yesterday I wrote,
> This is a Stata and Perl question.
In fact, much more Perl than Stata, but still a Stata problem. Apologies if
anyone deems this to be a misuse of the list.
> 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.
>
> <snip>
>
> 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.
I found the solution. The hex -> binary conversion wouldn't work yesterday
since by
syswrite DTA, pack("f", 0x0008007f);
the hex string got converted to decimal prior to being packed as a binary
IEEE single-precision float. The solution is to
syswrite DTA, pack("H*", "0000007f");
syswrite DTA, pack("H*", "0008007f");
syswrite DTA, pack("H*", "0010007f");
syswrite DTA, pack("H*", "00d0007f");
which properly maps missing values ., .a, .b, and .z to their binary values.
This conversion uses the Perl template character H which stands for
'hexadecimal string, high nybble first'. This surprises me a little since I
thought I was feeding a *LOHI* hex string. I thought the template character
'h' (low nybble first) would be the proper one.
At any rate, I presume that for portability, I should code
syswrite DTA, pack("h*", "7f000000");
syswrite DTA, pack("h*", "7f000800");
syswrite DTA, pack("h*", "7f001000");
syswrite DTA, pack("h*", "7f00d000");
for HILO system. Comments are welcome.
Patrick Joly
[email protected]
[email protected]
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