Set Ydata in Matlab, how does it work?

Hello, i came across this code.

plot_handle=plot(zeros(num_samples,2));

then in another function which is called update the plot.

i saw this.

data = getdata(daq_object,num_samples);
for i = 1:length(plot_handle)
  set(plot_handle(i),'YData',data(:,i));
end

I don't get how the set(plot_handle(i),'YData',data(:,i)); works, beacuse in the plot_handle function, i only have num_samples as input.

Answer 1

[Oleg Komarov]

I think it's a real time update (since it ivolves a daq object) of your plot.

A general example is:

h = plot(rand(10,1))

for n = 1:20
    set(h,'Ydata',rand(10,1))
    pause(.1)
end

[rave]

i tired to expand this to, more variables, since i will be doing a spectrogram function and surf.

i tried:

h=plot(rand(10,3),rand(10,1));

for n=1:20

set(h,'Ydata',rand(10,1),rand(10,4));

end

but getting errors

[Oleg Komarov]

Your 3 lines have the same Ydata but only the Xdata changes.

So it's again set(h,'Ydata',rand(10,1)); in the loop.

[Oleg Komarov]

Or if you wanna give it 3 different Ydatas:

set(h,{'Ydata'},num2cell(rand(3,10),2));

Answer 2

[Fangjun Jiang]

Plot a figure:

h=plot(1:10);

type get(h) to see a list of properties. You can get the data back by:

y=get(h,'YData');

You can also change the figure by:

set(h,'YData',rand(1,10);

[rave]

i tired to expand this to, more variables, since i will be doing a spectrogram function and surf.

i tried:

h=plot(rand(10,3),rand(10,1));

for n=1:20

set(h,'Ydata',rand(10,1),rand(10,4));

end

but getting errors

[Fangjun Jiang]

In your code, h=plot(rand(10,3),rand(10,1)) returns 3 handles indicating 3 curves. When you try to set(h), you need to provide proper data. Check out the other property such 'XData','ZData', 'CData'(for image). You'll get the idea.

Answer 3

[Walter Roberson]

When the Y value is not a vector, plot() will produce one curve for each column of Y. Your initial Y has two columns (unless numsamples is only 1), so plot() will produce two lineseries objects.

If you have more than one channel in your daq_object, then if you request to read numsamples samples, you are requesting to read that many per channel.

The rest of the code is then appropriate to update the respective plot lines according to the data received for each channel.

[rave]

what do you mean by "you are requesting to read that many per channel", i am hoping to read in 2 channels in the daq_object.