R help - Jan 2003 - problems with by()

Hello, another problem.
 > x<-rep(1,10)
 > y<-rep(c(1,2),c(5,5))
 > z<-seq(1:10)
 > ab<-data.frame(x,y,z)
#
    now I want to do some work by the value of 'y'
 > by(ab,y,mean)
y: 1
x y z
1 1 3
------------------------------------------------------------
y: 2
x y z
1 2 8
#
    I do not want all the means, only the mean of 'z'
 > by(ab,y,function(x) mean(z))
y: 1
[1] 5.5
------------------------------------------------------------
y: 2
[1] 5.5
 > by(ab,y,function(x) mean(z,data=x))
y: 1
[1] 5.5
------------------------------------------------------------
y: 2
[1] 5.5
 >
#
    so, how can I get the function(x) to be applied to each level
of the index variable y.
Actually I use my own function but the same happens, it is applied to all
the data and there is no partition of the data acording to index
Do not tell me that this version of R is completely buggy, I was waiting
for the 1.7 to be out before upgrading

R : Copyright 2002, The R Development Core Team
Version 1.6.1  (2002-11-01)

R.Heberto.Ghezzo Ph.D.
Meakins-Christie Labs
McGill University
Montreal - Canada

> x<-rep(1,10)
> y<-rep(c(1,2),c(5,5))
> z<-seq(1:10)
> ab<-data.frame(x,y,z)
> tapply(ab$z,ab$y,mean)
1 2 
3 8 


-------Original Message-------
From: Heberto Ghezzo <heberto.ghezzo at mcgill.ca>
Sent: 01/29/03 03:24 PM
To: r-help <r-help at stat.math.ethz.ch>
Subject: [R] problems with by()

so, how can I get the function(x) to be applied to each level
of the index variable y.

Actually I use my own function but the same happens, it is applied to all the
data and there is no partition of the data acording to index

On Wed, 29 Jan 2003, Heberto Ghezzo wrote:
> Hello, another problem.
>  > x<-rep(1,10)
>  > y<-rep(c(1,2),c(5,5))
>  > z<-seq(1:10)
>  > ab<-data.frame(x,y,z)
> #
>     now I want to do some work by the value of 'y'
>  > by(ab,y,mean)
> y: 1
> x y z
> 1 1 3
> ------------------------------------------------------------
> y: 2
> x y z
> 1 2 8
> #
>     I do not want all the means, only the mean of 'z'
>  > by(ab,y,function(x) mean(z))
> y: 1
> [1] 5.5
> ------------------------------------------------------------
> y: 2
> [1] 5.5
>  > by(ab,y,function(x) mean(z,data=x))
> y: 1
> [1] 5.5
> ------------------------------------------------------------
> y: 2
> [1] 5.5
>  >
> #
>     so, how can I get the function(x) to be applied to each level
> of the index variable y.
> Actually I use my own function but the same happens, it is applied to all
> the data and there is no partition of the data acording to index
The function you are applying is

	function(x) mean(z)

That is, no matter what x is supplied, it calculates the mean of the
variable z, which is in your global workspace. The mean of z is 5.5

What you want is
	function(x) mean(x$z)
That is, take a supplied data frame and compute the mean of its `z'
column.

I try to use argument names that remind me what is happening in functions
like by()

	by(ab,y, function(df) mean(df$z))
or even
	by(ab, y, function(subset) mean(subset$z))
> Do not tell me that this version of R is completely buggy, I was waiting
> for the 1.7 to be out before upgrading
I think it's fair to characterise this sort of commment as `unhelpful'.

	-thomas