Check if handle to axes exists at all [R2017b]

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TJ 2018-1-17

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评论： TJ 2018-1-18

采纳的回答： Jan

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Hi,

I am trying to check if an axes handle exists and if not create it, e.g.:

X = [1:100];
Y=sin(X);
if handle exists
a=line(h, X, Y);
else 
f=figure();
h=axes(figure);
end

In the case where it hasn't been created I keep getting an error 'Undefined function or variable' when I use ishandle or any get statement.

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Jan 2018-1-17

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编辑：Jan 2018-1-17

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The code is not meaningful. You can always know, if the axes has been created before or not. Therefore it is not clear, what you are exactly asking for. Maybe this helps:

f = gobjects(0);
h = gobjects(0);
% Then a magic section, which does not let you know if the axes is
% existing or not:
if rand > 0.5
  f = figure;
  h = axes;  % 
end
...
if ~isgraphics(h)   % Better than ishandle
  f = figure;
  h = axes;  % 
end
line(x, 1:10, rand(1, 10))

So define the variable holding the handle as an empty graphics object at first. The empty matrix would work also. Then you can check if the variable contains a valid graphics object.

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Jan 2018-1-17

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In both scenarios using the method I've suggested works well. Example:

FigH  = figure;
axes;
LineH = gobjects(0);
for k = 1:1000
  if isgraphics(h)  % of ishghandle(h)
    if isgraphics(LineH)
      LineH = plot(1:10, rand(1, 10));
    else
      set(LineH, 'YData', rand(1, 10));
      pause(1);
    end
  else
    disp('User has closed the figure');
    break;
  end
end

Here isgraphics is used to check, if the figure is still existing and if the line object has been created before, to allow a cheaper setting of YData instead of a re-creation.

Does this help?

TJ 2018-1-18