Error when using distributive property

D G
2011 7 26
1 个回答
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Hello!
The function below compares the output of two functions which run the same calculation. However, in one function the calculation's multiplications are distributed. It should output a matrix of 1s. However, it doesn't!
What's up with that?
function errorTest()
compute(1:100) == distributed(1:100)
function out = compute(a)
out = 10.1 * (a - 1);
end
function out = distributed(a)
out = 10.1 * a - 10.1;
end
end
Thanks!

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 采纳的回答

Oleg Komarov
Oleg Komarov 2011-7-26

0 个投票

The propagation of floating approximations is different:
idx = abs(compute(1:100) - distributed(1:100)) > 2*eps
nnz(idx)
How many values over 2eps?

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D G
D G 2011-7-26
function out = errorTest()
out = zeros(100, 2);
out(:, 1) = (compute(1:100) == distributed(1:100));
out(:, 2) = ((compute(1:100) - distributed(1:100)) > (2 * eps));
function out = compute(a)
out = 10.1 .* (a - 1);
end
function out = distributed(a)
out = 10.1 .* a - 10.1;
end
end
There is the updated code. It produces quite a few values over 2eps. There are even some over 500eps.
Oleg Komarov
Oleg Komarov 2011-7-26
Well, 500*eps = 1.11022302462516e-013; if you need precision to that point then it becomes an issue.
D G
D G 2011-7-26
I guess I had that backwards. If you remove the 2*, a lot of the answers are still different.
Oleg Komarov
Oleg Komarov 2011-7-26
Cannot really help you more than that, I don't have experience with distributed server.

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