[R] Question: Beginner stuck in a R cycle

From: Daniela Ottaviani <d.ottaviani_at_yahoo.it>
Date: Tue, 08 Jul 2008 13:18:13 +0000 (GMT)


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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