How to return empty vector of figures

Hi all,

I have a function draw_plots with an input vector plot_configs that should return a vector of figures. If the plot_configs vector is empty it should return an empty figure vector. But with the code underneath I get this error:

Output argument "figures" (and possibly others) not assigned a value in the execution with "draw_plots" function.

That makes perfect sense. So I tried to initialize figures with figures = Figure.empty but that doesn't work either:

Unable to resolve the name 'Figure.empty'.

So how to initialize/return an empty vector of figures? Thanks in advance!

Please note that we're not interested in showing the figures, just saving them to disk.

function figures = draw_plots(plot_configs, tsc, masks, n_rows, n_cols)

plot_configs = plot_configs([plot_configs.enable]); % filter enabled plot configs

color_order = [

0.0 0.0 0.8;

0.0 0.5 0.0;

0.8 0.0 0.0;

0.0 0.8 0.8;

0.8 0.0 0.8;

0.8 0.8 0.0;

0.2 0.2 0.2];

n_plots = length(plot_configs);

n_plots_per_page = n_rows * n_cols;

% figures = Figure.empty; <-- How I hoped to fix the error

for pc_idx = 1:n_plots

plot_idx = mod(pc_idx, n_plots_per_page);

if (plot_idx == 0) % Fill up figure with subplots until max number of plots per page

plot_idx = n_plots_per_page;

elseif (plot_idx == 1)

fig = figure('defaultAxesColorOrder', color_order, 'visible', 'off');

inner_w = 1200;

inner_h = 400 * n_rows;

fig.InnerPosition(3) = inner_w;

fig.InnerPosition(4) = inner_h;

fig_idx = ceil(pc_idx / n_plots_per_page);

figures(fig_idx) = fig;

end

sub = subplot(n_rows, n_cols, plot_idx);

hold on;

box on;

grid on;

pc = plot_configs(pc_idx);

mask = masks.get(pc.filter);

data_x = pc.signal_x.getter(tsc);

data_x = data_x(mask);

if (strcmp(pc.signal_x.generic_name, 'Time'))

data_x = datetime(data_x, 'ConvertFrom', 'datenum', 'Format', 'yyyy-MM-dd HH:mm:ss');

end

for signal_y = pc.signals_y

data_y = signal_y.getter(tsc);

data_y = data_y(mask);

plot = scatter(sub, data_x, data_y, 1, 'filled');

fontsize(plot, 8, "pixels")

end

title_str = pc.title;

xlabel_str = pc.signal_x.generic_name + ' [' + pc.unit_x + ']';

ylabel_str = pc.unit_y;

title(title_str, 'Interpreter', 'none');

xlabel(xlabel_str, 'Interpreter', 'none');

ylabel(ylabel_str, 'Interpreter', 'none');

end

end

[Garmit Pant]

Can you please provide the complete code to reproduce this and use the 'Insert Code' option to write your code in the comments?

[Erwin Werkhoven]

Ok I updated the code sample, but I'm not sure if the problem becomes more clear this way... The trick is that we want for instance 8 subplots in a figure, and then continue filling up the next figure.

[Bruno Luong]

May be you don''t need at all cost return an empty array of figure. You could initialized

figures = [];

in your function as default value and returns it when nplots == 0.

That may be sufficient to deal it outside.

Remember that matlab can deal figure handle array as array of figure numbers, which is standard numerical array. This is for compatibility with HG1.

Otherwise follow @Rik answer that create a dummy figure with 'visible' off, close it ad use the handle as template to create an empty figure array. This is a little cumbersome though for a small variable creation.

[Erwin Werkhoven]

Ok I did

if n_plots == 0

figures = [];

return

end

% other code

That seems to work. Thank you!

Just for the sake of curiosity, is there no Figure class since you cannot call figures = Figure.empty?

[Bruno Luong]

MATLAB figure class is

matlab.ui.Figure

[Bruno Luong]

You might even do this:

%if n_plots == 0
figures = []; % instead of figures = Figure.empty
%    return
%end

Answer 1

[Rik]

plot_configs=[];
figures = draw_plots(plot_configs)

figures = 
  1×0 empty Figure array.

plot_configs=ones(1,2);
figures = draw_plots(plot_configs)

figures = 
  1×2 Figure array:

    Figure    Figure

function figures = draw_plots(plot_configs, varargin)

n_plots=numel(plot_configs);

if n_plots==0
    figures=figure('visible', 'off');
    close(figures)
    figures(1)=[];
    return
end

for n=1:n_plots
    fig = figure(varargin{:}, 'visible', 'off');
    figures(n) = fig;
end
end

[Erwin Werkhoven]

Bruno Luong's suggestion is simpler than this one and seems to work too. Thanks for thinking along though!

Answer 2

[Steven Lord]

Preallocate the array using gobjects.

F = gobjects(1, 3)

F = 
  1×3 GraphicsPlaceholder array:

    GraphicsPlaceholder    GraphicsPlaceholder    GraphicsPlaceholder

F(2) = figure;
F

F = 
  1×3 graphics array:

    GraphicsPlaceholder    Figure                 GraphicsPlaceholder

To check if an element has been filled in, ask if it is a matlab.ui.Figure.

isa(F(3), 'matlab.ui.Figure') % false

ans = logical
   0

isa(F(2), 'matlab.ui.Figure') % true

ans = logical
   1

Or if they've all been filled in you can ask if the array as a whole is a matlab.ui.Figure.

F(3) = figure;
F(1) = figure;
F

F = 
  1×3 Figure array:

    Figure    Figure    Figure

isa(F, 'matlab.ui.Figure')

ans = logical
   1