Indexing and slicing of arrays in python.

Indexing and slicing of arrays.

What is Indexing in python?

·                 Indexing in Python means referring to an element of an iterable by its position within the iterable.

·                  Each character can be accessed using their index number.

·                       To access characters in a string we have two ways:

·        Positive index number

·        Negative index number

 Positive indexing example in Python

In Python Positive indexing, we pass a positive index that we want to access in square brackets. The index number starts from 0 which denotes the first character of a string.

Example:

my_str = "Python Guides"

print(my_str[0])

output:

P

Negative indexing example in Python

In negative indexing in Python, we pass the negative index which we want to access in square brackets. Here, the index number starts from index number -1 which denotes the last character of a string.

Example:

my_str = "Python Guides"

print(my_str[-1])

 output:

s

Indexing in python string

String indexing in python allows you to access individual characters from the string directly by using the index. Here, “(str[3])” is used to get “c”.

Example:

str = 'Welcome'

print(str[3])

 

output:

c

Slicing arrays

Slicing in python means taking elements from one given index to another given index.

We pass slice instead of index like this: [start:end].

We can also define the step, like this: [start:end:step].

If we don't pass start its considered 0

If we don't pass end its considered length of array in that dimension

If we don't pass step its considered 1

Program:

import numpy as np

 arr = np.array([1, 2, 3, 4, 5, 6, 7])

 print(arr[1:5])

output:

[2 3 4 5]

Note: The result includes the start index, but excludes the end index.

 Example

Slice elements from index 4 to the end of the array:

import numpy as np

arr = np.array([1, 2, 3, 4, 5, 6, 7])

print(arr[4:])

output:

[5 6 7]

  Accessing array elements

We can access the array elements using the respective indices of those elements.

import array as arr  

a = arr.array('i', [2, 4, 6, 8])  

print("First element:", a[0])  

print("Second element:", a[1])  

print("Second last element:", a[-1])  

 How to change or add elements

Arrays are mutable, and their elements can be changed in a similar way like lists.

import array as arr  

numbers = arr.array('i', [1, 2, 3, 5, 7, 10])     

# changing first element  

numbers[0] = 0     

print(numbers)    

# Output: array('i', [0, 2, 3, 5, 7, 10])     

# changing 3rd to 5th element  

numbers[2:5] = arr.array('i', [4, 6, 8])    

print(numbers)  

 # Output: array('i', [0, 2, 4, 6, 8, 10])  

 basic array operations

insertion opration

• By using + operator: The resultant array is a combination of elements from both the arrays.
• By using append() function: It adds elements to the end of the array.
• By using insert() function: It inserts the elements at the given index.

import array 

s1 = array.array('i', [1, 2, 3])

s2 = array.array('i', [4, 5, 6]) 

print(s1) 

print(s2)   

s3 = s1 + s2

print(s3) 

s1.append(4)

print(s1) 

s1.insert(0, 10)

print(s1)