[R] How to get minimum value by group

[David Winsemius]

On Jan 11, 2010, at 7:58 PM, JustinNabble wrote:

>
> I'd like to get a long data set of minimum values from groups in  
> another data
> set.
> The following almost does what I want. (Note, I'm using the word  
> factor
> differently from it's meaning in R; bad choice of words)
>
> myframe = data.frame(factor1 = rep(1:2,each=8), factor2 =
> rep(c("a","b"),each=4, times=2), factor3 = rep(c("x","y"),each=2,  
> times=4),
> y=1:16)
> attach(myframe)

# with(myframe,   ....) would be a better construction
> minimums = by(y, list(factor1, factor2,factor3), min)
> detach(myframe)

It's a table, which IS like an array, You want:

 > as.data.frame.table(minimums)
   Var1 Var2 Var3 Freq
1    1    a    x    1
2    2    a    x    9
3    1    b    x    5
4    2    b    x   13
5    1    a    y    3
6    2    a    y   11
7    1    b    y    7
8    2    b    y   15

>
> The problem is that "minimums" is object of class "by", which looks  
> like
> some kind of array with the number of dimension equal to the number of
> factors. I just want two dimensions though, with factors in columns.  
> I can't
> figure out how to reformat it to something like this:
>
> factor1 factor2 factor2 y
> 1         a         x         1
> 2         a         x         9
> 1         b         x         5
> ...
>
> I could make nested for loops, but I'd like something that will work  
> for an
> arbitrary number of factors. I've seen some functions that will  
> rearrange
> data (e.g. reshape), but they all seem to require a data.frame to  
> start
> with.
> -- 
-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT