[R] sequential row selection in dataframe

[Michael Kubovy]

On Dec 26, 2006, at 12:07 AM, Pedro Mardones wrote:

> I'm wondering if there is any 'efficient' approach for selecting a
> sample of 'every nth rows'  from a dataframe. For example, let's use
> the dataframe GAGurine in MASS library:
>
>> length(GAGurine[,1])
> [1] 314
>
> # select an 75% of the dataset, i.e. = 236 rows, every 2 rows starting
> from row 1
>> test<-GAGurine[seq(1,314,2),]
>> length(test[,1])
> [1] 157
>
> # so, I still need another 79 rows, one way could be:
> test2<-GAGurine[-seq(1,314,2),]
>> length(test2[,1])
> [1] 157
>> test3<-test2[seq(1,157,2),]
>
> # and then
> final<-rbind(test2,test3)
>> length(final[,1])
> [1] 236
>
> Does anyone have a better idea to get the same results but without
> creating different datasets like test2 and test3?

A probabilistic approach:

len <- length(GAGurine[,1])
GAGu <- GAGurine[sample(1:len, round(.75 * len)), ] # 236 rows

A deterministic one:

nr <- 1 #or 2
GAGu2 <- GAGurine[-seq(nr, len, 4),] # drop every 4th, giving 235 rows
nr <- 3 # or 4
will give 236 rows.
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