[R] array searches

[Murali Menon]

Folks,

I have a dataframe comprising a column of dates and a column of signals (-1, 
0, 1) that looks something like this:

30/01/2007	0
31/01/2007	-1
01/02/2007	-1
02/02/2007	-1
03/02/2007	1
04/02/2007	1
05/02/2007	1
06/02/2007	1
07/02/2007	1
08/02/2007	1
09/02/2007	0
10/02/2007	0
11/02/2007	0
12/02/2007	1
13/02/2007	1
14/02/2007	1
15/02/2007	0
16/02/2007	0

What I need to do is for each signal *in reverse chronological order* to 
find the date that it first appeared. So, for the zero on 16/02/2007 and 
15/02/2007, the 'inception' date would be 15/02/2007, because the day 
before, the signal was 1. Likewise, the 'inception' date for the signal 1 on 
08/02/2007 and the five days prior, would be 03/02/2007. I need to create a 
structure of inception dates that would finally look as follows:

-1	31/01/2007
-1	31/01/2007
-1	31/01/2007
1	03/02/2007
1	03/02/2007
1	03/02/2007
1	03/02/2007
1	03/02/2007
1	03/02/2007
0	09/02/2007
0	09/02/2007
0	09/02/2007
1	12/02/2007
1	12/02/2007
1	12/02/2007
0	15/02/2007
0	15/02/2007

Is there a clever way of doing this? My sadly C-oriented upbringing can only 
think in terms of for-loops.

Thanks!

Murali

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