How do unpack templates work

unlinker has asked for the wisdom of the Perl Monks concerning the following question:

I am a bit confused about how an unpack template works. For example:


#!/usr/bin/env perl

use Modern::Perl;
use Data::UUID;

my $ug = Data::UUID->new;
my $uuid = $ug->create_bin();
say unpack('H*', $uuid); # 767adecd8ed2e11191f4b1907aa0b7e4
 
my ($uu1, $uu2) = unpack('C4 C*', $uuid);
say unpack('H*', $uu1); # Got 313138 Expected:767adecd
say unpack('H*', $uu2); # Got 313232 Expected:8ed2e11191f4b1907aa0b7e4
[download]

I expected that 'C4 C*' would extract the first four bytes into $uu1 and C* would extract the rest into $uu2. Obviously I am not getting something here. Would appreciate an explanation

Thank you

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Re: How do unpack templates work by BrowserUk (Patriarch) on Jul 20, 2012 at 18:47 UTC
Try: `my( $bit1, $bit2 ) = unpack 'a4 a*', $uuid;` [download] But, regarding your "more efficient than substr" remarks. For the sake of a single operation like this, you won't notice the difference; even if there actually is any. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. The start of some sanity?	[reply] [d/l]
Re^2: How do unpack templates work by unlinker (Monk) on Jul 20, 2012 at 19:06 UTC
Yes - that works! Thanks. I agree that the efficiency/speed argument is not particularly compelling in this case. But a single unpack (when written correctly ;) ) is way more elegant that a bunch of substr calls Will wait for a later day to better understand why 'C' does not work	[reply]
Re^3: How do unpack templates work by BrowserUk (Patriarch) on Jul 20, 2012 at 19:44 UTC
Will wait for a later day to better understand why 'C' does not work Don't wait. Try a few simple tests: `print unpack 'H*', $uuid;; 767adecd8ed2e11191f4b1907aa0b7e4 print unpack 'a4', $uuid;; vzÌ═ print unpack 'C4', $uuid;; 118 122 222 205` [download] Look up what the 'C' template does. (Hint: see also ord ). With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. The start of some sanity?	[reply] [d/l]
Re^4: How do unpack templates work by unlinker (Monk) on Jul 20, 2012 at 20:12 UTC
Re^5: How do unpack templates work by BrowserUk (Patriarch) on Jul 20, 2012 at 20:35 UTC
Some notes below your chosen depth have not been shown here
Re: How do unpack templates work by daxim (Curate) on Jul 20, 2012 at 17:39 UTC
The `C` template does not help you here. You're probably making a wrong assumption about it. Just stay with `H` if you want to make hex digits out of octets. `my $uuid = "vz\336\315\216\322\341\21\221\364\261\220z\240\267\344"; # hex 767adecd8ed2e11191f4b1907aa0b7e4 my ($uu1, $uu2) = unpack('H8 H*', $uuid); say $uu1; # 767adecd say $uu2; # 8ed2e11191f4b1907aa0b7e4` [download]	[reply] [d/l]
Re^2: How do unpack templates work by unlinker (Monk) on Jul 20, 2012 at 17:48 UTC
Thanks. I do not want hex digits - I just want to split the binary string into two parts. I was using substr until I read that unpack is faster and more efficient.	[reply]
Re^3: How do unpack templates work by jwkrahn (Abbot) on Jul 20, 2012 at 22:51 UTC
Or you could use a regular expression: `my ( $uu1, $uu2 ) = $uuid =~ /(\C{4})(\C+)/;` [download]	[reply] [d/l]
Re: How do unpack templates work by flexvault (Monsignor) on Jul 21, 2012 at 07:16 UTC
unlinker, "...I just want to split the binary string into two parts. I was using 'substr' until I read that unpack is faster and more efficient." First off, I luv 'pack/unpack', but it may not always be the most efficient. There are several way to determine performance, but an excellent tool to use is 'NYTprof'. I had a Perl script that I couldn't improve the performance no matter what I did. I profiled it and found that my problem was calling 'pack/unpack' too often. On a run of 50,000 records, the script was calling 'pack/unpack' over 5,000,000 times. The 'pack/unpack' was in a tight loop, working on 32 byte substrings of a 4K string. What came to the rescue was 'substr'. Using the 4 parameter as `my $val = substr( $data, 0, 32, "" ); # $data already defin +ed` [download] In this situation, 'substr' just moves the pointer to the new beginning of the string, and was more efficient. I then used the 3 parameter non-destructive 'substr' to see it this was the substring I wanted, and then call 'pack/unpack'. Immediately, I saw a 300% improvement in performance and 'pack/unpack' was now called about 150,000 times. Perl has lots of powerful tools! Good Luck! "Well done is better than well said." - Benjamin Franklin	[reply] [d/l]

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