Why does a'*b yield a (slightly) different result than dot(a,b)?
3 Ansichten (letzte 30 Tage)
Ältere Kommentare anzeigen
EQ
am 28 Nov. 2017
Beantwortet: Walter Roberson
am 28 Nov. 2017
I was working an building a matrix entry-wise and checking two different methods against each other and eventually found that dot(a,b) is giving me a different answer than a'*b, where a and b are both real 9948x1 column vectors. I can see that the implementation of dot uses conj(a)*b, but my vectors are real. In one example, dot(a,b) = -1.0351e-11 and a'*b = -8.6402e-12.
I think it's likely a floating point precision issue, but I'm curious about the implementation of the "*" operator and what might be leading to it. Can anyone shed some light on this?
3 Kommentare
Guillaume
am 28 Nov. 2017
Indeed with your inputs I get the same discrepancy in R2017b. No idea why.
Akzeptierte Antwort
Walter Roberson
am 28 Nov. 2017
For sufficiently large arrays, a'*b is going to invoke the high performance libraries, which are going to calculate the multiplications in multiple threads, doing partial summations, before eventually doing a final summation. Because the summations could be done in a different order than sum(conj(a).*b), the round-off can be different.
0 Kommentare
Weitere Antworten (0)
Siehe auch
Kategorien
Mehr zu Logical finden Sie in Help Center und File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!