Outer product of tall arrays

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Z 2022-9-24

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https://ww2.mathworks.cn/matlabcentral/answers/1811470-outer-product-of-tall-arrays

编辑： Z 2022-9-25

I have two column vectors x and y which I cast as tall arrays. I want to take their outer product x*y', however I cannot transpose a tall array. What other way is there to perform the computation x*y'?

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John D'Errico 2022-9-24

编辑：John D'Errico 2022-9-25

NO. You did not listen to me. Think about what you are asking to do. If your arrays really are so tiny, then just convert them to regular, boring vectors. Then compute the outer product. Of course then, why in the name of god and little green apples are you using tall arrays in the first place if they are small?

But just suppose that your vectors REALLY do need to be tall arrays, as they are too large to store in memory. That is why you would use a tall array. For example, perhaps x and y are each 1e9 elements long.

Then the trivially simple rank 1 array the outer product would generate has 1e9*1e9=1e18 elements, each of which uses 8 bytes of RAM to store. Do you have that much disk storage available, so around 8e18 bytes, or roughly 8e9 GIGABYTES? That would be 8e6 terabytes. Or 8e3 petabytes. Or roughly 8 exabytes.

Do you, honestly, seriously have an 8 exabyte drive? Probably more than that of course, as things are never quite that easy. I suppose, if you work for the NSA, and you have full access to some seriously massive computers, you might have it. But then, if you do work there, you surely do not need me to explain this!

The computation you want to perform would have an immense result. You CANNOT DO IT, EVEN IF YOU COULD TAKE THE TRANSPOSE.