Thanks a lot, finally It works! I was wondering why my for() did not work.
Now I now the difference between i-3 and (i-3)

Thanks again for all your help.

Marlene.

> x
    y         vr
1   2         NA
2   1         NA
3   5         NA
4   8   4.333333
5  11  12.333333
6   3   9.000000
7   1  16.333333
8   7  28.000000
9  50   9.333333
10 21 714.333333
11 33 481.000000
12  7 212.333333
13 60 169.333333


2009/9/30 Eik Vettorazzi <E.Vettorazzi@uke.uni-hamburg.de>

> Hi Marlene,
>
> x=data.frame(y,vr=NA)
> for(i in 4:nrow(x)) x[i,"vr"] =var(y[(i-3):(i-1)])
>
> will do the trick, solving both problems with overwriting subsequent
> x-values and obeying the precedence of R operators.
> For the latter, see ?Syntax and compare
> i<-5
> i-3:i-1
> (i-3):(i-1)
>
> hth.
>
> marlene marchena schrieb:
>
>> Dear R-user
>>
>> Suppose I have the following data
>>
>>  y=c(2,1,5,8,11,3,1,7,50,21,33,7,60)
>>
>> x=data.frame(y)
>>
>> for(i in 4:nrow(x)) x[i,] =var(x[i-3:i-1,])
>>
>> I'm trying to get a new variable with the variance of the 3 previous
>> values
>> (just an example) and with NA in the three first positions. I know that my
>> for() is wrong
>> but I'm not able to find my error.
>>
>> Any idea?
>>
>> Thanks,
>>
>> Marlene.
>>
>>        [[alternative HTML version deleted]]
>>
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>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>
> --
> Eik Vettorazzi
> Institut für Medizinische Biometrie und Epidemiologie
> Universitätsklinikum Hamburg-Eppendorf
>
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