Pagemtimes accuracy versus for loop
10 views (last 30 days)
Show older comments
I was using pagemtimes for some calculations an I get results which are not quite what I expect, for example
N = 1e3;
a = randn(3, 3, N);
b = randn(2, 3);
c1 = zeros(2, 2, N);
for i=1:N
c1(:,:,i) = b * a(:,:,i) * b';
end
c2 = pagemtimes(pagemtimes(b, a), b');
c3 = pagemtimes(b, pagemtimes(a, b'));
isequal(c1,c2)
isequal(c1,c3)
returns a logical no for both isequal queries.
I assume this is due to numerical deviations? Or is there something non-equivalent between the different fomulations of the "c" matrix.
0 Comments
Answers (1)
Matt J
on 5 May 2021
Edited: Matt J
on 5 May 2021
I assume this is due to numerical deviations?
Yes, there is no expectation that both approaches will produce the same floating point noise. Clearly the percent errors are very small, though:
N = 1e3;
a = randn(3, 3, N);
b = randn(2, 3);
c1 = zeros(2, 2, N);
for i=1:N
c1(:,:,i) = b * a(:,:,i) * b';
end
c2 = pagemtimes(pagemtimes(b, a), b');
c3 = pagemtimes(b, pagemtimes(a, b'));
%Percent errors
relError=@(a,b)norm(a(:)-b(:),inf)/norm(b(:),inf)*100;
relError(c1,c2)
relError(c1,c3)
See Also
Categories
Find more on Logical in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!