How to unpack numbers with varying sizes?

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

I need to extract the integers contained in a binary file. The format of the file first specifies how many bytes (1..8) are used to represent each integer (in big-endian). I then use read to get the specified bytes into a variable. But how can I unpack the bytes properly so they are interpreted as a number? I have already read through pack and perlpacktut and could only find a solution for integers of size 1, 2, 4, and 8 bytes.

Comment on How to unpack numbers with varying sizes?

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Re: How to unpack numbers with varying sizes? by Anonymous Monk on Feb 13, 2011 at 04:13 UTC
I'm sure there's a more efficient and succinct method, but this should work: `# my $bytes = read ... my $bitstring = unpack 'B', $bytes; $bitstring = sprintf '%.64d', $bitstring; my $num = unpack 'Q', pack 'B', $bitstring;` [download]	[reply] [d/l]
Re^2: How to unpack numbers with varying sizes? by Anonymous Monk on Feb 13, 2011 at 04:16 UTC
`'Q'` should be `'Q>'` if it's big endian.	[reply] [d/l] [select]
Re: How to unpack numbers with varying sizes? by Anonymous Monk on Feb 13, 2011 at 02:36 UTC
and could only find a solution for integers of size 1, 2, 4, and 8 bytes Why is that a problem? Maybe youwant Math::BigInt?	[reply]
Re^2: How to unpack numbers with varying sizes? by Anonymous Monk on Feb 13, 2011 at 02:49 UTC
Because the data includes numbers encoded in 3, 5, 6, and 7 bytes as well.	[reply]
Re^3: How to unpack numbers with varying sizes? by ikegami (Patriarch) on Feb 13, 2011 at 06:16 UTC
`unpack` doesn't support those sizes directly. You could extract them in parts, then assemble them. `# 3-bytes, network order my ($o1, $o2, $o3) = unpack('CCC', $_); my $n = ($o1 << 16) \| ($o2 << 8) \| $o3; -or- my ($n1, $n2) = unpack('Cn', $_); my $n = ($n1 << 16) \| $n2;` [download] Alternatively, you could add padding. `# 3-bytes, network order my $n = unpack('N', "\x00".$_);` [download]	[reply] [d/l] [select]
Re^3: How to unpack numbers with varying sizes? by Anonymous Monk on Feb 13, 2011 at 06:06 UTC
Because the data includes numbers encoded in 3, 5, 6, and 7 bytes as well. And why do you think that is a problem? a 3 byte integer fits into a 4 byte, or 8 byte integer without problems	[reply]