In MATLAB, I have a variable proba and I have a parfor loop as showing below:
parfor f = 1:N
proba = (1/M)*ones(1, M);
% rest of the code
end
pi_proba = proba;
MATLAB said that: "The temporary variable 'proba' is used after the PARFOR loop, but its value is nondeterministic"
I do not understand how to correct this error. I need to use a parallel loop and I need proba after the loop. How to do this?
When using parfor the classes are classified according to these categories. Make sure every variable matches one of these categories. For non-writing access to proba a Broadcast-Variable would be the best choice:
proba = (1/M)*ones(1, M);
parfor f = 1:N
% rest of the code
end
pi_proba = proba;
In case of writing access within the loop, a sliced variable is nessecary:
proba=cell(1,N)
parfor f = 1:N
%now use proba{f} inside the loop
proba{f}=(1/M)*ones(1, M);
% rest of the code
end
%get proba from whatever iteration you want
pi_proba = proba{N};
Related
Can we configure MATLAB let variable have minium local scope?
I want matlab something similiar like C below.
% after some configure ...
for i=1:1:100
a=i*i
end
% here we can not using 'a' any more for it have local scope in for loop.
Why I want it becase the scope in whole script sometimes leds to bug hard to find.
For example:
% get accumulate of b via 100 times of x_0
b=0;
for i=1:1:100
x0=100
b=b+x0
end
% get accumulate of a via 100 times of x_0
a=0
for i=1:1:100
x_0=200
a=a+x0 %mistype x_0 to x0, and hard to find
end
Thanks advance.
I don't think there is any way to force a local scope in a script/loop. However, you can create a function, in a separate file or in the same file. Each function will have it's own local scope. So for your example you can create a file myScript.m with the following:
% get accumulate of b via 100 times of x_0
accum_b(100)
% get accumulate of a via 100 times of x_0
accum_a(200)
function a = accum_a(x0)
a = 0;
for k = 1:100
a = a + x0;
end
end
function b = accum_b(x0)
b = 0;
for k = 1:100
b = b + x0;
end
end
In this particular example, you can of course call the accum_a function twice, with different x0 inputs. But each function you define in a file will have it's own local scope, and will thus result in an error when mistyping x_0/x0.
I have a function which runs a parfor loop. Within the loop, I call another function which generates a structure as a result. I need to store all the structures.
function myFunction(arguments)
% do some preliminary calcultions
parfor i = 1:num_sim % number of simulations
name = sprintf('result_%i',i)
% do some calculations and generate a structure as a result called "struct_result"
total_results.(name) = struct_result
end
end
This gives me an error message:
The variable total_results in a parfor cannot be classified.
How can I store the structure "struct_result" from all the simulations? It is a nested structure.
The problem here is that you are assigning to part of total_results during the loop, but not in a "sliced" manner. It's probably simpler to collect up the names and values separately, and then use cell2struct after the parfor loop, like this:
N = 10;
names = cell(1, N);
results = cell(1, N);
parfor i = 1:10
name = sprintf('result_%i',i)
names{i} = name;
results{i} = struct('a', rand(i), 'b', rand(i));
end
total_results = cell2struct(results, names, 2);
EDIT another way of slicing the outputs (as suggested in the comments) is to use
parfor i = 1:10
total_results(i).result = struct('a', rand(i), 'b', rand(i));
end
In this case, this works because the first-level indexing expression is a "sliced" expression.
I am trying to set up a parfor nested loop in MatLab R2016a as below.
N = size(A,1);
M = size(v,1);
in = zeros(N*M,1);
parfor i=1:N
for j=1:M
k = (i-1)*M+j;
if sqrt(sum((A(i,:)-v(j,:)).^2))<=tol
in(k) = i;
end
end
end
However, I am getting the following error Valid indices for 'in' are restricted in PARFOR loops. Is there some way I can correct this because both arrays A and v are considerably large, over 40,000 rows for A and 8,000 v? The variable tol is 0.0959.
The problem is that MATLAB doesn't recognize that the variable k is slicing the matrix in correctly. The solution should be to index in using i and j separately:
N = size(A,1);
M = size(v,1);
in = zeros(M,N);
parfor i=1:N
for j=1:M
if sqrt(sum((A(i,:)-v(j,:)).^2))<=tol
in(j,i) = i;
end
end
end
in = in(:); % reshape to a column vector, as the output in the question's code
The other alternative, but it requires more intermediate memory, is to compute this without a loop at all:
A = reshape(A,1,N,[]);
v = reshape(v,M,1,[]);
in = sum(bsxfun(#minus,A,v).^2,3) < tol*tol;
in = in(:);
(Or something similar to that, I have not run this code... Please let me know, or fix the post, if there is a typo or other mistake.)
