sum(w) and ones(1,size(w,2))*w' results totally different numbers

5 visualizaciones (últimos 30 días)
Abalfazl Zareei
Abalfazl Zareei el 10 de Abr. de 2013
hi, I have a vector w and it is a matrix with one row and 10 columns.
>> sum(w)
ans =
Then, I calculate the following, which is the product of a Ones vector and w, in which in matrix algebra, this is equivalent to summation of w's elements. >> ones(1,size(w,2))*w'
ans =
Both results are different. Would you please let me know why?
w =
1.0e+15 *
Columns 1 through 6
-0.0497 2.4484 -3.2273 0.1944 0.1592 0.4407
Columns 7 through 10
-0.4389 0.1548 0.1592 0.1592

Respuestas (3)

James Tursa
James Tursa el 11 de Abr. de 2013
Editada: James Tursa el 11 de Abr. de 2013
The answer is on the order of eps of the numbers you are dealing with. You have massive cancellation going on in the operation, and the order of the operation will make a difference (apparently sum and mtimes are doing the calculation in a slightly different order). The answer will have a lot of garbage bits. E.g., what do you get when you do this:
And then compare that result with your answer and you will see that they are about the same size.
  1 comentario
Jan el 11 de Abr. de 2013
Exactly. Try this (insert all digits, if you have them):
w = [-0.0497e15, 2.4484e15, -3.2273e15, 0.1944e15, 0.1592e15, ...
0.4407e15, -0.4389e15, 0.1548e15, 0.1592e15, 0.1592e15];
format long g
[dummy, index] = sort(abs(w));
Do you see a difference already? For a sum with error correction see FEX: XSum

Iniciar sesión para comentar.

bym el 10 de Abr. de 2013
  2 comentarios
Abalfazl Zareei
Abalfazl Zareei el 10 de Abr. de 2013
The same answer:
>> ones(size(w))*w.'
ans =

Iniciar sesión para comentar.

Ahmed A. Selman
Ahmed A. Selman el 10 de Abr. de 2013
The line
means creating a ones matrix with dimensions (1,size(w,2)) multiplied with w', and
means summation of the vector w.
I really don't know if it is right to compare these two, entirely different things.
So use
and compare :)
  2 comentarios
Ahmed A. Selman
Ahmed A. Selman el 11 de Abr. de 2013
Indeed it is the same. I thought w was N-by-N, my bad. Thank you for the correction.

Iniciar sesión para comentar.


Más información sobre Matrix Indexing en Help Center y File Exchange.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by