Select an element or sub array by index from a Numpy Array

The elements of a NumPy array are indexed just like normal arrays. The index of the first element will be 0 and the last element will be indexed n-1, where n is the total number of elements.

Selecting a single element from a NumPy array

Each element of these ndarrays can be accessed using its index number.

Example: The following code shows how to access an element of a NumPy array.

import numpy as np

# NumPy Array
numpyArr = np.array([1, 2, 3, 4])
print("numpyArr[0] =", numpyArr[0])
print("numpyArr[-1] =", numpyArr[-1])

Output:

numpyArr[0] = 1
numpyArr[-1] = 4

In the first case, we accessed the first element of the array using its index number. In the second case we accessed the last element of the array by using negative indexes.

Selecting a sub array from a NumPy array (Slicing)

To get a sub-array, we pass a slice in place of the element index.

Syntax:

numpyArr[x:y]

Here x and y are the starting and last index of the required sub-array.

Example:

import numpy as np

# NumPy Array
numpyArr = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9])

# the slice 3:6 is passed instead 
# of index
print("Sub-Array=", numpyArr[3:6])

Output:

Sub-Array= [4 5 6]

A sub-array starting from the 3rd index up to the 6th index ( excluding the last the 6th index ) was selected. You can slice a sub-array starting from the first element by leaving the starting index blank.

Example: The following code selects a sub-array starting from the first element. 

import numpy as np

numpyArr = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9])

# works same as 0:6
print("Sub-Array=", numpyArr[:6])

Output:

Sub-Array= [1 2 3 4 5 6]

Similarly, leaving the left side of the colon blank will give you an array up to the last element.

Example: The following code selects a sub-array starting from a particular index up to the last index.

import numpy as np

numpyArr = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9])

# same as 3:9 or 3:n, where n is
# the length of array
print("Sub-Array=", numpyArr[3:])

Output:

Sub-Array= [4 5 6 7 8 9]