MATLAB Answers

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

Abalfazl Zareei 님이 질문을 제출함. 10 Apr 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

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James Tursa 님의 답변 11 Apr 2013
James Tursa 님이 편집함. 11 Apr 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.

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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

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bym 님의 답변 10 Apr 2013


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It's not that; it is related to the way MATLAB computes the product between a vector whose elements are ~1e15 and an other whose elements are 1's.
The same answer:
>> ones(size(w))*w.'
ans =

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Ahmed A. Selman 님의 답변 10 Apr 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 :)

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Multiplying an [1xN] with an [Nx1] vector means the dot-product. This is mathematically the same as summing the multiplied elements. Using a vector of 1's as one of the vectors results in a sum. Therefore the shown procedures are not different in theory.
Indeed it is the same. I thought w was N-by-N, my bad. Thank you for the correction.

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