Need help with pack
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From: kj on 1 Oct 2005 19:00 I want to write the function to_bits that takes a floating point number as argument and returns an array of 32 ones or zeroes, corresponding to the bits of the input argument. I have been staring at the docs for pack and unpack for a while now trying to figure out how to use them to do this and I am as clueless about it as I was when I first started. Also, perlpacktut was no help either [1]. If someone could please tell me how to generate an array of one's and zeros corresponding to the bits in a floating point number I'd be very grateful. kj [1] Lest I appear very lazy or very stupid, I should point out that I generally have *no problem* understanding the documentation for Perl built-in functions. pack is a very exceptional case. It is the only Perl function that I consider mysterious in the least. I wish I could convey my bewilderment upon reading the docs for pack. From the very first sentences it is as if I were reading some English-like code in which all words mean something different from what I think they should mean. As I said, the tutorial is no help, because ultimately the tutorial just gives cookbook recipes which I can follow, but I have no idea what I'm doing. If what I need to do is not described in one of the recipes, then *I have no idea* of how to devise my own recipe. -- NOTE: In my address everything before the first period is backwards; and the last period, and everything after it, should be discarded.
From: John W. Krahn on 1 Oct 2005 19:26 kj wrote: > I want to write the function to_bits that takes a floating point > number as argument and returns an array of 32 ones or zeroes, > corresponding to the bits of the input argument. > > I have been staring at the docs for pack and unpack for a while > now trying to figure out how to use them to do this and I am as > clueless about it as I was when I first started. Also, perlpacktut > was no help either [1]. > > If someone could please tell me how to generate an array of one's > and zeros corresponding to the bits in a floating point number I'd > be very grateful. $ perl -le' print for unpack q/a/ x 32, unpack q/b*/, pack q/f/, 5.678 ' 1 0 1 1 0 1 0 0 0 1 0 0 1 1 0 1 1 0 1 0 1 1 0 1 0 0 0 0 0 0 1 0 John -- use Perl; program fulfillment
From: tlm on 1 Oct 2005 19:29 >I want to write the function to_bits that takes a floating point >number as argument and returns an array of 32 ones or zeroes, >corresponding to the bits of the input argument. How about this (untested): sub to_bits { my $f = pack 'f', shift; return reverse map vec( $f, $_, 1 ), 0 .. 31; } HTH, tlm -- NOTE: In my address everything before the first period is backwards; and the last period, and everything after it, should be discarded.
From: A. Sinan Unur on 1 Oct 2005 19:36 kj <nomail(a)inval.id> wrote in news:dhn4dh$bqd$1(a)reader1.panix.com: > I want to write the function to_bits that takes a floating point > number as argument and returns an array of 32 ones or zeroes, > corresponding to the bits of the input argument. > > I have been staring at the docs for pack and unpack for a while > now trying to figure out how to use them to do this and I am as > clueless about it as I was when I first started. Also, perlpacktut > was no help either [1]. > > If someone could please tell me how to generate an array of one's > and zeros corresponding to the bits in a floating point number I'd > be very grateful. #!/usr/bin/perl use strict; use warnings; print unpack 'b*', pack 'f', 3.45; Sinan -- A. Sinan Unur <1usa(a)llenroc.ude.invalid> (reverse each component and remove .invalid for email address) comp.lang.perl.misc guidelines on the WWW: http://mail.augustmail.com/~tadmc/clpmisc/clpmisc_guidelines.html
From: Anno Siegel on 1 Oct 2005 20:01
kj <nomail(a)inval.id> wrote in comp.lang.perl.misc: > > > > I want to write the function to_bits that takes a floating point > number as argument and returns an array of 32 ones or zeroes, > corresponding to the bits of the input argument. > > I have been staring at the docs for pack and unpack for a while > now trying to figure out how to use them to do this and I am as > clueless about it as I was when I first started. Also, perlpacktut > was no help either [1]. > > If someone could please tell me how to generate an array of one's > and zeros corresponding to the bits in a floating point number I'd > be very grateful. I'll show how to generate a string (not an array) of zeroes and ones. You need two steps. First use the "f" template (single-precision float) to get the four bytes that make up a 32-bit float into a string. If the number is in $_: my $str = pack 'f', $_; Next, unpack the result as a bit string, using the "b" template my $bits = unpack 'b32', $str; Run it for a few examples printf "%.3f -> %s\n", $_, unpack 'b32', pack 'f', $_ for 1/8, 1/4, 1/2, 1, 2, 4, 8; and see if you can make heads or tails of the output. The bytes may not be in the expected order. On my machine, I get: 0.125 -> 01111100000000000000000000000000 0.250 -> 01111100000000010000000000000000 0.500 -> 11111100000000000000000000000000 1.000 -> 11111100000000010000000000000000 2.000 -> 00000010000000000000000000000000 4.000 -> 00000010000000010000000000000000 8.000 -> 10000010000000000000000000000000 If you want an array, either split the result in single characters, or use map vec( $str, $_, 1), 0 .. 31 instead of unpacking with "b". Anno -- If you want to post a followup via groups.google.com, don't use the broken "Reply" link at the bottom of the article. Click on "show options" at the top of the article, then click on the "Reply" at the bottom of the article headers. |