Displaying bit-vectors with Most Significant Bits to the left?

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

I'm not often working with vec , pack and unpack but whats the way to display a 16 bit-vector with MSB to the left?

 
  DB<138> undef $m0; vec($m0,$_,1)=1 for  0,2,8,9,10

  DB<139> p unpack('b16', $m0)                    # ok LSB left
1010000011100000
  DB<140> p unpack('B16', $m0)                    # MSB, but inside LS
+ bytes chunks
0000010100000111
  DB<142> $x+= 2**$_ for  0,2,8,9,10

  DB<144> printf "%016b",$x                      # what I expected
0000011100000101
  DB<145>
[download]

update

well it's documented

   B  A bit string (descending bit order inside each byte).
[download]

but there must be a better way than

  DB<184> p reverse unpack ('B8B8',$m0)
0000011100000101
  DB<185>
[download]

Cheers Rolf
_{(addicted to the Perl Programming Language :)

Wikisyntax for the Monastery
FootballPerl is like chess, only without the dice}

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Re: Displaying bit-vectors with Most Significant Bits to the left? by ikegami (Patriarch) on Jan 05, 2020 at 08:05 UTC
One could say that `vec` produces a packed little-endian integer. The bytes are ordered from most significant to least significant. In other words, `my $m0; vec($m0, $_, 1) = 1 for 0,2,8,9,10;` [download] is equivalent to `my $m0l; vec($m0l, $_, 1) = 1 for 0,2; my $m0h; vec($m0h, $_-8, 1) = 1 for 8,9,10; my $m0 = $m0l . $m0h;` [download] Option 1: Reverse the order of the bytes then convert the bytes to binary one by one. `unpack "B16", scalar reverse $mo` [download] Option 2: Unpack the bytes into a number, and print the binary representation of that number `sprintf "%016b", unpack "v", $m0` [download]	[reply] [d/l] [select]
Re: Displaying bit-vectors with Most Significant Bits to the left? by johngg (Canon) on Jan 05, 2020 at 12:18 UTC
Creating the vector in one 16-bit operation rather than several 1-bit operations simplifies the unpack process. `johngg@shiraz:~/perl/Monks$ perl -Mstrict -Mwarnings -E ' my @bitPosns = ( 0, 2, 8, 9, 10 ); my $val; $val += 2 ** $_ for @bitPosns; my $vec; vec( $vec, 0, 16 ) = $val; say unpack q{B16}, $vec;' 0000011100000101` [download] Six of one, a half dozen of the other I suppose. Cheers, JohnGG	[reply] [d/l]
Re^2: Displaying bit-vectors with Most Significant Bits to the left? by ikegami (Patriarch) on Jan 05, 2020 at 14:21 UTC
The OP didn't give any indication that `$m0` was different than expected. Changing the value of `$m0` doesn't seem appropriate. But if it is, there are two improvements you can make. When doing bit arithmetic, most people prefer to use actual bit arithmetic instead of powers. Not only is it more intuitive to use operations that match one's mental model, your approach fails if a bit is already set. Replace `$val += 2 ** $_` [download] with `$val \|= 1 << $_` [download] As evidenced by these posts, `vec` is quite weird, so it's surely best to avoid it if it doesn't offer a benefit. `my $vec; vec( $vec, 0, 16 ) = $val;` [download] can be replaced with `my $vec = pack 'n', $val;` [download]	[reply] [d/l] [select]
Re^3: Displaying bit-vectors with Most Significant Bits to the left? by LanX (Saint) on Jan 05, 2020 at 15:18 UTC
> vec is quite weird, so it's surely best to avoid it if it doesn't offer a benefit. I think it's trying to hide the machine representation of a number. I "grew up" on motorola, that's why I'm still confused. But vec's approach might provide reproducible results on different architectures. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply]
Re^4: Displaying bit-vectors with Most Significant Bits to the left? by ikegami (Patriarch) on Jan 06, 2020 at 07:09 UTC
Re^5: Displaying bit-vectors with Most Significant Bits to the left? by LanX (Saint) on Jan 06, 2020 at 14:33 UTC
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Re: Displaying bit-vectors with Most Significant Bits to the left? by shmem (Chancellor) on Jan 05, 2020 at 14:31 UTC
but there must be a better way than `DB<184> p reverse unpack ('B8B8',$m0) 0000011100000101 DB<185>` [download] Sure, since `unpack 'B8B8'` yields a list with 2 bytes. I'd say `DB<185> p reverse unpack 'b16',$m0 0000011100000101 DB<186>` [download] perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply] [d/l] [select]
Re^2: Displaying bit-vectors with Most Significant Bits to the left? by LanX (Saint) on Jan 05, 2020 at 14:57 UTC
Using `reverse` on a single value in list context is a no-op. You posted untested code in a fake demo. So clever! `DB<315> undef $m1; vec($m1,$_,1)=1 for 0,2,8,9,10 DB<316> x unpack 'b16',$m1 0 1010000011100000 DB<317> p reverse unpack 'b16',$m1 1010000011100000 DB<318> p scalar reverse unpack 'b16',$m1 0000011100000101 DB<319>` [download] Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l]
Re^3: Displaying bit-vectors with Most Significant Bits to the left? by shmem (Chancellor) on Jan 05, 2020 at 17:11 UTC
You posted untested code in a fake demo. So clever! Thanks for calling me a fool. So nice! `p scalar reverse unpack 'b16',$m1` [download] Yeah, the `scalar` was missing, which you spotted and corrected. So what? perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply] [d/l] [select]
Re^4: Displaying bit-vectors with Most Significant Bits to the left? by LanX (Saint) on Jan 05, 2020 at 17:14 UTC
Re^5: Displaying bit-vectors with Most Significant Bits to the left? by shmem (Chancellor) on Jan 05, 2020 at 17:28 UTC
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