[Rd] (PR#9811) sequence(c(2, 0, 3)) produces surprising results,

[bill at insightful.com]

On Fri, 27 Jul 2007, Prof Brian Ripley wrote:

> This is as doumented, and I think you could say the same thing of seq().
> BTW, sequence() allows negative inputs, and I don't think you want
> sum(input) in that case.

help(sequence) says contradictory things about
the nvec[i]==0 case:
   For each element of 'nvec' the
   sequence 'seq(nvec[i])' is created.  ...
and
   nvec: an integer vector each element
   of which specifies the upper bound ...

0 is not the upper bound of seq(0).

In any case, a suitably general multisequence function
would probably want vectors of both to's and from's.

merge.data.frame() requires a combination of a vectorized
sequence function and rep.  It uses a .Internal to do
the job well.  (This is a case where the individual sequences
typically have length one or zero.)

  > .Internal(merge(rep(1:3, c(3,0,5)), rep(1:4, c(2,2,3,2)), T, T))
  $xi
   [1] 1 1 2 2 3 3 4 4 4 5 5 5 6 6 6 7 7 7 8 8 8

  $yi
   [1] 1 2 1 2 1 2 5 6 7 5 6 7 5 6 7 5 6 7 5 6 7

  $x.alone
  integer(0)

  $y.alone
  integer(0)

sequence and rep produce complementary outputs, except in the nvec[i]==0 case.
  > rep(1:3, c(5,2,7)) # identifies group
   [1] 1 1 1 1 1 2 2 3 3 3 3 3 3 3
  > sequence(c(5,2,7)) # which in group
   [1] 1 2 3 4 5 1 2 1 2 3 4 5 6 7

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Bill Dunlap
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