R devel - Jul 2018 - Inconsistencies when extracting with non-integer numeric indices near zero

Dear R-devel,

When I was playing around with different kind of indices when subsetting I
noticed some unexpected behaviours when using non-integer numeric indices,
especially near zero.
From the docs: ?Numeric values are coerced to integer as by
as.integer<http://127.0.0.1:14277/help/library/base/help/as.integer> (and
hence truncated towards zero).?
But some behaviour differs from that, and the behaviour also differs between [
and [[ :
c(1,2)[as.integer(.5)]               --> numeric(0) # As expected
c(1,2)[.5]                                 --> numeric(0) # As expected
c(1,2)[[as.integer(.5)]]            --> Error in c(1, 2)[[as.integer(0.5)]] :
attempt to select less than one element in integerOneIndex # Also as expected
c(1,2)[[.5]]                               --> [1] 1            # Not so
expected
c(1,2)[[1.5]]                             --> [1] 1            # As expected,
but this also means somevector[[n]] and somevector[[n+1]] give back the same
element for 0<n<1
c(1,2)[[as.integer(-1.5)]]         --> [1] 2            # As would be
expected, though negative subscript for [[ is of course sketchy
c(1,2)[[-1.5]]                            --> [1] 1            # But coerced
to -2 is the last thing I?d expect
c(1,2)[as.integer(-.5)]             --> numeric(0) # As expected
c(1,2)[-.5]                                --> [1] 2            # Coerced to
-1?, this also means that length(union(c(1,2)[-n], c(1,2)[n])) != 2 for
-1<n<1
c(1,2)[-1.5]                              --> [1] 2            # Again as
expected, but same problem as before: indexing with n and n+1 can give same
element back.

I suspect most of this behaviour is due to the case of special treatment of
zero, where first 0-indices are dropped, and only then the casting to integer is
done, and when that returns zero some unforeseen behaviour occurs.
Along with using negative indices when extracting with [[, which in any case
only succeeds with length-2 vectors (We need a length-one index resulting in the
return of a single element). For the last case I?d think we?d do best in
throwing an error whenever negative indices are used with [[, but for other
cases I think we need to change the underlying code, or at the very least update
documentation.
Any thoughts?

Best regards,
Emil Bode

Data-analyst

+31 6 43 83 89 33
emil.bode at dans.knaw.nl<mailto:emil.bode at dans.knaw.nl>

DANS: Netherlands Institute for Permanent Access to Digital Research Resources
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