[R] Selection on dataframe based on order of rows

[Bonfigli Sandro]

I have a dataframe with the following structure

id    date         value
-------------------------
1    22/08/2006     48
1    24/08/2006     50
1    28/08/2006     150
1    30/08/2006     100
1    01/09/2006     30
2    11/08/2006     30
2    22/08/2006     100
2    28/08/2006     11
2    02/09/2006     5
3    01/07/2006     3
3    01/08/2006     100
3    01/09/2006     100
4    22/08/2006     48
4    24/08/2006     50
4    28/08/2006     150
4    30/08/2006     100
4    01/09/2006     30
4    03/09/2006     100
4    06/09/2006     100


N.B.: dates in european format; ordered dataframe

For each ID I need to select the first occurrence of
all the rows which are the first of at least two with 
"value" >= 50.

Rather convoluted explication. I mean that for each id I have to select
the first row in which value is > 50 only if at least the following row 
has "value" > 50 too. If this is not true I repeat the test for all the 
following rows in which "value" > 50 untill I find a record that respects
the condition

this means that with my example dataframe the result is :
id    date         value
-------------------------
1    28/08/2006     150
3    01/08/2006     100
4    28/08/2006     150

It's clear that a for loop would work but I think that that is a better 
way.

I tried "by" and could obtain the first row for wich "value" is > 50.

I thought of an iterative process (delete the first row > 50, find the 
second row > 50, examine if there are rows in the middle) but it
is quite inelegant as if the first value is not the "good" one I have to 
repeat the process for a a priori unknown number of times.

Thanks in advance for Your help

  Sandro Bonfigli