how the code '-input[range(target.shape[0]),target]' works?

Yan W 0
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I'm learing pytorch.Reading the official tutorial,I met the preplexing code. input is a tensor, so is target.

def nll(input,target):
    return -input[range(target.shape[0]),target].mean()

And the pred is:
pred

target is:
target

the '-input[range(target.shape[0]),target]' is:
'input[range(target.shape[0],target]'

Output shows this is not substracting target from input or merging two tensors

pytorch python
1 Answers

The code input[range(target.shape[0]), target] simply picks, from each row i of input the element at column indicated by the corresponding element of target, that is target[i].
In other words, if out = input[range(target.shape[0]), target] then out[i] = input[i, target[i]].

This is very similar to torch.gather.

usero1c70
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