Question

I often use Octave to create data that I can plot from my lab results. That data is then fitted with some function in gnuplot:

f1(x) = a * exp(-x*g);
fit f1(x) "c_1.dat" using 1:2:3 via a,g

That creates a fit.log:

*******************************************************************************
Tue May  8 19:13:39 2012


FIT:    data read from "e_schwach.dat" using 1:2:3
        format = x:z:s
        #datapoints = 16
function used for fitting: schwach(x)
fitted parameters initialized with current variable values



 Iteration 0
 WSSR        : 12198.7           delta(WSSR)/WSSR   : 0
 delta(WSSR) : 0                 limit for stopping : 1e-05
 lambda   : 14.2423

initial set of free parameter values

mu2             = 1
omega2          = 1
Q2              = 1

After 70 iterations the fit converged.
final sum of squares of residuals : 46.0269
rel. change during last iteration : -2.66463e-06

degrees of freedom    (FIT_NDF)                        : 13
rms of residuals      (FIT_STDFIT) = sqrt(WSSR/ndf)    : 1.88163
variance of residuals (reduced chisquare) = WSSR/ndf   : 3.54053

Final set of parameters            Asymptotic Standard Error
=======================            ==========================

mu2             = 0.120774         +/- 0.003851     (3.188%)
omega2          = 0.531482         +/- 0.0006112    (0.115%)
Q2              = 17.6593          +/- 0.7416       (4.199%)


correlation matrix of the fit parameters:

               mu2    omega2 Q2     
mu2             1.000 
omega2         -0.139  1.000 
Q2             -0.915  0.117  1.000 

Is there some way to get the parameters and their error back into Octave? I mean I can write a Python program that parses that, but I hoped to avoid that.

Update

This question is not applicable to me any more, since I use Python and matplotlib for my lab work now, and it can does all this from a single program. I leave this question open in case somebody else has the same problem.

Answer 1

I don’t know much about the gnuplot-Octave interface, but what can make your (parsing) life easier is you can:

set fit errorvariables
fit a*x+g via a,g
set print "fit_parameters.txt"
print a,a_err
print g,g_err
set print

Now your variables and their respective errors are in the file “fit_parameters.txt” with
no parsing needed from python.

from the documentation on fit:

If gnuplot was built with this option, and you activated it using set
fit errorvariables, the error for each fitted parameter will be
stored in a variable named like the parameter, but with _err
appended. Thus the errors can be used as input for further
computations.