Double Sum Using Matlab


by adeeyo
Tags: double, matlab
adeeyo
adeeyo is offline
#1
Feb21-12, 10:15 PM
P: 19
Hi,
I have this double summation expression to solve as part of matlab code I am writing. I have searched matlab no syntax that can do it. Please assist.

Q=ƩƩxixjaij i.e double sum of xi xj aij, i=1:n, j=1:n

Please assist me

Thanks
adeeyo
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mathman
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#2
Feb22-12, 03:44 PM
Sci Advisor
P: 5,941
What do mean by solve? What you have written is simply an expression. To solve something you need to describe what is known and what you are trying to find.
adeeyo
adeeyo is offline
#3
Feb22-12, 05:12 PM
P: 19
Thanks Mathman,

What I mean is this. I wrote a matlab code for that expression and manual as seen below but got different answer. I don't know what is wrong either with matlab code or manual expression or both.

Q=∑∑xixjaij the first sigma has i=1:n, the second sigma j=1:n

for i=1:n
for j=1:n
Q=sum(sum(x(i).*x(j).*(a(i, j))));
end
end



Mannual

Q=x(1)*x(1)*(a(1)*a(1))+x(1)*x(2)*(a(1)*a(2))+x(1)*x(3)*a(1)*a(3)+x(2)* x(1)*a(2)*a(1))+x(2)*x(2)*a(2)*a(2))+x(2)*x(3)*a(2)*a(3)*+x(3)*x(1)*a(3 )*a(1))+x(3)*
x(2)*a(3)*a(2))+x(3)*x(3)*a(3)*a(3));

Thanks

mathman
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#4
Feb23-12, 03:45 PM
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P: 5,941

Double Sum Using Matlab


From what I recall about coding: You need to set Q = 0.0 before starting. Then the operating instruction should be Q=Q+x(i)*x(j)*a(i, j). To save a little time you could multiply by x(i) outside the j loop.
Wizlem
Wizlem is offline
#5
Feb24-12, 04:57 PM
P: 35
if a is nxn and x is nx1 try Q = sum(sum(a.*repmat(x,[1 n]).*repmat(x',[n 1])))
Office_Shredder
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#6
Feb24-12, 05:03 PM
Mentor
P: 4,499
Q=sum(sum(x(i).*x(j).*(a(i, j))));
This line is what is wrong with your code. Let's consider what happens on each loop:
i=1, j=1:
sum(sum(x(1)*x(1)*a(1,1))) = x(1)*x(1)*a(1,1) so Q=x(1)*x(1)*a(1,1)

i=1,j=2:
sum(sum(x(1)*x(2)*a(1,2))) = x(1)*x(2)*a(1,2) so Q=x(1)*x(2)*a(1,2) NOT x(1)*x(1)*a(1,1)+x(1)*x(2)*a(1,2)

If you replace the line with
Q=Q+x(i)*x(j)*a(i,j)
you should get the right answer


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