How to establish if the dates in a column are unique?

Question

I have a simple question to ask. I couldn't find a solution on SO.

I have a column vector of dates and I would like to see whether months are unique or not. I tried to use unique but I may have used it in the wrong way.

An example:

Date = structure(c(11690, 11725, 11753, 11781, 11809, 11844, 11872, 
11900, 11942, 11970, 11998, 12026, 12061, 12089, 12117, 12145, 
12180, 12208, 12243, 12264, 12265, 12299, 12327, 12362, 12390, 
12425, 12453, 12481, 12509, 12544, 12572, 12600, 12635, 12663, 
12698, 12726, 12754, 12796, 12817, 12845, 12880, 12907, 12936, 
12971, 12999, 13027, 13062, 13090, 13118, 13160, 13181, 13209, 
13244, 13272, 13307, 13335, 13363, 13392, 13426, 13454, 13489, 
13524, 13552, 13580, 13615, 13643, 13670, 13699, 13727, 13762, 
13790, 13825, 13853, 13888, 13916, 13944, 13979, 14007, 14035, 
14063, 14098, 14126, 14154, 14160, 14189, 14217, 14259, 14280, 
14308, 14336, 14371, 14399, 14427, 14462, 14490, 14525, 14553, 
14581, 14623, 14644, 14672, 14707, 14735, 14770, 14798, 14826, 
14854, 14889, 14917, 14945, 14987, 15008, 15036, 15071, 15099, 
15134, 15162, 15190, 15225, 15253, 15281, 15316, 15351, 15379, 
15407, 15434, 15463, 15497, 15526, 15554, 15589, 15617, 15652, 
15680, 15715, 15743, 15771, 15799, 15827, 15862, 15890, 15918, 
15953, 15980, 16016, 16044, 16079, 16107, 16135, 16163, 16198, 
16226, 16254, 16289, 16317, 16345, 16380, 16408, 16457, 16467, 
16499, 16540, 16556, 16589, 16632, 16648, 16681, 16730, 16740, 
16772, 16821, 16832, 16870, 16912, 16922, 16954, 17003, 17014, 
17052, 17094, 17106, 17143, 17185, 17198, 17234, 17283, 17287, 
17325, 17367, 17379, 17416, 17465, 17471, 17514, 17556, 17563, 
17598, 17647, 17652, 17696, 17738, 17744, 17787, 17829, 17836, 
17878, 17920, 17928, 17962, 17996, 18017, 18053, 18102, 18109, 
18151, 18193, 18201, 18242), class = "Date")

In this column vector I would like to see whether there are two observations in the same month (there are 2 for "2003-07" and "2008-10"). I can I check it with one line of command?

Thanks!

Answer 1

In base R, we can format the Date to get only year and month, use table to count their occurrences, Filter to select only those month-year which occur more than once.

names(Filter(function(x) x > 1, table(format(Date, "%Y-%m"))))
#[1] "2003-07" "2008-10"

Same logic using zoo::as.yearmon.

names(Filter(function(x) x > 1, table(zoo::as.yearmon(Date))))
#[1] "Jul 2003" "Oct 2008"