Introduction to Deep Learning with PyTorch > Extracting sub-tensors | Python Programming (70053 Autumn Term 2022/2023) | Department of Computing

Introduction to Deep Learning with PyTorch

Chapter 3: PyTorch Tensors

Extracting sub-tensors

face Luca Grillotti

All PyTorch tensors support slicing.

Let’s first define a tensor t to illustrate this:

import torch
t = torch.randn(size=(3, 4))

Here is the value of t:

tensor([[-1.2328, -0.2615, -0.6309,  1.0880],
        [-1.0982,  0.1157,  0.0263,  0.7285],
        [-0.5299, -0.7179, -0.9029,  0.8168]])

Extracting a single line or column

Suppose we just want to extract the second line of the tensor, then we can simply do:

index_row = 1  # remember that indexes start at 0!
row = t[index_row, :]

Here is the value of row:

tensor([-1.0982,  0.1157,  0.0263,  0.7285])

Similarly, if we want to get the 3rd column:

index_col = 2  # remember that indexes start at 0!
col = t[:, index_col]

Here is the value of `col`:

tensor([-0.6309,  0.0263, -0.9029])

Extracting several lines and/or columns:

For example, if we want to get the 2nd and 3rd column:

col_lower = 1  # remember that indexes start at 0!
col_upper = 3 
block = t[:, col_lower:col_upper]  # notice that the slicing does never include col_upper

Then block equals:

tensor([[-0.2615, -0.6309],
        [ 0.1157,  0.0263],
        [-0.7179, -0.9029]])

And if we want the same tensor as before, but only with the 2 first lines:

col_lower = 1  # remember that indexes start at 0!
col_upper = 3 
row_lower = 0
row_upper = 2
block = t[row_lower:row_upper, col_lower:col_upper]  # notice that slicing never includes col_upper

producing:

tensor([[-0.2615, -0.6309],
        [ 0.1157,  0.0263]])