Special treatment of "0/0"

1

I am implementing an algorithm that can lead to 0/0 by design with the convention that 0/0 -> 1. The relevant part of the algorithm is as follows

Tensor a = [a1, a2, ..., an]
Tensor b = [b1, b2, ..., bn]
Tensor c = (x - a) / (b - a)

Here, / denotes element wise division and x is a scalar. I can ensure that entries in b-a are equal to zero if and only if the relevant entries in x-a are zero as well. I also need to extend the above algorithm to a, b, and c being matrices and x being a vector of appropriate size.

What is the most efficient way to make the element wise division operator treat 0/0 -> 1?

2

How about using torch.where?

torch.where(a == b , 1 , (x - a) / (b - a) )
3

That also will depend on how you want to treat x/0. Assuming you want both to be =1, here is what I would do:

x=6
a=torch.cat([torch.tensor([1, 2, 3, 4])]*5)
b=torch.cat([torch.tensor([1, 2, 3, 4,5])]*4)


c =torch.where(b-a==0, 1, (x - a) / (b - a))
4

Hi Shivam (and Matthias)!

Two issues to be aware of:

First, if a and b are floating-point tensors (which I am guessing they
might be), you have the issue that a == b is an exact floating-point
comparison that won’t work (because of round-off error) for typical
use cases. You would probably need to substitute something like
close_enough_to_be_considered_equal_for_my_use_case (a, b)
for the exact equality test.

Second, torch.where() doesn’t backpropagate nicely when nans and
infs occur, even in the “branch not followed.” (See for example this
torch.where() github issue.) In instances where a and b are equal, you
will get nans or infs.

You could do something like:

mask = (almost_equal (x, b)).float()   # 1.0 if b is "equal" to x, otherwise 0.0
a = a - mask
c = (x - a) / (b - a)   # 0/0 has now become 1.0 / 1.0

Best.

K. Frank

2 Likes
5

This solution works perfectly. Thanks.