Error: The variable Calls in a parfor cannot be classified.

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I am trying to use the "parfor" command from the parallel processing toolbox in an app that I am designing, but I am getting some errors. This is my first time using the parallel processing toolbox and I am not really familiar with the common errors. could you help me out?
%% Quail Call Detection
parfor i = 1:4
Ispec= app.Spectrograms{i};
if ~isempty(app.Spectrograms{i})
spec_len=size(Ispec,2);
spec_width=size(Ispec,1);
% figure();imshow(Ispec)
band=round((spec_width-temp_width)/3);
spec_seg_size=ceil(spec_len/30);
call_candidates=[];
%call_locations=[];
no_seg=ceil(spec_len/spec_seg_size)*2-1;
for s=1:no_seg
spec_seg=Ispec(band:end-band,(s-1)*ceil(spec_seg_size/2)+1:min((s-1)*ceil(spec_seg_size/2)+1+spec_seg_size,spec_len));
cc=xcorr2(spec_seg,template);
cc_signal=mat2gray(max(cc));
cc_signal=cc_signal(round(temp_length/2)-1:end-round(temp_length/2));
TF = islocalmax(cc_signal,'MinProminence',0.25,'ProminenceWindow',temp_length/2);
locs=find(TF);
call=(((s-1)*ceil(spec_seg_size/2)+1)+locs)';
call_candidates=[call_candidates;call];
% figure;imshow(spec_seg)
% x=1:length(cc_signal);
% figure;plot(x,cc_signal,x(TF),cc_signal(TF),'r*')
% waitforbuttonpress;
end
if ~isempty(call_candidates)
if length(call_candidates)>1
[L,n]=bwlabel(squareform(pdist(call_candidates))<temp_length/2,4);
for k=1:n
[rows,~]=find(L==k);
rows=unique(rows);
spec_seg=Ispec(band:end-band,max(call_candidates(min(rows))-...
round(temp_length/2),1):min(call_candidates(min(rows))+round(temp_length/2),spec_len));
cc=xcorr2(spec_seg,template);
cc_signal=mat2gray(max(cc));
cc_signal=cc_signal(round(temp_length/2)-1:end-round(temp_length/2));
TF = islocalmax(cc_signal,'MinProminence',0.25,'ProminenceWindow',temp_length/2);
location=find(TF);
if call_candidates(min(rows)) <round(temp_length/2)
call=(time(1)+(spec_duration/spec_len)*location);
Calls{i}=[Calls{i};call'];
%call_locations=[call_locations;location'];
else
call=(time(1)+(spec_duration/spec_len)*(call_candidates(min(rows))+location-round(temp_length/2)));
Calls{i}=[Calls{i};call'];
%call_locations=[call_locations;call_candidates(min(rows))+location'-round(temp_length/2)];
end
% figure;imshow(spec_seg)
% x=1:length(cc_signal);
% figure;plot(x,cc_signal,x(TF),cc_signal(TF),'r*')
% waitforbuttonpress;
end
else
spec_seg=Ispec(band:end-band,max(call_candidates(1)-round(temp_length/2),1):min(call_candidates(1)+round(temp_length/2),spec_len));
cc=xcorr2(spec_seg,template);
cc_signal=mat2gray(max(cc));
cc_signal=cc_signal(round(temp_length/2)-1:end-round(temp_length/2));
TF = islocalmax(cc_signal,'MinProminence',0.25,'ProminenceWindow',temp_length/2);
location=find(TF);
if call_candidates(1) <round(temp_length/2)
call=(time(1)+(spec_duration/spec_len)*location)';
Calls{i}=[Calls{i};call'];
%call_locations=[call_locations;location'];
else
call=(time(1)+(spec_duration/spec_len)*(call_candidates(1)+location-round(temp_length/2)));
Calls{i}=[Calls{i};call'];
%call_locations=[call_locations;max(call_candidates(1)+location'-round(temp_length/2),1)];
end
% figure;imshow(spec_seg)
% x=1:length(cc_signal);
% figure;plot(x,cc_signal,x(TF),cc_signal(TF),'r*')
% waitforbuttonpress;
end
end
% For Drawing Bounding Boxes
%pos{i} = [Calls{i} 1200*ones(length(Calls{i}),1) ...
% 0.5*ones(length(Calls{i}),1) 1900*ones(length(Calls{i}),1)];
%% Place Call Times into corresponding variables
CallA = Calls{1};
CallA(find(CallA==0)) = [];
CallB = Calls{2};
CallB(find(CallB==0)) = [];
CallC = Calls{3};
CallC(find(CallC==0)) = [];
CallD = Calls{4};
CallD(find(CallD==0)) = [];
%% Drawing Lines on Calls
if app.OffButton.Value == 1 && strcmp(app.ModeSwitch.Value,"Online")
for row = 1:size(Calls{1},1)
line(app.UIAxes,Calls{1}(row,:)*ones(1,length(app.F)),app.F,'Color','red','LineWidth',1.5);
end
for row = 1:size(Calls{2},1)
line(app.UIAxes_2,Calls{2}(row,:)*ones(1,length(app.F)),app.F,'Color','red','LineWidth',1.5);
end
for row = 1:size(Calls{3},1)
line(app.UIAxes_3,Calls{3}(row,:)*ones(1,length(app.F)),app.F,'Color','red','LineWidth',1.5);
end
for row = 1:size(Calls{4},1)
line(app.UIAxes_4,Calls{4}(row,:)*ones(1,length(app.F)),app.F,'Color','red','LineWidth',1.5);
end
end
end
end
end
I am getting the following error:
Error: The variable Calls in a parfor cannot be classified.
Please let me know if you needed any exrta information.

Accepted Answer

Walter Roberson
Walter Roberson on 31 May 2020
%%Place Call Times into corresponding variables
CallA = Calls{1};
Earlier you index Calls{i} where i is the parfor variable. Loops are not done in order, so when you go to process i=4, producing Calls{4}, then Calls{1} might not be ready yet. Inside parfor if you index a variable with the parfor index then you cannot refer to the same variable indexed at any constant index or any index that is not derived from the parfor variable.
Please also note that inside a worker, you cannot make graphics changes to the display.
It looks to me as if all that drawing code should be moved after the parfor loop.
  2 Comments
Walter Roberson
Walter Roberson on 31 May 2020
Ispec= app.Spectrograms{i};
Before the loop, extract app.Spectrograms into a separate variable that is not within app, such as
Spectrograms = app.Spectrograms;
and use that variable instead of app.Spectrograms .
That is, do not have any reference to app inside your parfor execution.

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