5.28: perl How can I read in an entire file all at once?

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From: xlr82sas on 17 Mar 2010 14:25

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Hi SAS-lers,

I hope SAS-Lers do not take my intentions the wrong way. My goal would
be to enhance SAS by integrating perl.

SAS is like the universe of light and perl and R are like the world of
sound. Sound cannot travel past the earths atmosphere but light is can
travel through the universe.

Interesting concept to read all lines of a file into and array and
then access lines randomly.

5.28: perl How can I read in an entire file all at once?

$ - scaler
@ - array

You can use the File::Slurp module to do it in one step.


use File::Slurp;


$all_of_it = read_file($filename); # entire file in
scalar
@all_lines = read_file($filename); # one line per element


The customary Perl approach for processing all the lines in a file
is to
do so one line at a time:


open (INPUT, $file) || die "can't open $file: $!";
while (<INPUT>) {
chomp;
# do something with $_
}
close(INPUT) || die "can't close $file: $!";


This is tremendously more efficient than reading the entire file
into
memory as an array of lines and then processing it one element at
a
time, which is often--if not almost always--the wrong approach.
Whenever
you see someone do this:


@lines = <INPUT>;


you should think long and hard about why you need everything
loaded at
once. It's just not a scalable solution. You might also find it
more fun
to use the standard Tie::File module, or the DB_File module's
$DB_RECNO
bindings, which allow you to tie an array to a file so that
accessing an
element the array actually accesses the corresponding line in the
file.


You can read the entire filehandle contents into a scalar.


{
local(*INPUT, $/);
open (INPUT, $file) || die "can't open $file: $!";
$var = <INPUT>;
}


That temporarily undefs your record separator, and will
automatically
close the file at block exit. If the file is already open, just
use
this:


$var = do { local $/; <INPUT> };


For ordinary files you can also use the read function.


read( INPUT, $var, -s INPUT );


The third argument tests the byte size of the data on the INPUT
filehandle and reads that many bytes into the buffer $var.

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