Select every element except every nth

1

Hi,
I can select every 3rd. element using tensor[2::3], but how can I select every element except the 3rd.?

2

You can do that by creating the indexes of the elements that you want. I have the following example:

>>> a = torch.randn(100)
>>> inx = np.array([i for i in range(len(a)) if i%3!=0])
>>> inx
array([ 1,  2,  4,  5,  7,  8, 10, 11, 13, 14, 16, 17, 19, 20, 22, 23, 25,
       26, 28, 29, 31, 32, 34, 35, 37, 38, 40, 41, 43, 44, 46, 47, 49, 50,
       52, 53, 55, 56, 58, 59, 61, 62, 64, 65, 67, 68, 70, 71, 73, 74, 76,
       77, 79, 80, 82, 83, 85, 86, 88, 89, 91, 92, 94, 95, 97, 98])
>>> b = a[inx]
>>> b.shape
torch.Size([66])
3

As another approach, you can use reshape:

>>> a = torch.arange(12)
>>> a
tensor([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11])
>>> a.reshape(-1,3)[:,:2].reshape(-1)
tensor([ 0,  1,  3,  4,  6,  7,  9, 10])
4

@Tony-Y does it work it’s not divisible by the modolus? It shouldn’t, or does it?

5

It shouldn’t work. The following code works for any modulus.

def exceptEvery(nth, a):
     m = a.size(0) // nth * nth
     return torch.cat((a[:m].reshape(-1,nth)[:,:nth-1].reshape(-1), a[m:m+nth-1]))

>>> exceptEvery(2, torch.arange(11))
tensor([ 0,  2,  4,  6,  8, 10])
>>> exceptEvery(3, torch.arange(11))
tensor([ 0,  1,  3,  4,  6,  7,  9, 10])
>>> exceptEvery(4, torch.arange(11))
tensor([ 0,  1,  2,  4,  5,  6,  8,  9, 10])
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