How to rewrite parfor loop using arrayfun to compute on GPU?

2016 2 7
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更新时间：2016 2 19
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Is there a way to rewrite this parfor/for loops to compute on GPU using arrayfun? And will there be any difference?
parfor i = 1:size(A,1)
for j = i+1:size(A,1)
for k = j+1:size(A,1)
sumMatrix = [sumMatrix; [(A(i) + A(j) + A(k)) A(i) A(j) A(k)]];
end
end
end
A is a vector of integers.

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回答（1 个）

Joss Knight
Joss Knight 2016-2-15
编辑：Joss Knight 2016-2-15

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Seems like you're just trying to add together all the elements of A in every permutation, with the proviso that k > j > i. So really you shouldn't be using loops at all. Just form the list of numbers you want to add and prune out all the ones you don't want.
[Ak, Aj, Ai] = ndgrid(A);
[maskK, maskJ, maskI] = ndgrid(1:numel(A));
mask = maskJ > maskI & maskK > maskJ;
Acat = [Ai(mask) Aj(mask) Ak(mask)];
sumMatrix2 = [sum(Acat,2) Acat];
In answer to your last question, yes, it makes an astronomical difference. With even 100 numbers the looping method takes several minutes while the vectorized method takes a fraction of a second. Although you could speed up the looping method a massive amount by preallocating the output.

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Stepan Ulyanin
Stepan Ulyanin 2016-2-19
编辑：Stepan Ulyanin 2016-2-19
Hi! Your version works perfectly on arrays of moderate sizes, but I tried to run the code on an array with 1000000 elements and it resulted in a 100% utilization of memory and crush afterwards, do you have any ideas?
I also changed the code a little since there are some bounding conditions, so I check for a condition now and if it works then I add to the array, if it doesn't I disregard the entry. My CPU runs it fairly fast but when I run for 1 000 000 000 000 numbers it takes a long time, and I wondered if there is more efficient algorithm or if use of GPU would make the computation faster?
Here is the code I am using right now:
parfor i = 1:(N-2)
%draw the progress bar
parfor_progress;
%sum int at i with int at i+1 and with int at i+2
for j = i+1:(N-1)
for k = j+1:N
%sum of 3 perfect squares
sum = arrayOfPerfectSquares(i) + arrayOfPerfectSquares(j) + arrayOfPerfectSquares(k);
%sumMatrix = [sumMatrix; [(A(i) + A(j) + A(k)) A(i) A(j) A(k)]];
% ---------
%if at least one of the perfect squares * 3 are equal to the sum ->
%append the sum and the squares to the matrix
if (3 * arrayOfPerfectSquares(i) == sum || 3 * arrayOfPerfectSquares(j) == sum || 3 * arrayOfPerfectSquares(k) == sum) && (arrayOfPerfectSquares(i) + arrayOfPerfectSquares(j) == 2 * arrayOfPerfectSquares(k) || arrayOfPerfectSquares(i) + arrayOfPerfectSquares(k) == arrayOfPerfectSquares(j) || arrayOfPerfectSquares(j) + arrayOfPerfectSquares(k) == 2 * arrayOfPerfectSquares(i))
sumMatrix = [sumMatrix; [sum arrayOfPerfectSquares(i) arrayOfPerfectSquares(j) arrayOfPerfectSquares(k)]];
end
%----------
end
end
end
I am sorry but I am fairly new to MATLAB. Thanks for your response!

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