Re: [R] subset data frame with condition

From: Petr Savicky <savicky_at_praha1.ff.cuni.cz>
Date: Fri, 18 Mar 2011 20:16:00 +0100

On Fri, Mar 18, 2011 at 10:48:44AM -0700, Nicolas Gutierrez wrote:
> Hello,
>
> One more question.. I have the data.frame "pop":
>
> xloc yloc gonad ind Ene W Area
> 1 23 20 516.74 1 0.02 20.21 1
> 2 23 20 1143.20 1 0.02 20.21 1
> 3 23 20 250.00 1 0.02 20.21 1
> 4 22 15 251.98 1 0.02 18.69 2
> 5 22 15 598.08 1 0.02 18.69 2
> 6 21 19 250.00 1 0.02 20.21 3
> 7 22 20 251.98 1 0.02 18.69 4
> 8 22 20 598.08 1 0.02 18.69 4
>
> and I need to extract 50% (or rounded) of the rows for each Area (from
> Area 1 to 3 only):
>
> xloc yloc gonad ind Ene W Area
> 1 23 20 516.74 1 0.02 20.21 1
> 2 23 20 1143.20 1 0.02 20.21 1
> 4 22 15 251.98 1 0.02 18.69 2
> 6 21 19 250.00 1 0.02 20.21 3

Let me suggest one more solution, which is a modification of the solution by Henrique Dallazuanna

  pop <- data.frame(Area=c(1,1,1,1,1,2,2,2,2,3,3,3,4,4))   g <- function(x) { seq(along=x) <= ceiling(length(x)/2) }   subset(pop, ave(Area, Area, FUN=g) & Area %in% 1:3)

     Area
  1     1
  2     1
  3     1
  6     2
  7     2

  10 3
  11 3

Hope this helps.

Petr Savicky.



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