[R] manipulating "by" lists and "ave()" functions

[William Dunlap]

Q1.  simplify2array(b) gives the transpose of what
     I think you want.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com  

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of ivo welch
> Sent: Friday, July 08, 2011 3:43 PM
> To: r-help
> Subject: [R] manipulating "by" lists and "ave()" functions
> 
> dear R wizards---more ignorance on my part, exacerbated by too few
> examples in the function documentations.
> 
> > d <- data.frame( id=rep(1:3,3), x=rnorm(9), y=rnorm(9))
> 
> Question 1: how do I work with the output of "by"?  for example,
> 
> > b <- by( d, d$id, function(x) coef(lm( y ~ x, data=x ) ))
> > b
> 
> d$id: 1
> (Intercept)           x
>      0.2303      0.3618
> --------------------------------------------------------------
> -------------------------------------
> d$id: 2
> (Intercept)           x
>     0.05785    -0.40617
> --------------------------------------------------------------
> -------------------------------------
> d$id: 3
> (Intercept)           x
>       0.269      -0.378
> 
> getting the categories is easy:
> 
> >  names(b)
>  [1] "1" "2" "3"
> 
> but how do I transform the non-name info in this by() data structure
> into a matrix with dimensions 3 by 2?  (presumably, there is some
> vector operator that can do this kind of magic.)
> 
> 
> Question 2:  Let's say I want to add only one of the two coefficients
> to d.  The naive approach
>   a <- ave( d, d$id, FUN=function(x) coef(lm( y ~ x, data=x ))[2] )
> gives me the right coefficient in each row, but overwrites every
> entry.  I guess I can keep only the first column of a, and add it to
> d, but this seems a rather ugly and inefficient way.  How is this done
> better?
> 
> Question 3: repeat question 2, but keep both the intercept 
> and the slope.
> 
> 
> thanks in advance, as always, for any advice.
> 
> /iaw
> ----
> Ivo Welch (ivo.welch at gmail.com)
> 
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