On Sun, Apr 3, 2011 at 11:05 PM, Jane Shevtsov <jane.eco@gmail.com> wrote:

> Thanks, but that doesn't quite work. Using your toy example gives an
> error about variable lengths.
>
> > z <- matrix(1:9,nrow=3,dimnames=list(1:3,4:6))
> > z.melt <- melt(z)
> > z.melt
>  X1 X2 value
> 1  1  4     1
> 2  2  4     2
> 3  3  4     3
> 4  1  5     4
> 5  2  5     5
> 6  3  5     6
> 7  1  6     7
> 8  2  6     8
> 9  3  6     9
> > lm(z ~ z.melt$X1 + z.melt$X2)
>

In this example the syntax for your model is:
> lm(z.melt$value ~ z.melt$X1 + z.melt$X2)

Call:
lm(formula = z.melt$value ~ z.melt$X1 + z.melt$X2)

Coefficients:
(Intercept)    z.melt$X1    z.melt$X2
        -12            1            3

Best,

Chris
-------

> Error in model.frame.default(formula = z ~ z.melt$X1 + z.melt$X2,
> drop.unused.levels = TRUE) :
>  variable lengths differ (found for 'z.melt$X1')
> I'm using R 2.12.0 on Windows and just installed the reshape package.
>
> Jane
>
> On Sun, Apr 3, 2011 at 3:40 PM, Ben Bolker <bbolker@gmail.com> wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> >
> > On 11-04-02 10:39 PM, Jane Shevtsov wrote:
> >> I have data in matrix form from a parameter scan of a food web
> >> simulation. (The matrix entry M[i,j] gives the variable of interest
> >> for a web size i and connectance j.) How would I find an equation
> >> expressing the variable of interest to web size and connectance? Can
> >> lm() do it?
> >>
> >> Thanks,
> >> Jane Shevtsov
> >>
> >
> >  suppose your size and connectance vectors were sizevec and connvec.
> > Then something like
> >
> > dimnames(M) <- list(sizevec,connvec)
> > library(reshape)
> > X <- melt(M)
> > lm(value~X1+X2)
> >
> >  will do it.
> >
> >  Mini example:
> >
> >> z <- matrix(1:9,nrow=3,dimnames=list(1:3,4:6))
> >> library(reshape)
> > Loading required package: plyr
> >
> > Attaching package: 'reshape'
> >
> > The following object(s) are masked from 'package:plyr':
> >
> >    round_any
> >
> >> str(melt(z))
> > 'data.frame':   9 obs. of  3 variables:
> >  $ X1   : int  1 2 3 1 2 3 1 2 3
> >  $ X2   : int  4 4 4 5 5 5 6 6 6
> >  $ value: int  1 2 3 4 5 6 7 8 9
> > -----BEGIN PGP SIGNATURE-----
> > Version: GnuPG v1.4.10 (GNU/Linux)
> > Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/
> >
> > iEYEARECAAYFAk2Y90MACgkQc5UpGjwzenOmPQCdFAOGcxnzADXZNv9ikgDzJcPQ
> > 3/IAn1ayn1HVOQOdlqFDPoq+Et6rR5nt
> > =cfIQ
> > -----END PGP SIGNATURE-----
> >
> > _______________________________________________
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> > R-sig-ecology@r-project.org
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> >
>
>
>
> --
> -------------
> Jane Shevtsov
> Ecology Ph.D. candidate, University of Georgia
> co-founder, <www.worldbeyondborders.org>
> Check out my blog, <http://perceivingwholes.blogspot.com>Perceiving Wholes
>
> "In the long run, education intended to produce a molecular
> geneticist, a systems ecologist, or an immunologist is inferior, both
> for the individual and for society, than that intended to produce a
> broadly educated person who has also written a dissertation." --John
> Janovy, Jr., "On Becoming a Biologist"
>
> _______________________________________________
> R-sig-ecology mailing list
> R-sig-ecology@r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
>



-- 
Christian Roy, Biol. Msc.
Étudiant au Doctorat / Phd Student
courriel: christian.roy.22@ulaval.ca <c_roy@ducks.ca>
Université Laval, Faculté de foresterie, de géographie et de géomatique

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