Matlab: Cell contents assignment to a non-cell array object

Question

I am getting the above mentioned error while going through multiple loops. I really don't know how to explain the problem, But i will try my best

code:

function this = tempaddfilt(this,varargin)
            fc = linspace(1,200,(200/0.5));
            main = struct('seg_err',{},'sig_err',{},'filt_err',{},'fc',{});
            for a = 1:length(fc) % fc
                q = 0;
                w = 0
                for i = 1:length(this.segments) % total number signal
                    for k = 1:length(this.segments{i}) % total number of segments
                         filt_sig = eval(this.segments{i}(k).signal,this.segments{i}(k).signal(1)); % apply filter to the ith singal and kth segemnt
                        filt_sig = filt_sig';
                        main{i}(k).seg_err(a) = std(filt_sig-this.segments{i}(k).ref); % calculate the standard divitation of the filtered signal and previously calculated signal.
                        q = q+main{i}(k).seg_err(a); add all the error of the segments for the same FC
                        
                    end
                    
                    main{i}(1).sig_err(a) = q; % assign the sum of all error of the all segemnts of the same signal
                    w = w+main{i}(1).sig_err(a); % add all the error of the signals
                end
                main.filt_err = w; % assign the sum of all error of the all signals
            end
            this.error_norm = [this.error_norm ;main];
        end
        
    end

Basically I have 3 loops, 1st loop is for fc, 2nd loop is for signal and 3rd loop is for segemnts of the singal. Program works fine when fc = 1.

But when fc is 2, i get the following error:

Cell contents assignment to a non-cell array object.

in the line:

main{i}(k).seg_err(a) = std(filt_sig-this.segments{i}(k).ref);

that is when i =1 , k=1 ,a = 2

Answer 1

The problem seems to be with the way how you want to access the members of the main struct dynamically. You declared main as a struct, with

main = struct('seg_err',{},'sig_err',{},'filt_err',{},'fc',{});

BUT struct members can not be accessed by using braces {}. Here is a reference to a previous discussion related to dynamic indexing of struct arrays. So, basically, the problem is with "main{i}", which is not a valid way of dynamically indexing members of a struct.

Try following. Change the struct declaration to explanation

main = struct('seg_err',[],'sig_err',[],'filt_err',[],'fc',[]);

Then, extract the field names by

FieldNames = fieldnames(main);

Then, you can reference the struct members like in

for 
loopIndex = 1:numel(FieldNames) 
main.(FieldNames{loopIndex})(1).seg_err(1) = 1; 
end