Array Indexing and Slicing operations

Indexing

Ex1. Python program for indexing one D array

import numpy as np

 arr = np.array([10,20,30,40,50])

print("Element at index 0: ",arr[0])

print("Element at last position: ",arr[‒1])

Output

Element at index 0: 10

Element at last position: 50

Ex 2. Python program for indexing two D array

import numpy as np

arr = np.array([[1,2,3],

[4,5,6],

[7,8,9]])

print(arr[0,2])

print(arr[2,1])

Output

3

8

Code explanation: In above code,

Using first print statement, we can access the element 3 which is at position 0th row and 2nd  column.

Using second print statement, we can access element 8 which is at position at position 2nd row and first column.

Slicing

Syntax

array[start: end: step]

where

start: Index where slice begins.

end: Index where slice ends.

step: How many elements to skip

Following python programs illustrate the use of slicing in NumPy.

Ex 3. Python program for slicing a one D array

import numpy as np

arr = np.array([10,20,30,40,50,60])

print("Displaying slice(elements from 1 to 3)")

print(arr[1:4])

print("Displaying first three elements as slice")

print(arr[:3])

print("Displaying every alternate element as slice")

print(arr[::2])

Output

Displaying slice (elements from 1 to 3)

[20 30 40]

Displaying first three elements as slice

[10 20 30]

Displaying every alternate element as slice

[10 30 50]

The working of above code can be illustrated by following figure ‒

Ex 4. Python program for slicing a two D array

import numpy as np

arr = np.array([[1,2,3,4],

[5,6,7,8],

[9,10,11,12],

 [13,14,15,16]])

print("Slice second and third rows and first three columns")

print(arr[1:3,0:3])

Output

slice second and third rows and first three columns

 [[ 5 6 7]

[ 9 10 11]]

Following figure illustrates how to slice a two dimensional array ‒

Iterating

Ex 5. Python program for iterating one D array

import numpy as np

arr = np.array([10,20,30,40,50])

for x in arr:

       print(x)

Output

10

20

30

40

50

Code explanation: In above code,

There is a for loop that iterates over each element in the arr array. For each element x, it prints the value to the console,

Ex 6. Python program for iterating two D array

import numpy as np

arr = np.array([[1,2,3],

 [4,5,6],

[7,8,9]])

for row in arr:

print("Row: ",row)

Output

Row: [1 2 3]

Row: [4 5 6]

Row: [7 8 9]

In above code, the code

for row in arr:

print("Row: ", row)

• This is a for loop that iterates over each row of the 2D array.

• In each iteration, the variable row holds one row of the matrix.

• It prints each row with the label "Row: ".