On 14/04/2008, at 2:36 PM, zerfetzen wrote:
>
> Hi,
> I'm very new to R and absolutely love it. Does anyone know how to use
> something in R that functions like a BY command in SAS?
>
> For example, let's say you have a variable x, and you want to see
> the mean.
> Easy...
>
>> mean(x)
>
> But what if you want to see the mean of x conditional on another
> discrete
> variable? My best attempts so far are something like...
>
>> mean(x, y_cat=1)
>
> ...which of course doesn't work. I have downloaded plenty of R
> user guides
> that are very informative, but am not seeing much on detailed
> descriptives
> or data manipulation (for my life, I can't figure out how to sort an
> attached data frame, but that's another issue). Thanks.
> set.seed(42)
> x <- rnorm(100)
> y <- sample(letters[1:3],100,TRUE)
> by(x,y,mean)
INDICES: a
[1] 0.1089523
------------------------------------------------------------
INDICES: b
[1] -0.2253035
------------------------------------------------------------
INDICES: c
[1] 0.2997985
Or:
> tapply(x,y,mean)
a b c
0.1089523 -0.2253035 0.2997985
cheers,
Rolf Turner
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