index_select and then reshaping is faster than just indexing?

index_select_example.py

import torch

if  __name__ == '__main__':
    X = torch.randn(100)
    out_shape = (100,100)
    idxs = torch.randint(high=100, size=out_shape).long()
    assert torch.abs(X[idxs] - X.index_select(0, idxs.view(-1)).view(*out_shape)).max() < 1e-3
    from timeit import timeit
    setup = 'import torch; X = torch.randn(100); out_shape=(100,100); idxs = torch.randint(high=100, size=out_shape).long()'
    print("X[idxs]: ", timeit("_ = X[idxs]", setup=setup, number=100))
    print("X.index_select and reshape: ", timeit("_ = X.index_select(0, idxs.view(-1)).view(*out_shape)", setup=setup, number=100))

Results:

X[idxs]:  0.021933638956397772
X.index_select and reshape:  0.00275177089497447