[R] conditional selection of dataframe rows

[Marc Schwartz]

On Aug 12, 2010, at 2:11 PM, Toby Gass wrote:

> Dear helpeRs,
> 
> I have a dataframe (14947 x 27) containing measurements collected 
> every 5 seconds at several different sampling locations.  If one 
> measurement at a given location is less than zero on a given day, I 
> would like to delete all measurements from that location on that day.
> 
> Here is a toy example:
> 
> toy <- data.frame(CH = rep(3:5,3), DAY = c(rep(4,5), rep(5,4)), 
> SLOPE = c(seq(0.2,0.6, .1),seq(0.2, -0.1, -0.1)))
> 
> In this example, row 9 has a negative measurement for Chamber 5, so I 
> would like to delete row 6, which is the same Chamber on the same 
> day, but not row 3, which is the same chamber on a different day.  In 
> the full dataframe, there are, of course, many more days.
> 
> Is there a handy R way to do this?
> 
> Thank you for the assistance.
> 
> Toby



Not fully tested, but here is one possibility:

> toy
  CH DAY SLOPE
1  3   4   0.2
2  4   4   0.3
3  5   4   0.4
4  3   4   0.5
5  4   4   0.6
6  5   5   0.2
7  3   5   0.1
8  4   5   0.0
9  5   5  -0.1


> subset(toy, ave(SLOPE, CH, DAY, FUN = function(x) any(x < 0)) == 0)
  CH DAY SLOPE
1  3   4   0.2
2  4   4   0.3
3  5   4   0.4
4  3   4   0.5
5  4   4   0.6
7  3   5   0.1
8  4   5   0.0


See ?ave and ?subset


HTH,

Marc Schwartz