[R] R newbie: logical subsets

[Gabor Grothendieck]

Try this, using the built in anscombe data set:

anscombe[!rowSums(abs(scale(anscombe)) > 2),]



On 7/11/06, Joshua Tokle <jtokle at math.washington.edu> wrote:
> Hello!  I'm a newcomer to R hoping to replace some convoluted database
> code with an R script.  Unfortunately, I haven't been able to figure out
> how to implement the following logic.
>
> Essentially, we have a database of transactions that are coded with a
> geographic locale and a type.  These are being loaded into a data.frame
> with named variables city, type, and price.  E.g., trans$city and all
> that.
>
> We want to calculate mean prices by city and type, AFTER excluding
> outliers.  That is, we want to calculate the mean price in 3 steps:
>
> 1. calculate a mean and standard deviation by city and type over all
> transactions
> 2. create a subset of the original data frame, excluding transactions that
> differ from the relevant mean by more than 2 standard deviations
> 3. calculate a final mean by city and type based on this subset.
>
> I'm stuck on step 2.  I would like to do something like the following:
>
> fs <- list(factor(trans$city), factor(trans$type))
> means <- tapply(trans$price, fs, mean)
> stdevs <- tapply(trans$price, fs, sd)
>
> filter <- abs(trans$price - means[trans$city, trans$type]) <
>             2*stdevs[trans$city, trans$type]
>
> sub <- subset(trans, filter)
>
> The above code doesn't work.  What's the correct way to do this?
>
> Thanks,
> Josh
>
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