Clip or threshold a tensor using condition and zero pad the result in PyTorch
let's say I have a tensor like this

w = [[0.1, 0.7, 0.7, 0.8, 0.3],
    [0.3, 0.2, 0.9, 0.1, 0.5],
    [0.1, 0.4, 0.8, 0.3, 0.4]]


Now I want to eliminate certain values base on some condition (for example greater than 0.5 or not)

w = [[0.1, 0.3],
     [0.3, 0.2, 0.1],
     [0.1, 0.4, 0.3, 0.4]]


Then pad it to equal length:

w = [[0.1, 0.3, 0, 0],
     [0.3, 0.2, 0.1, 0],
     [0.1, 0.4, 0.3, 0.4]]


and this is how I implemented it in pytorch:

w = torch.rand(3, 5)
condition = w &lt;= 0.5
w = [w[i][condition[i]] for i in range(3)]
w = torch.nn.utils.rnn.pad_sequence(w)


But apparently this is going to be extremely slow, mainly because of the list comprehension.
is there any better way to do it?

Here's one straightforward way using boolean masking, tensor splitting, and then eventually padding the splitted tensors using torch.nn.utils.rnn.pad_sequence(...).

# input tensor to work with
In [213]: w 
Out[213]: 
tensor([[0.1000, 0.7000, 0.7000, 0.8000, 0.3000],
        [0.3000, 0.2000, 0.9000, 0.1000, 0.5000],
        [0.1000, 0.4000, 0.8000, 0.3000, 0.4000]])

# values above this should be clipped from the input tensor
In [214]: clip_value = 0.5 

# generate a boolean mask that satisfies the condition
In [215]: boolean_mask = (w &lt;= clip_value) 

# we need to sum the mask along axis 1 (needed for splitting)
In [216]: summed_mask = boolean_mask.sum(dim=1) 

# a sequence of splitted tensors
In [217]: splitted_tensors = torch.split(w[boolean_mask], summed_mask.tolist())  

# finally pad them along dimension 1 (or axis 1)
In [219]: torch.nn.utils.rnn.pad_sequence(splitted_tensors, 1) 
Out[219]: 
tensor([[0.1000, 0.3000, 0.0000, 0.0000],
        [0.3000, 0.2000, 0.1000, 0.5000],
        [0.1000, 0.4000, 0.3000, 0.4000]])


A short note on efficiency: Using torch.split() is super efficient since it returns the splitted tensors as a view of the original tensor (i.e. no copy is made).
