Random dot product tricks

Greetings again,

I have two arrays:

x = [1.3, 5.6, 7.8, -6.5, 0.3]
y = [134.2, 8786.98, -22.4, 4553518.6, 0.0019]

   Now, I know how to do the dot product

a=dot(x,y)

and the reverse (i.e, the first dot product has the summation from 1 to 5, but the reverse is 5 to 1),

so

xback = fliplr(x);
yback = fliplr(y);

b=dot(x,y)

but what about largest-to-smallest order dot produc? Is there anyway to organize that in MATLAB from the original arrays?

I didn't know if this was with the sort command.

Thanks!

Answer 1

[Walter Roberson]

    xy = x.*y;
    dotxy = sum(sort(xy(xy>0), 'descend')) + sum(sort(xy(xy<0)));

[Jesse]

Yes - that's what I was looking for. Very tight too!

Thanks Walter!

Answer 2

[Azzi Abdelmalek]

If you sort a or b or both the result will be different.

Maybe you want to do other thing. Clarify what you want to achieve

% dot(a,b) is the same then sum(a.*b)

[Azzi Abdelmalek]

[Jesse commented]

Basically I want to add the positive number is order from largest to smallest, then add the negative numbers in order from most to least negative, then add the partial sums. I hope this clarifies what I am trying to achieve.

Your suggestions?

Thanks Azzi!

[Azzi Abdelmalek]

No this is not clear, illustrate with a numeric example

[Jan]

What is your purpose of creating the partial sums at first? Do you want to increase the accuracy of the result? Then better use FEX: XSum: XSum(a .* b).