[GRASS-dev] r.mapcalc operator question

[Glynn Clements]

Michael Barton wrote:

> In r.mapcalc docs there is an operator >>> for an unsigned right shift.
> 
> Is there a corresponding <<< that is not mentioned?

No. A left shift is neither signed nor unsigned; it always inserts
zeros into the least significant bits.

An unsigned right shift inserts zeros into the most significant bits;
a signed right shift inserts copies of the most signficant bit. This
ensures that shifting right by N places is equivalent to division by
2^N.

E.g. 0xFFFFFFF0 is either -16 or 4294967280, depending upon whether it
is signed or unsigned. If it's unsigned, division by 2 should yield
0x7FFFFFF8 = 2147483640; if it's signed, it should be 0xFFFFFFF8 = -8.

Technically, CELL is a signed type, with a range of -2147483647 to
2147483647 inclusive (-2147483648 is used for null). But r.mapcalc's
>>> operator allows you to treat it as an unsigned 32-bit integer with
a range of 0 to 4294967295 inclusive (excluding 2147483648, which is
null; maybe that should be changed?).

-- 
Glynn Clements <glynn at gclements.plus.com>