| Re: multiple non-linear regression analysis [message #24685 is a reply to message #24684] |
Tue, 17 April 2001 14:39   |
thompson
Messages: 584
Registered: August 1991
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Senior Member |
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"Kenlo Nishida" <kenlo@ntsg.umt.edu> writes:
> Dear IDL news group:
> How can I make multiple non-linear regression analysis with IDL?
> I know "LMFIT" command can make a non-linear regression analysis
> for a single independent variable. However, I want to know
> an appropriate command or function of IDL which provide me with
> a fitting of an arbitrary non-linear function with two or more
> independent variables. I mean, I want to determine the following
> three parameters (a, b, c):
> y=f(x1, x2, x3; a, b, c)
> Here x1, x2, and x3 are arrays of independent variables each
> containing n data. y is an array of dependent variable with
> n data. a, b, and c are scalars (parameters) which determine
> the non-linear function f(x1, x2, x3).
The simplest way to do this is to define your function so that the input array
is a structure. For example, suppose that x1, x2, x3 are all floating point
arrays of size N. You could then define your structure via
s = {x1: 0.0, x2: 0.0, x3: 0.0}
s = replicate(s, n_elements(x1))
s.x1 = x1
s.x2 = x2
s.x3 = x3
Thus, s is a structure array of N elements, and an individual element s(i)
contains x1(i), x2(i), and x3(i). For example
IDL> help,s
S STRUCT = -> <Anonymous> Array[100]
IDL> help,s(0),/str
** Structure <403eeb48>, 3 tags, length=12, refs=2:
X1 FLOAT -0.923884
X2 FLOAT 0.192019
X3 FLOAT -0.277066
You could then write your function along the lines of
function myfunc, s, a
return, a[0] + a[1]*s.x1 + a[2]*s.x2 + a[3]*s.x3
end
You can then put this function into a fitting routine in the normal manner.
(With appropriate changes for whatever peculiarities a particular fitting
routine may require. For example, LMFIT wants the partial derivatives returned
as extra dimensions in the result.)
Bill Thompson
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