N = size(A,1);
M = size(v,1);
in = cell(N,1);
parfor i=1:N
s=v;
p=zeros(1:M,1);
for j=1:M
k = (i-1)*M+j;
if sqrt(sum((A(i,:)-s(j,:)).^2))<=tol
p(k) = i;
end
end
in{i}=single(p);
end
in=cell2mat(in);
in=reshape(in,[N*M,1]);
sometime matlab doesnt recognize a variable in parfor loop as "sliced variable"
, a sliced variable is a variable that has a reference out of parfor loop and each of its element only accessed by a single worker (in parfor parallel workers)
so you could use a temporary variable and collect results after the parfor loop,
NOTE 1: It is better to vectorise code in the older versions because loops didn't use to be as good as they are now in R2017 referring to (this).
NOTE 2: if most elements of "in" are zero try using "sparse matrix" which could save a lot of memory;
I have a code in MATLAB in which I'm running monte-carlo simulations using parfor instead of simple for loop to convert the code from sequential to parallel. Following is the piece of code which is inside the parfor loop.
But MATLAB gives an error saying "Valid indices for local_Q_mega_sub_seed are restricted in parfor loop". Suggested action says to "Fix the index" and it suggests to use "Sliced Variables". I've been struggling to use this concept. I have read https://blogs.mathworks.com/loren/2009/10/02/using-parfor-loops-getting-up-and-running/#12 and https://www.mathworks.com/matlabcentral/answers/123922-sliced-variables-in-parfor-loop-restricted-indexing along with MATLAB documentation https://www.mathworks.com/help/distcomp/sliced-variable.html and https://www.mathworks.com/help/distcomp/parfor.html but I'm not getting it right.
Could anyone please let me know how can I use sliced variables in the given piece of code so that I could get an idea?
index_f = 1;
subseed_step = (sub_seed_transmitted_at/fs_local)*sintablen_mega_frequency;
for i = 1 : fs_local
local_Q_mega_sub_seed(i) = SINTAB(round(index_f));
local_I_mega_sub_seed(i) = COSTAB(round(index_f));
index_f = index_f + subseed_step;
if index_f>sintablen_mega_frequency
index_f = index_f - sintablen_mega_frequency;
end
You're not showing enough context here, but I bet the problem here is similar to this one:
parfor ii = 1:10
for jj = 1:10
tmp(jj) = rand
end
out(ii) = sum(tmp);
end
In this case, the parfor machinery cannot categorically prove that the way tmp is being used is independent of the order of iterations of the parfor loop. This is because it appears as though values assigned to tmp in one iteration of the parfor loop are still being used in the next iteration.
Fortunately, there's a very simple workaround for this case - convince parfor that you are not doing anything dependent on the order of evaluation of the iterations of the loop by resetting the variable. In the simple case above, this means:
parfor ii = 1:10
% reset 'tmp' at the start of each parfor loop iteration
tmp = [];
for jj = 1:10
tmp(jj) = rand
end
out(ii) = sum(tmp);
end
I am having trouble using struct arrays in Matlab's parfor loop. The following code has 2 problems I do not understand:
s=struct('a',{},'b',{});
if matlabpool('size')==0
matlabpool open local 2
end
for j = 1:2
parfor k=1:4
fprintf('[%d,%d]\n',k,j)
s(j,k).a = k;
s(j,k).b = j;
end
end
matlabpool close
It fails with an error Error using parallel_function (line 589)
Insufficient number of outputs from right hand side of equal sign to satisfy assignment.
On output, variable s is a vector, not an array (as it should be, even if the code breaks before finishing).
EDIT the problem is solved if I initialize the struct arrays to the correct size, by:
s=struct('a',cell(2,4),'b',cell(2,4));
However, I would still be happy to get insights about the problem (e.g is it rally a bug, as suggested by Oleg Komarov)
It was originally working fine for me but then I don't know what happens. In general you need to be careful with parfor loops and there are ample documentation on how to align everything. Two different words of advice.
First and more importantly, the parfor loop is on the outside loop:
function s = foo
s=struct('a',{},'b',{});
parfor j = 1:2
for k=1:4
fprintf('[%d,%d]\n',k,j)
s(j,k).a = k;
s(j,k).b = j;
end
end
Two, Matlab gets very picky about writing the main exit variable (i.e. the variable contained in the parfor loop which is indexed to the loop, in your case, s). You first want to create a dummy variable that holds all the innerloop information, and then writes to it once at the end of the loops. Example:
function s = khal
s=struct('a',{},'b',{});
parfor j = 1:2
dummy=struct('a',{},'b',{});
for k=1:4
fprintf('[%d,%d]\n',k,j)
dummy(k).a = k;
dummy(k).b = j;
end
s(j,:) = dummy;
end
You don't have a problem here, but it can get complicated in other instances