link

sherpa.astro.ui.link(par, val)

Link a parameter to a value.

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.

Parameters:

• 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.

See also

freeze()

Fix model parameters so they are not changed by a fit.

set_par()

Set the value, limits, or behavior of a model parameter.

thaw()

Allow model parameters to be varied during a fit.

unlink()

Unlink a parameter value.

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)

Examples

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)

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