CODEWITHSUNDEEP
pythonpython-3.xnumpy
Hunk_
Hunk_

Reputation: 1

Inquiry about NumPy boolean arrays / indexing with boolean arrays

import numpy as np

G1 = np.array([[0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 10, 11]],)
G1_Compare = G1 > 4
print(G1[G1_Compare])

Output:

[ 5  6  7  8  9 10 11]

I do understand what this does, but I just don't understand what is the operation happening in here.

What is exactly happening behind the scenes?

Also, why doesn't it print the false elements?

Upvotes: 0

Views: 46

Answers (1)

fountainhead
fountainhead

Reputation: 3722

Conceptual understanding of G1 > 4:

G1 is a shape (3,4) numpy array.

In G1 > 4, G1 is participating as an operand in a comparison operation (>).

The other operand is 4, which is not an array at all, and has no dimensions.

Now, Broadcasting occurs.

Due to broadcasting, conceptually, the scalar 4 is broadcast into a shape (3,4) array consisting of just several 4s as its elements.

So, now, conceptually speaking, the comparison operation > occurs element-wise between two similarly shaped arrays, each of shape (3,4).

This results in a boolean array (G1_Compare), of shape (3,4), consisting of True and False values. The True value occurs at those element-coordinates where the corresponding element of G1 is greater than that of the broadcasted array consisting of 4s.

Note that broadcasting of array does not actually create any new array (even temporarily) with the broadcasted shape. It is only a concept or metaphor to explain/understand the defined behavior and the result. Internally, numpy uses index manipulations to achieve the effect of broadcasting, without actually creating any new arrays in the broadcasted shape.

Conceptual understanding of G1[G1_Compare]:

Here, boolean indexing occurs.

Your question "why doesn't it print the false elements?" doesn't really make sense -- what would be the point of using a boolean index if it were to select the True-position elements as well as the False-position elements? The whole point of using a boolean index is to select and return the elements in the True positions, and exclude the elements in the False positions.

Upvotes: 1

Related Questions