[datatable-help] For each row, aggregate an external vector based on a column value

[tjonesd289]

Hello,

Say I have data.table:  
>> dt <- data.table(a = c(as.Date('2014-01-01'),
>> as.Date('2014-06-01'),as.Date('2014-12-31')))  
and a vector of dates:  
>> my.dates <-
>> sapply(c('2014-02-21','2014-04-03','2014-10-21','2015-01-20'), as.Date)  
  
In dt, I want to create a column (b) with the number of items in my.dates
that are on or before the date given in dt$a. In the above example, the
result would be:  
>> print(dt$b)  
0 2 3  
   
The following two methods work outside a data.table:  
>> d1 = as.Date('2014-06-01')  
>> length(which(my.dates <= d1))   
2  
  
>> d2 = as.Date('2014-12-31')  
>> sum(my.dates <= d2)  
3  
  
However, I cannot get either of these to work inside of data.table.  
When I do:  
>> dt[, b:=sum((my.dates)<=a)] # or dt[, b:=length(which((my.dates)<=a))]  
Warning message:  
In `<.default`((my.dates), a) :  
  longer object length is not a multiple of shorter object length  
>> print(dt$b)  
2 2 2  
  
Can anyone advise an easy way to do this?  
  
I was thinking I might first have to move the vector into it's own column in
the data.table, but am not sure how I would even do that.  
  
Thanks!  
-TJ



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