Linear Model of Data in Matrix Form

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

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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
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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
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