[R] "sequeeze" a data frame

[array chip]

Thank you David. I probably didn't make my question clear enough. In the end, I 
want a "shorter" version of the original data frame, basically with variable 
"rep" removed, then just one row per id, time and mode combination. The original 
data frame has 54 rows, I wish to have a data frame with only 27 rows after 
average.

Using ave() would produce the following 3 rows for example:
      id    time    mode   rep            y                 avg.y
1     1      0          R      1    -1.089540576    -0.21924078
19   1      0          R      2     -0.743374679   -0.21924078
37   1      0          R      3      1.175192908   -0.21924078

But I only need one row:

     id      time      mode            avg.y
1    1        0           R        -0.21924078

Thanks again,

John



----- Original Message ----
From: David Winsemius <dwinsemius at comcast.net>
To: array chip <arrayprofile at yahoo.com>
Cc: r-help at r-project.org
Sent: Thu, September 9, 2010 3:21:15 PM
Subject: Re: [R] "sequeeze" a data frame


On Sep 9, 2010, at 5:47 PM, array chip wrote:

> Hi, suppose I have a data frame as below:
> 
>dat<-cbind(expand.grid(id=c(1,2,3),time=c(0,3,6),mode=c('R','L'),rep=(1:3)),y=rnorm(54))
>)
> 
> 
> I kind of want to "squeeze" the data frame into a new one with averaged "y" 
>over
> "rep" for the same id, time and mode. taking average is easy with tapply:
> 
> tapply(dat$y, list(dat$id, dat$time, dat$mode), mean)

Try:

dat$avg.y <- ave(dat$y, dat$id, dat$time, dat$mode, FUN=mean)

?ave

The syntax of ave is different than "tapply" or "by". The grouping factors are 
not presented as a list and the FUN argument comes after the ,..., so it needs 
to be named if different than the default of "mean".

> 
> But I want the result to be in the same format as "dat". Certainly, we always
> can transform the result (array) into the data frame using a lot of codes. But
> is there a simple way to do this?
> 
> Thanks

David Winsemius, MD
West Hartford, CT