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From: allbery@NCoast.ORG (Brandon S. Allbery KB8JRR/AA)
Newsgroups: comp.lang.perl
Subject: Re: file append/read (+>>)
Message-ID: <1991Apr3.030016.19788@NCoast.ORG>
Date: 3 Apr 91 03:00:16 GMT
References: <6086@beryl12.UUCP>
Reply-To: allbery@ncoast.ORG (Brandon S. Allbery KB8JRR/AA)
Followup-To: comp.lang.perl
Distribution: comp
Organization: North Coast Public Access Un*x (ncoast)
Lines: 38

As quoted from <6086@beryl12.UUCP> by mostek@motcid.UUCP (Frank B. Mostek):
+---------------
| Also, is there a way to grep a file within perl?  Perl's grep needs a
| list, I currently use system("grep...") which is much slower.  I just
| need to know if an expression occurs in a file.  Is there an elegant
| way to do this using Perl's constructs?
+---------------

open(F,$file) || die "can't open $file: $!\n";

print grep(/re/, <F>);
# <F> being in an array context, it puts the file into a temporary array and
# then greps that.  This behaves like grep(1) with one file argument and no
# other options.

# To simply determine the existence of an expression in a file:

$found = grep(/re/, <F>);
# grep returns an array of all the "true" values (the lines on which the /re/
# succeeded); in a scalar context, an array produces the length of the array,
# which is 0 if no line contains /re/.  Since 0 is false and non-0 is true,
# this works as we expect.  If you want to use the grep as an expression and
# it might accidentally be taken as an array value, use "scalar" or one of
# the other tricks (concatenate the empty string, add 0, etc.) to force a
# scalar context.

# Be warned that either of these on a large file will take a LOT of memory.
# If this is a problem, do it one line at a time:

$found = 0;
$found += /re/ while $_ = <F>; # or while (<F>) { $found += /re/; }

close(F);
-- 
Me: Brandon S. Allbery			  Ham: KB8JRR/AA on 2m, 220, 440, 1200
Internet: allbery@NCoast.ORG		(QRT on HF until local problems fixed)
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