how do I use the array first element to do some math in python

Question

I have an

array([[5.1, 3.5, 1.4, 0.2],
       [4.9, 3. , 1.4, 0.2],
       [4.7, 3.2, 1.3, 0.2],
       [4.6, 3.1, 1.5, 0.2])

I want to get the sum of the equation:

(5.1 – 1)

(4.9 – 1)

(4.7 – 1)

(4.6 – 1)

How do I get the every arrays’ first element?

Asked By: Zhang

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Source

Answer 1

Assuming this is a Numpy array, you can just subtract from the first column and let broadcasting do the work. If you want the sum of that result, just use sum():

import numpy as np

arr = np.array([
    [5.1, 3.5, 1.4, 0.2],
    [4.9, 3. , 1.4, 0.2],
    [4.7, 3.2, 1.3, 0.2],
    [4.6, 3.1, 1.5, 0.2]
])

a = arr[:, 0] - 1
#array([4.1, 3.9, 3.7, 3.6])

a.sum()
15.299999999999999

If you are bothered by the inexact sum, make sure you read Is floating point math broken?

Answered By: Mark

Answer 2

There’s an axis argument in array.sum that you can set to sum the array vertically.

(arr-1).sum(axis=0)
array([15.3,  8.8,  1.6, -3.2])

Answered By: Kevin Choon Liang Yew

how do I use the array first element to do some math in python

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