Inconsistencies when extracting with non-integer numeric indices near zero
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 # Not so expected
c(1,2)[[1.5]] -->  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)]] -->  2 # As would be expected, though negative subscript for [[ is of course sketchy
c(1,2)[[-1.5]] -->  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] -->  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] -->  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.
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