Would ave() do what you want?
Rashid
On Tue, 8 Jul 2008, Daniela Ottaviani wrote:
> Dear All,
>
> I have a database of 200 observations named myD.
> In the dataframe there are a column named code (with codes varying from 1 to
> 77), a column named "prevalence" with some quantitative measurements are
> given and an column named Pr_mean, with no values.
>
> I would like to set a cycle to compute the average of prevalence values for
> each different code and store the averages under the empty field Pr_mean.
>
> This is what I wrote:
>
> # Set a cycle
> for (i in 1:nrow(myD)) {
> mycode = myD$code[i]
> mymean[i] = mean(prevalence)
> myD$Pr_mean[i] = mymean[i]
> }
>
> With the above cycle I am able to compute the average of all 200 observations
> which is then written in every cell.
> I understand that a condition is missing, that indicates that the average has
> to be computed amongst the observations showing the same codes values.
>
> Could you please help me ?
>
>
> D.
>
>
>
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