NumPy: Calculate cumulative sum of the elements along a given axis

Last update on August 29 2025 12:48:25 (UTC/GMT +8 hours)

27. Cumulative Sum and Row/Column Sums

Write a NumPy program to calculate cumulative sum of the elements along a given axis, sum over rows for each of the 3 columns and sum over columns for each of the 2 rows of a given 3x3 array.

Sample Solution:

Python Code:

# Importing the NumPy library
import numpy as np

# Creating a 2D array
x = np.array([[1, 2, 3], [4, 5, 6]])

# Displaying the original array
print("Original array: ")
print(x)

# Calculating the cumulative sum of all elements in the array
print("Cumulative sum of the elements along a given axis:")
r = np.cumsum(x)
print(r)

# Calculating the cumulative sum over rows for each of the 3 columns
print("\nSum over rows for each of the 3 columns:")
r = np.cumsum(x, axis=0)
print(r)

# Calculating the cumulative sum over columns for each of the 2 rows
print("\nSum over columns for each of the 2 rows:")
r = np.cumsum(x, axis=1)
print(r)

Sample Output:

Original array: 
[[1 2 3]
 [4 5 6]]
Cumulative sum of the elements along a given axis:
[ 1  3  6 10 15 21]

Sum over rows for each of the 3 columns:
[[1 2 3]
 [5 7 9]]

Sum over columns for each of the 2 rows:
[[ 1  3  6]
 [ 4  9 15]]

Explanation:

x = np.array([[1,2,3], [4,5,6]]) – x is a two-dimensional NumPy array with shape (2, 3). The first row of the array is [1, 2, 3] and the second row is [4, 5, 6].

np.cumsum(x): This line computes the cumulative sum of all elements in the flattened array and returns [ 1 3 6 10 15 21]

np.cumsum(x, axis=0): This line computes the cumulative sum of elements along the rows of the array x and returns [[1, 2, 3], [5, 7, 9]]

np.cumsum(x, axis=1): This line computes the cumulative sum of elements along the columns of the array x and returns [[ 1, 3, 6], [ 4, 9, 15]].

For more Practice: Solve these Related Problems:

Implement a function that computes the cumulative sum of a 2D array using np.cumsum and reshapes the result to the original array dimensions.
Test the function to calculate both the row-wise and column-wise cumulative sums and verify against manual totals.
Create a solution that returns the cumulative sum along with overall row and column sums as separate arrays.
Apply the function on a transposed array and compare the results with the original cumulative sums for consistency.

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