R help - Jul 2014 - how to subset based on other row values and multiplicity

Hi R experts,
 
I have a dataset as sampled below. Values are only regarded as Œconfirmed¹
in an individual (Œid¹) if they occur
more than once at least 30 days apart.

 
id   date value
a    2000-01-01 x
a    2000-03-01 x
b    2000-11-11 w
c    2000-11-11 y
c    2000-10-01 y
c    2000-09-10 y
c    2000-12-12 z
c    2000-10-11 z
d    2000-11-11 w
d    2000-11-10 w

 
I wish to subset the data to retain rows where the value for the
individual is confirmed more than 30 days apart. So, after deleting all
rows with just one occurrence of id and value, the rest would be the
earliest occurrence of each value in each case id, provided 31 or more
days exist between the dates. If >1 value is present per id, each value
level needs to be assessed independently. This example would then reduce
to:

 
id   date           value
a    2000-01-01 x
c    2000-09-10 y
c    2000-10-11 z

 
 
I can do this via some crude loops and subsetting, but I am looking for as
much efficiency as possible
as the dataset has around 50 million rows to assess. Any suggestions
welcomed.

Thanks in advance
 
Scott Williams MD
Melbourne, Australia



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