[R] Question: Beginner stuck in a R cycle

[(Ted Harding)]

On 08-Jul-08 13:18:13, 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.

I think something on the following lines would do what you want
(I think it is wise to call the final column "Pr.mean", as below,
rather than "Pr_mean"):

  for( Code in unique(myD$code) ){
    ix <- (myD$code == Code )
    myD$Pr.mean[ix] <- mean(myD$prevalence[ix])
  }

Ted.

> 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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Date: 08-Jul-08                                       Time: 17:01:38
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