[R] aggregate data.frame based on column class

[Ista Zahn]

Please see in line.

On Fri, Jan 11, 2013 at 10:07 AM, Martin Batholdy
<batholdy at googlemail.com> wrote:
> Hi,
>
> When using the aggregate function to aggregate a data.frame by one or more grouping variables I often have the problem, that I want the mean for some numeric variables but the unique value for factor variables.
>
> So for example in this data-frame:
>
> data <- data.frame(x = rnorm(10,1,2), group = c(rep(1,5), rep(2,5)), gender =c(rep('m',5), rep('f',5)))
> aggregate(data, by=list(data$group), FUN=mean)
>
>
> I would like to have 'm' and 'f' in the third column, not NA.
>
>
> I see the problem, that it could happen that there is no unique factor level in a group –
> but is there an alternative function who at least tries what I am aiming at?
>
> That is;
>
> "aggregate the data.frame by a list of grouping variables,
> for numeric variables compute the mean,
> for factor variables return the unique factor value"

R is a language, so you just have to do the translation:

mt <- function(x) {
  if(is.numeric(x)) { # if x is numeric
    return(mean(x)) # compute the mean
  } else { # otherwise
    tab <- table(x) # tabulate x
    return(paste(paste(names(tab), # and format it for display
                       tab, sep=": "),
                 collapse=", "))
  }
}

aggregate(Dat, by=list(Dat$group), FUN=mt)

Best,
Ista
>
>
> Thanks!
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