy
Exercise
1. To find sum, average of elements in 2D array
2. To sort the Matrix row wise and column wise
3. To find the transpose of the matrix
4. To accept two matrices and find thier sum
5. To accept two matrices and find their product
Note: Read the matrices from the user and make the program user friendly

Python programming : Arrays

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