## how to calculate a serie of a function with two variables

on 22 Feb 2015

on 24 Feb 2015
hi everybody. i want to calculate the sum of a serie that has a function which has two variables named 'xi' and 'eta'. 'xi' and 'eta' are the coordinates of 4 points. for example i want to calculate the sum of the serie from point 1 to point 4 and in each point 'xi' and 'eta' has a specific value. can anyone help me how to write the code??

### dpb (view profile)

on 22 Feb 2015

xi=[list of four values];
eta=[list of four more presumably different values];
func=yourfunction(xi,eta); % compute the function
sumFunc=cumsum(func);

on 22 Feb 2015
thanks but it didn't work.
dpb

### dpb (view profile)

on 22 Feb 2015
How do we Know that? You didn't show any more of your work than your original question... :(

### Shoaibur Rahman (view profile)

on 22 Feb 2015

% xi eta
x =[1 2;...
3 4;...
4 5;...
6 7];
y = sum(x) % only sum of all elements along each column
y = cumsum(x) % cumulative sum along each column ### Image Analyst (view profile)

on 22 Feb 2015

Be more specific. Why can't you give us the series? Is it homework or something? If you don't get more specific then you'll just get more answers like dpb's (which is all your fault rather than dpb's because you were not specific enough to be able to answer your question).
What's wrong with using the sum() function, or even a brute force for loop()?

on 22 Feb 2015
ok i try to describe more. yes it is my homework in finite element. i am trying to write a code that meshes a 2d shape with quadrilateral elements. i have 4 functions named shape functions. these functions has two variable "xi" and "eta". you can assume x and y, it doesn't matter. the xi and eta are the coordinates of 4 points. it means that each shape function should be calculated in these 4 points. but i can not assign values to xi and eta until i do some calculations on these shape functions and obtain a final function that is also a function of xi and eta. at last i reach a matrix named j(n,m,z). For obtaining each element of this matrix, i should calculate the sum of that final function in those 4 points.also i should mention the final function is f(n,m,z) that has two variables xi and eta. i tries my best to explain it to you. sorry
Image Analyst

### Image Analyst (view profile)

on 22 Feb 2015
If you want to sum a 3D matrix "j" between certain rows and columns, do this
subArray = j(row1:row2, col1:col2, :);
theSum = sum(subArray(:));