[R] Matching Dates Closest without going over

[ampc]

Hi,

I have 2 date vectors d1 and d2. 

d1 <- structure(c(14526, 14495, 14464, 14433, 14402, 14371, 14340, 14309,
14278, 14247, 14216, 14185), class = "Date")
d2 <- structure(c(14526, 14509, 14488, 14466, 14453, 14441, 14396, 14388,
14343, 14333, 14310, 14281), class = "Date")

I would like to create another dataframe with columns d1 and d2, where d1 is
the original d1 and d2 is the max(d2) such that d1 > d2. And NA's where not
applicable.
(See desired result below)

Thanks,
Ampy



d1
2009-10-09
2009-09-08
2009-08-08
2009-07-08
2009-06-07
2009-05-07
2009-04-06
2009-03-06
2009-02-03
2009-01-03
2008-12-03
2008-11-02


d2
2009-10-09
2009-09-22
2009-09-01
2009-08-10
2009-07-28
2009-07-16
2009-06-01
2009-05-24
2009-04-09
2009-03-30
2009-03-07
2009-02-06

result:

d1                         d2
2009-10-09	     2009-10-09
2009-09-08	     2009-09-01
2009-08-08	     2009-07-28
2009-07-08	     2009-06-01
2009-06-07	     2009-06-01
2009-05-07	     2009-04-09
2009-04-06	     2009-03-30
2009-03-06	     2009-02-06
2009-02-03	     NA
2009-01-03	     NA
2008-12-03	     NA
2008-11-02	     NA

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