This is a Stata and Perl question. I am generating a Perl module to read
Stata data files and I need some clarification with regard to the conversion
of floating-point numbers into decimal and hexadecimal values.
Specifically, I am able to convert values from binary to decimal (and
vice-versa) but I am having trouble with the hexadecimal to binary
conversion.
Lets take single-precision floats for instance. When I read 4-bytes of data
from a Stata .dta file which I know represent a missing value (i.e. the
4-bytes are represented by 00 00 00 7f in hexadecimal (on a LOHI system))
as,
read DTA, my ($val), 4;
$val = unpack("f", $val);
This results in the following mappings:
Stata value (bytes read) maps to (Perl variable $val)
------------------------ ----------------------------
. (0000007f) '1.70141183460469e+038'
.a (0008007f) '1.70182721835338e+038'
The reverse (decimal -> binary) conversion also works when I write a .dta
file thusly,
# de-reference object data
my $val = $self->{DATA}{"$vlist[$k]"}{CELLS}[$n];
syswrite DTA, pack("f", $val), 4;
So far, I am delighted, everything works as expected.
The puzzling thing is when I try to write a missing value by 'packing' it
from its hexadecimal representation, according to the following table
(pasted from -help .dta-)
V value HILO LOHI
---------------------------------------------------------------
<snip>
. +1.000000X+7f 7f000000 0000007f
.a +1.001000X+7f 7f000800 0008007f
.b +1.002000X+7f 7f001000 0010007f
.z +1.01a000X+7f 7f00d000 00d0007f
---------------------------------------------------------------
I don't get the expected results, e.g. if I try either
syswrite DTA, pack("f", 0x0008007f), 4;
or
syswrite DTA, pack("f", 0x7f000800), 4;
what gets written to DTA is F0070049 and 1000FE4E respectively, not the
0008007F I expected .
Would anyone happen to know why I am having trouble with the hex -> binary
conversion? Thanks in advance.
Patrick Joly
[email protected]
[email protected]
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