Parfor sliced variable error

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Shawn Fernandes
Shawn Fernandes 2017-3-5
编辑： Shawn Fernandes 2017-3-7
Dear Matlab,
I have an issue with sliced variable declaration in Matlab.
The below code works perfectly fine
Block - 1
x=zeros(2,2,10);
for i = 1:10
x(:,:,i)= [i,3*i;5*i,7*i];
end
parfor i = 1:10
x(:,:,i)= [i,2*i;4*i,6*i];
end
disp(x)
In my main code, original for loop implementation works as expected
Block - 2
%Pre initialising par for loop variables
modes_points=zeros(max_iter,length(data_sort(1,:)),length(data_sort(:,1)));
%convert mat 2 cell
modes_points = mat2cell(modes_points,max_iter,length(data_sort(1,:)),length(data_sort(:,1)));
for k = 1: n
%Pre initialising par for loop variables
mode=zeros(max_iter,length(data_sort(1,:)));
if(k*Range <= length(data_sort(1,:)))
%performing first iteration
mode = FindMode(data_sort,1+(k-1)*Range,R,max_iter);
modes_points(: ,:,1+(k-1)*Range) = zeros([size(mode)]);
However, if I implement the above with parfor loop, I get an error.
Block-3
%Pre initialising par for loop variables
modes_points=zeros(max_iter,length(data_sort(1,:)),length(data_sort(:,1)));
%convert mat 2 cell
modes_points = mat2cell(modes_points,max_iter,length(data_sort(1,:)),length(data_sort(:,1)));
parfor k = 1: n
%Pre initialising par for loop variables
mode=zeros(max_iter,length(data_sort(1,:)));
if(k*Range <= length(data_sort(1,:)))
%performing first iteration
mode = FindMode(data_sort,1+(k-1)*Range,R,max_iter);
modes_points(: ,:,1+(k-1)*Range) = zeros([size(mode)]);
Output of check code:
L 111 (C 1-6): The PARFOR loop cannot run due to the way variable 'modes_points' is used.
L 125 (C 9-20): Valid indices for 'modes_points' are restricted in PARFOR loops.
But, this code in parfor works
Block-4
%Pre initialising par for loop variables
modes_points=zeros(max_iter,length(data_sort(1,:)),length(data_sort(:,1)));
%convert mat 2 cell
modes_points = mat2cell(modes_points,max_iter,length(data_sort(1,:)),length(data_sort(:,1)));
parfor k = 1: n
%Pre initialising par for loop variables
mode=zeros(max_iter,length(data_sort(1,:)));
if(k*Range <= length(data_sort(1,:)))
%performing first iteration
mode = FindMode(data_sort,1+(k-1)*Range,R,max_iter);
modes_points = cat(3,modes_points,mode);
Runtime performance of block-2(for loop) & block-4(parfor loop with cat) is same.
Please clarify why Block-3 is causing parfor error, although it is similar to Block-1 in terms of sliced variables.

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回答（1 个）

Walter Roberson
Walter Roberson 2017-3-6
In Block 4, MATLAB can tell that modes_points is a reduction variable and handles that specially.
In Block 3, with
parfor k = 1: n
modes_points(: ,:,1+(k-1)*Range) = zeros([size(mode)]);
then MATLAB might not be able to prove that 1+(k-1)*Range will definitely be unique or contained within
modes_points=zeros(max_iter,length(data_sort(1,:)),length(data_sort(:,1)));
I would suggest,
modes_points = zeros(max_iter, length(data_sort(1,:)), Range, n);
and in the loop,
modes_points(: ,:, 1, k) = zeros([size(mode)]);
as it is clear that 1+(k-1)*Range is emulating a Range by n matrix.
  1 个评论
Shawn Fernandes
Shawn Fernandes 2017-3-6
编辑：Shawn Fernandes 2017-3-6
Dear Walter Roberson,
The variable 'Range' is a constant, and equals length(data)/n.
In the expression 1+(k-1)*Range, only k=1,2, 3,4....n changes for the parfor loops, remaining all values remain constant.
Variable mode is a 2d matrix. mode_points is a 3D matrix. mode_points is an array of mode, which are 2D matrices.

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