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NumPy Summations


Summations

What is the difference between summation and addition?

Addition is done between two arguments whereas summation happens over n elements.

Example

Add the values in arr1 to the values in arr2:

import numpy as np

arr1 = np.array([1, 2, 3])
arr2 = np.array([1, 2, 3])

newarr = np.add(arr1, arr2)

print(newarr)
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Returns: [2 4 6]

Example

Sum the values in arr1 and the values in arr2:

import numpy as np

arr1 = np.array([1, 2, 3])
arr2 = np.array([1, 2, 3])

newarr = np.sum([arr1, arr2])

print(newarr)
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Returns: 12


Summation Over an Axis

If you specify axis=1, NumPy will sum the numbers in each array.

Example

Perform summation in the following array over 1st axis:

import numpy as np

arr1 = np.array([1, 2, 3])
arr2 = np.array([1, 2, 3])

newarr = np.sum([arr1, arr2], axis=1)

print(newarr)
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Returns: [6 6]


Cummulative Sum

Cummulative sum means partially adding the elements in array.

E.g. The partial sum of [1, 2, 3, 4] would be [1, 1+2, 1+2+3, 1+2+3+4] = [1, 3, 6, 10].

Perfom partial sum with the cumsum() function.

Example

Perform cummulative summation in the following array:

import numpy as np

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

newarr = np.cumsum(arr)

print(newarr)
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Returns: [1 3 6]