In r.mapcalc docs there is an operator >>> for an unsigned right shift.
Is there a corresponding <<< that is not mentioned?
MIchael
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@gclements.plus.com>
Thanks Glynn.
Michael
____________________
C. Michael Barton
Director, Center for Social Dynamics & Complexity
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University
voice: 480-965-6262 (SHESC), 480-727-9746 (CSDC)
fax: 480-965-7671 (SHESC), 480-727-0709 (CSDC)
www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu
On Aug 25, 2010, at 3:38 AM, Glynn Clements wrote:
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'soperator 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@gclements.plus.com>