# Compute double sum or double integral throug sum

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F.O on 30 Oct 2017
Answered: Roger Stafford on 30 Oct 2017
x=[-20:0.2:10];
y=[-10:0.2:5];
u2=-3*exp(-sqrt(x.^2+y.^2)/2)
I want to sum this function from y=1 to the number of elements of y vector then the sum of this result from x=1 to the number of elements in x. IT is actually a double integral in form of sum.

You want to sum from y = 1 up to the number of elements in y even though y begins at -10 and runs to 5? How is that expected to work exactly?
F.O on 30 Oct 2017
Here we are dealing with the position of the elements not the elements themseves. Maybe to use for loop command here. IT is actually a double integral which I should integrate with respect to from y= -10to y=5in the first step then with respect to x from x=-20 to x=10 I have dx=0.2 dy=0.2

Honglei Chen on 30 Oct 2017
If the ultimate goal is to compute the integral, you can consider using integral2
If you have to do it with sum, then since you already computed u2 for all values, it is just
x=[-20:0.2:10]; y=[-10:0.2:5]'; u2=-3*exp(-sqrt(x.^2+y.^2)/2);
sum(u2(:))*0.2*0.2
Note that you have to transpose x or y so it forms a grid
HTH

F.O on 30 Oct 2017
Could you explain more why we should use transpose here and what this the sum here works?
Guillaume on 30 Oct 2017

Roger Stafford on 30 Oct 2017
[X,Y] = meshgrid(x,y);
M = -3*exp(-sqrt(X.^2+Y.^2)/2);
S = sum(M(:))*.2*.2; % The desired sum