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NumPy Array Indexing and Slicing

NumPy for Scientific Computing with Python
NumPy for Scientific Computing with Python

NumPy Array Indexing and Slicing


NumPy arrays can be indexed and sliced similar to that of Python Lists. In this lesson, we will be understanding these NumPy array operations in detail.


NumPy Array Indexing

Indexing refers to the way elements are accessed in a NumPy array.

The indexing of elements in a NumPy Array starts at 0 and you can access any element of an n-dimensional array by using the index number of the elements.

You’ll understand this better with the help of the following examples of 1D and 2D NumPy arrays.

Accessing elements of a 1D NumPy Array

The syntax for accessing the elements of a 1D NumPy array is as follows:

numpy.array([elements])[i]

Here, numpy.array([elements]) is a 1D NumPy array and [i] refers to the index position of the ith element.

The following example showcases how to access elements of a 1D NumPy array:

NumPy Array Indexing and Slicing
# Importing the NumPy library as np
import numpy as np

# Creating a NumPy array from a Python List
num_array = np.array([1, 2, 3])

# Printing the content
print("The content of the NumPy array: ", num_array)

# Printing the first element by accessing index 0
print("The first element of the NumPy array: ", num_array[0])

# Printing the second element by accessing index 1
print("The second element of the NumPy array: ", num_array[1])

# Printing the third element by accessing index 2
print("The third element of the NumPy array: ", num_array[2])
The content of the NumPy array: [1 2 3]
The first element of the NumPy array: 1
The second element of the NumPy array: 2
The third element of the NumPy array: 3

You can also use negative indexing to access elements of a NumPy array. This means that the last element of a NumPy array is indexed as -1, the second last element of a NumPy array is indexed as -2, and so on.

So, the same 1D NumPy array with negative indexing can be understood as the following.

NumPy Array Indexing and Slicing
# Importing the NumPy library as np
import numpy as np

# Creating a NumPy array from a Python List
num_array = np.array([1, 2, 3])

# Printing the content
print("The content of the NumPy array: ", num_array)

# Printing the first element by accessing index -3
print("The first element of the NumPy array: ", num_array[-3])

# Printing the second element by accessing index -2
print("The second element of the NumPy array: ", num_array[-2])

# Printing the third element by accessing index -1
print("The thrird element of the NumPy array: ", num_array[-1])
The content of the NumPy array: [1 2 3]
The first element of the NumPy array: 1
The second element of the NumPy array: 2
The third element of the NumPy array: 3

Accessing elements of a 2D NumPy Array

The syntax for accessing the elements of a 2D NumPy array is as follows:

numpy.array([[elements]])[i][j]

Here, numpy.array([[elements]]) is a 2D NumPy array and [i] refers to the index position of the ith element in the first axis and [j] refers to the index position of the jth element in the second axis.

The following example showcases how to access elements of a 2D NumPy array:

2D NumPy Array Indexing
# Importing the NumPy library as np
import numpy as np

# Creating a NumPy array from a Python List
num_array = np.array([[1, 2, 3], [4, 5, 6]])

# Printing the content
print("The content of the NumPy array: ", num_array)

# Printing the element at position 0
print("The element at position 0 of the NumPy array: ", num_array[0])

# Printing the element at position (0, 0)
print("The element at position (0,0) of the NumPy array: ", num_array[0][0])

# Printing the element at position (0, 1)
print("The element at position (0,1) of the NumPy array: ", num_array[0][1])

# Printing the element at position (1)
print("The element at position (1) of the NumPy array: ", num_array[1])

# Printing the element at position (1, 0)
print("The element at position (1, 0) of the NumPy array: ", num_array[1][0])

# Printing the element at position (1, 1)
print("The element at position (1, 1) of the NumPy array: ", num_array[1][1])
The content of the NumPy array:  [[1 2 3]
                                  [4 5 6]]
The element at position 0 of the NumPy array:  [1 2 3]
The element at position (0,0) of the NumPy array:  1
The element at position (0,1) of the NumPy array:  2
The element at position (1) of the NumPy array:  [4 5 6]
The element at position (1, 0) of the NumPy array:  4
The element at position (1, 1) of the NumPy array:  5

You can take this concept of indexing even further and generalize it for n-dimensional arrays.


NumPy Array Slicing

Slicing a NumPy array refers to accessing/retrieving elements in between a starting and ending index position of an array. The general syntax for NumPy array slicing is as follows:

numpy.array()[start:end:step]

Here, numpy.array() is a NumPy array and,

  • start refers to the starting index position from where the slice should happen. If start is not specified, it is set as 0 by default.
  • end refers to the ending index position till where the slice should happen. If end is not specified, it is set as the length of the array by default.
  • step refers to the distance between two adjacent values in the array to be sliced. The default step size is 1.

The following example showcases how to slice a 1D NumPy array in different ways:

# Importing the NumPy library as np
import numpy as np

# Creating a 1D NumPy array
num_array = np.array([1, 2, 3, 4, 5])

print("The content of the NumPy array: ", num_array)

print("Elements starting from index 1:", num_array[1:])

print("Elements from index 1 to index 5: ", num_array[1:5])

print("Elements starting from index 1 to index 5 with a step of 2: ", num_array[1:5:2])

print("Elements starting from index 1 with a step of 2: ", num_array[1::2])

print("Elements starting with a step of 2: ", num_array[::2])

print("Elements till and not including index 1: ", num_array[:1])

print("Elements till and not including index 4: ", num_array[:4])

print("Elements till and not including index 4 with a step of 2: ", num_array[:4:2])

print("Elements except the last two indexes: ", num_array[:-2])

print("Elements starting from the second last index: ", num_array[-2:])

print("All elements of the array: ", num_array[:])
The content of the NumPy array: [1 2 3 4 5]
Elements starting from index 1: [2 3 4 5] 
Elements from index 1 to index 5: [2 3 4 5] 
Elements starting from index 1 to index 5 with a step of 2: [2 4] 
Elements starting from index 1 with a step of 2: [2 4] 
Elements starting with a step of 2: [1 3 5] 
Elements till and not including index 1: [1] 
Elements till and not including index 4: [1 2 3 4] 
Elements till and not including index 4 with a step of 2: [1 3] 
Elements except the last two indexes: [1 2 3] 
Elements starting from the second last index: [4 5] 
All elements of the array: [1 2 3 4 5]

That is it for this lesson on indexing and slicing NumPy array.

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