How do I index a 2-D matrix row wise?

1

I would like to index a 2-D matrix row-wise and re-assign values.

For example, first consider a 1-D vector case where we have three 1-D tensors t1, indexes, t2 with the same shape. We can do this indexing and re-assignment as follows:

indexes = torch.tensor([0, 2, 1, 3])
t1 = torch.tensor([0.0, 0.0, 0.0, 0.0])
t2 = torch.tensor([0.1, 0.2, 0.3, 0.4])

t1[indexes] = t2

Now, say that t1, indexes, t2 are 2-D matrices instead of 1-D vectors and have the same shape (R X C). I would like to do similar indexing as above for every row in these matrices where:

for i in range(R):
    t1[i][indexes[i]] = t2[i]

I would like to vectorize this operation instead of using a for loop. How do I do this?

2

.scatter_ should work in this case:

indexes = torch.tensor([[0, 2, 1, 3],
                        [1, 0, 3, 2]])
t1 = torch.zeros_like(indexes).float()
t2 = torch.tensor([[0.1, 0.2, 0.3, 0.4],
                   [0.5, 0.6, 0.7, 0.8]])
t1.scatter_(1, indexes, t2)
1 Like