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From: PerlFAQ Server on 30 Apr 2010 00:00
This is an excerpt from the latest version perlfaq5.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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5.8: How can I manipulate fixed-record-length files?

The most efficient way is using pack() and unpack(). This is faster than
using substr() when taking many, many strings. It is slower for just a
few.

Here is a sample chunk of code to break up and put back together again
some fixed-format input lines, in this case from the output of a normal,
Berkeley-style ps:

# sample input line:
# 15158 p5 T 0:00 perl /home/tchrist/scripts/now-what
my $PS_T = 'A6 A4 A7 A5 A*';
open my $ps, '-|', 'ps';
print scalar <$ps>;
my @fields = qw( pid tt stat time command );
while (<$ps>) {
my %process;
@process{@fields} = unpack($PS_T, $_);
for my $field ( @fields ) {
print "$field: <$process{$field}>\n";
}
print 'line=', pack($PS_T, @process{@fields} ), "\n";
}

We've used a hash slice in order to easily handle the fields of each
row. Storing the keys in an array means it's easy to operate on them as
a group or loop over them with for. It also avoids polluting the program
with global variables and using symbolic references.



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The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.

If you'd like to help maintain the perlfaq, see the details in
perlfaq.pod.
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