[datatable-help] About roll in data.table

[statquant3]

What I thought about is the following,

whenever you use y[x,roll={-Inf,-a,a,Inf}] (say keyed by {t,u,v}) my
understanding is that data.table will

case 1) if there is a match on all keys do a regular join
case 2) if all but the last key match select the unique row if any such that
v of x is the  prevailing value of y is rolled iif it is within the bounds
defined by the roll argument.

What I though about is there might be several rows in y that might be within
the roll bounds. Because of this and because we have a mult parameter that
can be {first,last,all} why not return the first/last/all the rows of y that
where in the bounds ?

This is what I call a window join, it is useffull if, say you want to
calculate a moving average or a function over all the past elements in the
last 2minutes...
I wrote a SO post about it (with no answer...) 
http://stackoverflow.com/questions/17233973/is-it-possible-to-compute-any-window-join-in-data-table



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