[R] subset data frame with condition

[Petr Savicky]

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.