Reading and unpacking on one line

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

In "man perlfunc," the unpack command has the heading:

unpack TEMPLATE,EXPR

The following code snippet works:

read FILNAM, $_, 16;
$val = unpack 'H32';
[download]

But this does not:

$val = unpack 'H32', (read (FILNAM, $_, 16));
[download]

With the latter code, $val gets assigned 3136 (the hex string for 16). While I rank as a lowly novice, I still expected the one liner to work. Can someone explain (purely for my edification) why it doesn't?

Comment on Reading and unpacking on one line Select or Download Code

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Re: Reading and unpacking on one line by choroba (Cardinal) on Nov 05, 2020 at 16:19 UTC
From read: > Returns the number of characters actually read, 0 at end of file, or undef if there was an error Therefore, the second version is equivalent to `$val = unpack 'H32', 16;` [download] because the read returned 16. If you only specify the template, unpack uses the default variable $_ as the value to unpack. You can use do to read and return the value you have just read: `$val = unpack 'H32', do { read FILNAM, $_, 16; $_ };` [download] But you should check what `read` returned to handle possible errors. Also, calling a file handle FILNAM, which rather seems to be an abbreviation of "file name", is misleading. `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l] [select]
Re: Reading and unpacking on one line by tybalt89 (Monsignor) on Nov 05, 2020 at 16:31 UTC
`#!/usr/bin/perl use strict; # https://perlmonks.org/?node_id=11123425 use warnings; open FILNAM, '<', \<<END or die $!; 1234567890 abcdefghijkl END my $val = unpack 'H32', do { local $/ = \16; <FILNAM> }; print "$val\n";` [download] UPDATED: changed input to an in memory file to make program completely self contained.	[reply] [d/l]
Re^2: Reading and unpacking on one line by Anonymous Monk on Nov 05, 2020 at 22:18 UTC
Thanks to all for the answers. I understand what happened now. :) I will ask you this, since you slightly changed the method used to read the data (not that I need to know for this particular case, but just want to increase my understanding in general) - how do I specify reading 16 characters (perhaps UTF-8, for example) versus reading 16 bytes (as in pure binary data? Thank you again	[reply]
Re^3: Reading and unpacking on one line by choroba (Cardinal) on Nov 05, 2020 at 22:33 UTC
The behaviour depends on the filehandle. It's documented at the end of read > depending on the status of the filehandle, either (8-bit) bytes or characters are read. Similarly, sysread: > Note that if the filehandle has been marked as ":utf8", Unicode characters are read instead of bytes So, to read UTF-8, open the file handle with the `:encoding(UTF-8)` layer. `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l] [select]
Re^3: Reading and unpacking on one line by tybalt89 (Monsignor) on Nov 05, 2020 at 22:20 UTC
I don't do UTF. Maybe someone else will answer this.	[reply]
Re: Reading and unpacking on one line by BillKSmith (Monsignor) on Nov 05, 2020 at 16:15 UTC
In the first case. you read 16 characters into $_ and then unpack the contents of $_. In the second case, you do the same read, but you unpack the return value of read (the number of characters actually read, 16). Bill	[reply]


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