Saving a Structured Array (containing plot parameters) to a mat file

Hi, Im trying to save plot data by using a structured arra

            ax=app.UIAxes;
            ax.Children;
            h1 = findall(ax, 'type', 'line');      % Line objects
            nlines=numel(h1);

Assign the desired plot parameters to a struct "S"

            for i=1:nlines
                thisLine=h1(i);  
                S(i).Xdata=thisLine.XData; S(i).Ydata=thisLine.YData;
                S(i).Colour=thisLine.Color;
                S(i).LineStyle=thisLine.LineStyle;
                S(i).LineWidth=thisLine.LineWidth;
                S(i).DName=thisLine.DisplayName;
            end 
            

Save to a .MAT file

            try
                [file,folder]=uiputfile({'*.mat','Matlab Files'},'Save Data',app.startfolder);
            catch
                [file,folder]=uiputfile({'*.mat','Matlab Files'},'Save Data','C:\');
            end    
            app.startfolder=folder;
            savepath=fullfile(folder,file);      
            
            save(savepath,"-struct",'S')

But I keep getting this error:

            Error using save
            The argument to -STRUCT must be the name of a scalar structure variable.

I have checked the class of S and the fieldnames contained within it:

            fields = fieldnames(S)
            class(S)
            
            fields =            
            6×1 cell array            
            {'Xdata'    }
            {'Ydata'    }
            {'Colour'   }
            {'LineStyle'}
            {'LineWidth'}
            {'DName'    }
            
            
            ans =            
            'struct'

Thanks for any help

Answer 1

[Stephen23]

Explanation: your structure is not scalar: it has nlines elements. The -struct option only works with scalar structures.

Error using save
The argument to -STRUCT must be the name of a scalar structure variable.
%                                             ^^^^^^

Solutions:

• either get rid of the -struct option, or
• create a scalar structure (by making the fields arrays, not the structure).

Tip: always LOAD into an output variable!

[Jason]

Thanks Stephen, so for this option " create a scalar structure (by making the fields arrays, not the structure)", you mean

for i=1:nlines
    thisLine=h1(i);  
    S.Xdata(i)=thisLine.XData; S.Ydata(i)=thisLine.YData;
    S.Colour(i)=thisLine.Color;
    etc
end

Thanks for tip, but Im not sure I follow.

Thanks

Jason

[Steven Lord]

Since several of the properties you're trying to store in the struct are non-scalar, it won't work as you wrote it. But you could have the fields of the scalar struct contain cell arrays, where each cell in the array can have data of different sizes depending on the properties of the specific line you're trying to store inside it.

h = plot(1:10, 1:10, 'r-', 1:7, 10:-1:4, 'k:');

for k = 1:numel(h)
    S.XData{k} = h(k).XData;
    S.YData{k} = h(k).YData;
end
disp(S)

    XData: {[1 2 3 4 5 6 7 8 9 10]  [1 2 3 4 5 6 7]}
    YData: {[1 2 3 4 5 6 7 8 9 10]  [10 9 8 7 6 5 4]}

isscalar(S) % yes

ans = logical
   1

isstruct(S) % also yes

ans = logical
   1

cd(tempdir)
save('mydata.mat', '-struct', 'S')
whos -file mydata.mat

  Name       Size            Bytes  Class    Attributes

  XData      1x2               344  cell               
  YData      1x2               344  cell               

P = load('mydata.mat')

P = struct with fields:
    XData: {[1 2 3 4 5 6 7 8 9 10]  [1 2 3 4 5 6 7]}
    YData: {[1 2 3 4 5 6 7 8 9 10]  [10 9 8 7 6 5 4]}

isequal(P, S)

ans = logical
   1

[Jason]

Thats fantastic, thankyou!