Re: Multi Column Data file to plot, gradient plot, or gradient of gradient plot

[Paul Kienzle]

On Mar 26, 2004, at 1:39 PM, Timothy Milstead wrote:

> I have a data file with x values in one column and corresponding y 
> values in
> the other columns of which there are one or more.
> One set of values per line.
> Values separated by comma (this can be changed if need be)

z=load('datafile');
x=z(:,1);
y=z(:,2:end);

> I am completely and utterly new to octave.
> I usually learn a new package by example, I'm not very good at 
> RTFM'ing.
> I would like to xy plot each column as a separate (coloured?) line and 
> then
> xy plot the gradient of the data as lines and then finally the 
> gradient of
> the gradient as xy plot.

subplot(311); plot(x,y);
subplot(312); plot(x(1:end-1),diff(y)./repmat(diff(x),1,columns(y)));
subplot(313); 
plot(x(2:end-1),diff(y,2)./repmat(diff(x(2:end)).^2,1,columns(y)));

This is the simple formula for a gradient,

        dy/dx ~= (y_i+1 - y_i)  /  (x_i+1 - x_i)
        d2y/dx2 ~= (y_i+1 - 2y_i - y_i-1) / (x_i+1 - x_i)^2

You can come up with more sophisticated approximations.



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