Add head, tail, and/or slice verbs about arquero HOT 6 CLOSED

It looks like these work fine, with respect for sorting and grouping ?
tb.filter(d => op.row_number() < 10)
tb.groupby('groupvar').filter(d => op.row_number() < 10)

but of course a more compact writing could prove convenient

from arquero.

It certainly works, but it's verbose and hard to keep straight for a pretty common operation in exploratory analysis particularly.
E.g., since row_number is one-indexed, that code tb.filter(d => op.row_number() < 10) actually returns the first 9 items, not the first 10! And tail operations aren't symmetric with respect to the index put in:tb.filter(d=>op.row_number() > op.count() - 10 does return 10.

from arquero.

yes indeed, the tail operation is more complex to write:

tb.params({nrows: naiss_tb.numRows()}).filter((d,$) => op.row_number() > $.nrows - 10)
or

let nrows = tb.numRows();
tb.filter(d => op.row_number() >= nrows - 10) ;

from arquero.

I'm open to adding a slice() method to tables that provides syntactic sugar over window based filtering (or something more performant for ungrouped tables), followed by a reify() call.

The most delicate aspects concern grouped tables. I'm assuming each group should be sliced separately. Still, the ordering of rows in the output would be solely determined by any orderby criteria, oblivious to groups.

Following JavaScript conventions, the method could have a table.slice(start[, stop]) signature, where negative values indicate an offset from the last row of the group. Output tables would retain the groupby criteria of the input.

Note that standard JavaScript does not support "inside out" specifications like the slice(-1, 0) described above. I'm unconvinced that Arquero should go down that route. An alternative would be to take two slices and concatenate them, or use a non-sugared windowed filter.

How does this sound?

from arquero.

I would be satisfied with a slice operator that follows JS slice syntax while respecting (and preserving) group_by. Selection of non-contiguous subsets is not very important and can be done inside filter as described above. tbl.slice(-5) for tail(5) works great.

from arquero.

Added slice() verb in v2.2.0.

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