[R] conditional selection of dataframe rows

[Toby Gass]

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