Last modified: December 2020

URL: https://cxc.cfa.harvard.edu/sherpa/ahelp/link.html
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AHELP for CIAO 4.13 Sherpa v1

link

Context: modeling

Synopsis

Link a parameter to a value.

Syntax

link(par, val)

Description

A parameter can be linked to another parameter value, or function of that value, rather than be an independent value. As the linked-to values change, the parameter value will change.


Examples

Example 1

The fwhm parameter of the g2 model is set to be the same as the fwhm parameter of the g1 model.

>>> link(g2.fwhm, g1.fwhm)

Example 2

Fix the pos parameter of g2 to be 2.3 more than the pos parameter of the g1 model.

>>> gauss1d.g1
>>> gauss1d.g2
>>> g1.pos = 12.2
>>> link(g2.pos, g1.pos + 2.3)
>>> g2.pos.val
14.5
>>> g1.pos = 12.1
>>> g2.pos.val
14.399999999999999

PARAMETERS

The parameters for this function are:

Parameter Definition
par The parameter to link.
val The value - wihch can be a numeric value or a function of other model parameters, to set `par` to.

Notes

The link attribute of the parameter is set to match the mathematical expression used for `val` .

For a parameter value to be varied during a fit, it must be part of one of the source expressions involved in the fit. So, in the following, the src1.xpos parameter will not be varied because the src2 model - from which it takes its value - is not included in the source expression of any of the data sets being fit.

>>> set_source(1, gauss1d.src1)
>>> gauss1d.src2
>>> link(src1.xpos, src2.xpos)
>>> fit(1)

One way to work around this is to include the model but with zero signal: for example

>>> set_source(1, gauss1d.src1 + 0 * gauss1d.src2)

Bugs

See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.

See Also

contrib
renorm
data
get_bkg_chisqr_plot, get_bkg_delchi_plot, get_bkg_fit_plot, get_bkg_model_plot, get_bkg_plot, get_bkg_ratio_plot, get_bkg_resid_plot, get_bkg_source_plot
info
list_model_ids, show_bkg_model, show_bkg_source
modeling
add_model, add_user_pars, clean, create_model_component, delete_bkg_model, delete_model, delete_model_component, freeze, get_model, get_model_autoassign_func, get_model_component, get_model_component_image, get_model_component_plot, get_model_plot, get_num_par, get_num_par_frozen, get_num_par_thawed, get_order_plot, get_par, get_pileup_model, get_source, get_source_component_image, get_source_component_plot, get_source_contour, get_source_image, get_source_plot, get_xsabund, get_xscosmo, get_xsxsect, get_xsxset, guess, image_model, image_model_component, image_source, image_source_component, integrate, load_table_model, load_template_interpolator, load_template_model, load_user_model, normal_sample, paramprompt, reset, save_model, save_source, set_bkg_model, set_bkg_source, set_full_model, set_model, set_model_autoassign_func, set_par, set_pileup_model, set_source, set_xsabund, set_xscosmo, set_xsxsect, set_xsxset, t_sample, thaw, uniform_sample, unlink
plotting
get_cdf_plot, get_pdf_plot, get_pvalue_plot, get_pvalue_results, plot_cdf, plot_model, plot_model_component, plot_pdf, plot_pvalue, plot_scatter, plot_source, plot_source_component, plot_trace
psfs
delete_psf, load_conv
saving
save_delchi, save_resid
utilities
calc_chisqr, calc_energy_flux, calc_model_sum, calc_photon_flux, calc_source_sum, calc_stat, eqwidth
visualization
contour_model, contour_ratio, contour_resid