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Matrix summation rounding error?

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Giuseppe Gallo
Giuseppe Gallo 2019-5-23
Dear all,
I'm having trouble with summation of matrices and difference of the two sum. Here some lines of code:
thetaMN_prev = cellfun(@nansum, theta_prev);
cost_prev = sum(thetaMN_prev,'all');
thetaMN_try = cellfun(@nansum, theta_try);
cost_try = sum(thetaMN_try,'all');
cost_prev - cost_try often returns 0, while
sum(thetaMN_try - thetaMN_prev,'all') ~= 0, but this should be the same calculation mathematically speaking.
When this happens, cost_try and cost_prev are of the order of 1e19.
Is this an error due to some rounding process in function sum(A,'all')?

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