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Trouble in rewriting the for loop as a parfor loop

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I want to reduce unnecessary calculations in the loop, and this is the sample code for "for" loop which reduce the number of calculations from 200 to 120 :
R_num = 20;
C_numhalf = 10;
row = 1:C_numhalf;
Ray = R_num/C_numhalf*2;
row_flip = flip(row);
a = zeros(R_num,C_numhalf);
for ii = 1:R_num
col = [row(1:ceil(ii/Ray)) flip(row_flip((1:ceil(ii/Ray))))];
for jj = col % 1:C_numhalf
a(ii,jj) = 1;
end
end
disp(a)
The problem is how to rewrite the "for" loop into "parfor" loop, the following code does not work properly and this is the help link:
R_num = 20;
C_numhalf = 10;
row = 1:C_numhalf;
Ray = R_num/C_numhalf*2;
row_flip = flip(row);
a = zeros(R_num,C_numhalf);
parfor ii = 1:R_num
col = [row(1:ceil(ii/Ray)) flip(row_flip((1:ceil(ii/Ray))))];
for jj = col
a(ii,jj) = 1;
end
end
disp(a)
I have poor knowledge of parallel computing and failed to find a solution to the problem. Any suggestions are helpful!
By the way, there is an error in the help link:

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Accepted Answer

Raymond Norris
Raymond Norris on 3 Nov 2020
This might work, but two clarifying points
  • These aren't the same
for jj = col % 1:C_numhalf
In your example code, col might be the vector [1 10], which when used as the range in a for loop is not the same as
1:10. I'm assuming you want the vector [1 x] and not the vector 1:x.
  • Depending on how you initialize a, what I'm showing may or may not work (unless you really do want to work with the vector 1:x and not [1 x]). If it's with zeros, you don't need to call zeros again in unit_of_work (just because of the way scalar expansion works). If it's any other helper function (e.g. nan), then you'll need to preallocate it with the same helper function (e.g. tempa = nan(1,C_numhalf)). If a is assigned some other way, for example a = myfcn(R_num,C_numhalf), what I'm suggesting most likely won't work. Based on your response, we might be able to write it differently.
function chenguang
R_num = 20;
C_numhalf = 10;
row = 1:C_numhalf;
Ray = R_num/C_numhalf*2;
row_flip = flip(row);
a = zeros(R_num,C_numhalf);
parfor ii = 1:R_num
a(ii,:) = unit_of_work(row,ii,Ray,row_flip,C_numhalf);
end
disp(a)
function tempa = unit_of_work(row,ii,Ray,row_flip,C_numhalf)
col = [row(1:ceil(ii/Ray)) flip(row_flip((1:ceil(ii/Ray))))];
tempa = zeros(1,C_numhalf);
for jj = col
tempa(1,jj) = 1;
end

  1 Comment

chenguang yan
chenguang yan on 3 Nov 2020
  • I want the vector [1 x] and not the vector 1:x, when ii == 1.
  • I preallocate a with a = sym(zeros(R_num,C_numhalf)) actually. Calculating the element of matrix a involves symbolic computation, which is why I hope to use parfor to speed up the calculation.
Following the method you provided, I placed the inner for loop in the local function in my own code, and the outer parfor loop seems to work normally. Thank you very much for your answers.

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