Hi Daniela,
There may be other more elegant ways of doing it, but here is one:
> myD <- data.frame(code = sample(3, 10, rep = T), prev = rnorm(10), Pr_mean =
> 0)
> myD
code prev Pr_mean
1 3 -0.06710968 0
2 2 -1.43422034 0
3 1 0.22717580 0
4 3 0.32703754 0
5 3 1.26254159 0
6 2 0.65104107 0
7 1 -0.74293152 0
8 3 0.45845330 0
9 2 -0.64206400 0
10 3 -0.48671646 0
> m <- tapply(myD$prev, myD$code, mean)
> myD$Pr_mean <- m[match(myD$code, names(m))]
> myD
code prev Pr_mean
1 3 -0.06710968 0.2988413
2 2 -1.43422034 -0.4750811
3 1 0.22717580 -0.2578779
4 3 0.32703754 0.2988413
5 3 1.26254159 0.2988413
6 2 0.65104107 -0.4750811
7 1 -0.74293152 -0.2578779
8 3 0.45845330 0.2988413
9 2 -0.64206400 -0.4750811
10 3 -0.48671646 0.2988413
Hope this helps.
Ciao,
Giovanni
> Date: Tue, 08 Jul 2008 13:18:13 +0000 (GMT)
> From: Daniela Ottaviani <[EMAIL PROTECTED]>
> Sender: [EMAIL PROTECTED]
> Precedence: list
> DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=yahoo.it;
>
> 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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>
--
Giovanni Petris <[EMAIL PROTECTED]>
Department of Mathematical Sciences
University of Arkansas - Fayetteville, AR 72701
Ph: (479) 575-6324, 575-8630 (fax)
http://definetti.uark.edu/~gpetris/
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