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Groups > comp.graphics.apps.gnuplot > #3666

Re: Using arrays

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John Edwards wrote:

> This may be a bit early, since the array functionality was only
> released so recently, but I've been trying to use arrays to replicate
> the functionality of plotting a multi-column file.
> 
> For example, if I have the following data.dat :
> 
> -90., -1.30e+01
> -60., -3.78e+01
> -30., -1.66e+01
> 0., -5.61e-03
> 30., -1.66e+01
> 60., -3.78e+01
> 90.,  -1.30e+01
> 
> And do plot "data.dat" then I get a graph we're all familiar with.
> 
> What I've come up with, to replicate using arrays, is :
> 
> array plt[7] = [ {-90., -1.30e+01}, {-60., -3.78e+01}, {-30.,
> -1.66e+01}, {0., -5.61e-03}, {30., -1.66e+01}, {60., -3.78e+01}, {90.,
> -1.30e+01} ]
> plot plt u 1:2
> 
> But it doesn't give me what I expected.
> 
> I would welcome any thoughts.
> 
> Thanks very much,
> John


Why are you storing complex numbers in the array?
Unless you do something fancy, gnuplot only plots the real component
of a complex value.  So your plot command amounts to 
   plot "data.dat" using 0:1
using the original data format. The second column of numbers
is never referenced.

If you really want to recover the original data after packing into a set
of complex values the plot command would have to be something like

   plot plt using (real($2)):(imag($2))

That doesn't work either (might be an actual bug lurking there) but at
least it tries to unpack the original 2 real values from a single
complex value. A variant that does work is

   plot sample [i=1:7:1] '+' using (real(plt[i])):(imag(plt[i]))

That variant is a tour-de-force of new syntax items but I don't see
anything gained over using the original data format.


Other thoughts that might or might not be relevant to what you want...

Using version 5.2 syntax I suppose you could create and plot
two parallel arrays:

  array plt_x[7] = [ 90., -60., -30., 0., 30., 60., 90. ]
  array plt_y[7] = [-13, -37.8, -16.6, -.00561, -16.6 -37.9, -13. ]

  plot sample [i=1:7:1] '+' using (plt_x[i]):(plt_y[i])

But why would you want to do this? 

The more natural representation would be a named data block,
not an array.

  $DATA << EOD
  -90., -1.30e+01
  -60., -3.78e+01
  -30., -1.66e+01
  0., -5.61e-03
  30., -1.66e+01
  60., -3.78e+01
  90.,  -1.30e+01
  EOD

  set datafile separator comma
  plot $DATA using 1:2

Can you explain what you are trying to accomplish by using an array
instead of a data block?   There are possible additions to the datablock
and array syntax that are kicking around as experimental patches because
it's not clear that they serve a real need.  If you have something in
mind that the current syntax can't handle I'd be interested to see if
one of these experimental extensions would.

	cheers,

		Ethan

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Thread

Using arrays John Edwards <johned0@gmail.com> - 2017-06-03 23:59 +0100
  Re: Using arrays Ethan A Merritt <EAMerritt@gmail.com> - 2017-06-03 23:22 -0700
    Re: Using arrays John Edwards <johned0@gmail.com> - 2017-06-04 09:30 +0100
    Re: Using arrays Ethan A Merritt <sfeam@users.sourceforge.net> - 2017-06-05 09:41 -0700

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