How to use Sum and Dot function in GPU computation with arrayfun?

I am trying to find a dot product of two arrays. The given code below is an example. I am new to GPU computing and any help is appreciated.

The main function is:

a = [1,2,3,4];
b = [5,6,7,8];
a=gpuArray(a);
b=gpuArray(b);

funOut = arrayfun(@protoDotProd,a,b);

The function where the dot product is below:

function funOut = protoDotProd(a,b)

cTemp = a+b; % Just an example. In reality, lots of operations

%% option1
% funOut = dot(a,cTemp);

%% option2
% funOutTemp = a.*cTemp;
% funOut = sum(funOutTemp);

%% option3
funOut = sum(a .* cTemp);

None of these options are working and giving me an error:

Error using dotTest
Function passed as first input argument contains unsupported or unknown function 'sum'  (or 'dot' for option 1).
For more information see Tips.
Error in 'protoDotProd' (line: 6)

[Voss]

https://www.mathworks.com/matlabcentral/answers/281288-manipulating-arrays-within-gpu-arrayfun#answer_219870

Answer 1

[Joss Knight]

You cannot perform vector operations in GPU arrayfun. The a and b arguments to your function are not the whole array, they are the scalar values of each element.

Do your element-wise operations ("lots of operations" in your words, followed by the element-wise multiplication) inside your arrayfun function and then do the sum.

Consult the documentation and examples for more info.

[Viraj Gandhi]

I see your point here in "The a and b arguments to your function are not the whole array, they are the scalar values of each element". arrayfun(@protoDotProd,a,b) is almost similar to write

for i=1:n
    funOut(i) = protoDotProd(a(i),b(i));
end

Am I correct?

Thanks for your help.

[Joss Knight]

Exactly correct, yes. Except that you can only perform scalar operations on those scalars (so you can't form an array by concatenating the two values for instance).