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When running the following code:

from platform import python_version
print(python_version())
    
import numpy as np
x = np.array([[1,2,3],[4,5,6],[7,8,9]])
x[1,:] = x[1,:] / 5
print(x)
y = np.array([1,2,3])
y = y / 5
print(y)

I get the following output:

3.8.6
[[1 2 3]
 [0 1 1]
 [7 8 9]]
[0.2 0.4 0.6]

Why does numpy / python use integer division when dividing a row in a matrix by a scalar while dividing a single row using regular division? I thought " / " division in numpy 3 was always regular?

Guillaume Jacquenot
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jgklsdjfgkldsfaSDF
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  • *"Why does numpy / python use integer division when dividing a row in a matrix"* - Hint: it doesn't. – superb rain Dec 06 '20 at 14:02
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    Try it with `x = np.array([[1,2,3],[4,5,6],[7,8,9]],dtype=float)`. – wwii Dec 06 '20 at 14:06
  • Related:[Numpy array being rounded? subtraction of small floats](https://stackoverflow.com/questions/31732728/numpy-array-being-rounded-subtraction-of-small-floats). [How can I multiply column of the int numpy array to the float digit and stays in int?](https://stackoverflow.com/questions/51466808/how-can-i-multiply-column-of-the-int-numpy-array-to-the-float-digit-and-stays-in). – wwii Dec 06 '20 at 14:14
  • @wwii Hmm, I'd say that rather obfuscates the reason. One might think the outcome is floats because it's then dividing floats by an int. – superb rain Dec 06 '20 at 14:15
  • Numpy documentation:[Assigning values to indexed arrays](https://numpy.org/doc/stable/user/basics.indexing.html#assigning-values-to-indexed-arrays) – wwii Dec 06 '20 at 14:23

2 Answers2

2

Why does numpy / python use integer division when dividing a row in a matrix by a scalar

It doesn't - the symptom you are seeing is due to the assignment.

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

Dividing by an integer produces an array of floats,

>>> z = x[1,:] / 5
>>> z
array([0.8, 1. , 1.2])

But assigning that array to a slice of an integer array causes the dtype conversion.

>>> x[1,:] = z
>>> x
array([[1, 2, 3],
       [0, 1, 1],
       [7, 8, 9]])
>>> z.dtype
dtype('float64')
>>> x.dtype
dtype('int32')
>>>

This is mentioned in the documentation - Assigning values to indexed arrays

Note that assignments may result in changes if assigning higher types to lower types (like floats to ints) or even exceptions (assigning complex to floats or ints):

wwii
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0

The trick here is in line:

x[1,:] = x[1,:] / 5

According to numpy documentation of dtype: https://numpy.org/doc/stable/reference/generated/numpy.dtype.html

A numpy array is homogeneous, and contains elements described by a dtype object

So when manually assigning the row, it's taking dtype of x matrix into account, which is of type dtype('int64').

The same will happen to you if you tried to manually assign an element to the y array:

y = np.array([1,2,3])
y[1] = 0.5
print(y)
# this will print array([1, 0, 3])

Why does numpy / python use integer division when dividing a row in a matrix by a scalar while dividing a single row using regular division?

So it's about enforcing the homogenous dtype of the np.array itself rather than dividing a row in a matrix, as shown in the line below:

x[1] / 5
>>> array([0.8, 1. , 1.2])
Turtlean
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