R help - Jun 2009 - tapply with cbinded x

Dear List,

why does this not work?

df <- data.frame(var1 = c(3,2,1), var2 = c(6,5,4), var3 = c(9,8,7),
	fac = c('A', 'A', 'B'))
tapply(cbind(df$var1, df$var2, df$var3), df$fac, mean)

Thank you,
Stefan

Stefan Uhmann wrote:
> Dear List,
>
> why does this not work?
>
> df <- data.frame(var1 = c(3,2,1), var2 = c(6,5,4), var3 = c(9,8,7),
>     fac = c('A', 'A', 'B'))
> tapply(cbind(df$var1, df$var2, df$var3), df$fac, mean)
because

    length(cbind(df$var1, df$var2, df$var3))
    # 9

    length(df$fac)
    # 3

and that's enough for it not to work, as far as i understand what
?tapply says.

here's another question:  why this *does* work (or "work"):

    d = data.frame(a=1:3, b=1:3, c=1:3)
    f = factor(1:3)

    tapply(d, f, c)
    # no issues

although

    is.atomic(d)
    # FALSE

and ?tapply says:

    " X: an atomic object, typically a vector."

note the 'atomic'.

vQ

On Tue, Jun 16, 2009 at 5:16 AM, Stefan Uhmann
<stefan.uhmann@googlemail.com
> wrote:
> why does this not work?
>
> df <- data.frame(var1 = c(3,2,1), var2 = c(6,5,4), var3 = c(9,8,7),
>        fac = c('A', 'A', 'B'))
> tapply(cbind(df$var1, df$var2, df$var3), df$fac, mean)
>
Because tapply is defined for atomic vectors and not for data frames.  Why?
I don't know.

Does this do what you want?:
> df <- data.frame(var1 = c(3,2,1), var2 = c(6,5,4), var3 = c(9,8,7))
> fac <- c('a','a','b')
> do.call(rbind, lapply(split(df,fac),mean))
  var1 var2 var3
a  2.5  5.5  8.5
b  1.0  4.0  7.0

Alternatively, you can use sapply, which returns the result in matrix form.
> sapply(split(df,fac),mean)
       a b
var1 2.5 1
var2 5.5 4
var3 8.5 7
> as.data.frame(t(sapply(split(df,fac),mean)))
  var1 var2 var3
a  2.5  5.5  8.5
b  1.0  4.0  7.0

Note that sapply's matrix output form (the so-called
'simplification') needs
to be transposed.

             -s

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