[R] subsetting data by specified observation number

[Greg Snow]

Here is one way:

> tmp1 <- data.frame(Species=c('setosa','virginica','versicolor'),
+ row=c(7,20,18) )
> 
> tmp.iris <- iris
> tmp.iris$row <- ave(iris$Sepal.Length, iris$Species, FUN=seq_along)
> 
> out.iris <- merge(tmp.iris, tmp1, by=c('Species','row'))
> 
> 
> out.iris
     Species row Sepal.Length Sepal.Width Petal.Length Petal.Width
1     setosa   7          4.6         3.4          1.4         0.3
2 versicolor  18          5.8         2.7          4.1         1.0
3  virginica  20          6.0         2.2          5.0         1.5
>

Another way would be to use the split function on your big data set, then use sapply to iterate over the list resulting and return just the rows from failed.3 in each group.  Need to think a bit more about how that would look.

You could also just loop through the rows of failed.3 and grab the corresponding pieces in the full dataset.

There are probably a few other ways as well.


-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Linh Tran
> Sent: Saturday, March 05, 2011 11:56 AM
> To: r-help at r-project.org
> Subject: [R] subsetting data by specified observation number
> 
> Hi members,
> 
> I'd like to thank you guys ahead of time for the help. I'm kind of
> stuck.
> 
> I have a data frame with ID and position numbers:
> 1> head(failed.3)
>          id position
> 1  10000997        2
> 4  1000RW_M        2
> 15 1006RW_G        2
> 24 1012RW_M        3
> 28 10160917        2
> 30 1016RW_M       13
> 
> I'd like to use this to subset out a large dataset and keep only the
> observation number corresponding to the position number. So for
> example,
> ID 10000997 has 10 observations. I want to keep the 2nd one only.
> 
> 
> Thanks,
> 
> -linh
> 
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