[R] Question: Beginner stuck in a R cycle

[Greg Snow]

Try:

> myD <- transform(myD,  Pr_mean = ave(prevalence, codes))

See ?ave and ?transform for details.

--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
(801) 408-8111



> -----Original Message-----
> From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org] On Behalf Of Daniela Ottaviani
> Sent: Tuesday, July 08, 2008 7:18 AM
> To: r-help at r-project.org
> Subject: [R] Question: Beginner stuck in a R cycle
>
> 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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