PERL Beginners - grep usage

Author

grep usage

Reginald Johnson

2008-03-26, 7:11 pm

I am trying to grep an array return the lines that match. From my
reading I see that grep returns the number of times an expression was
true not the actual expression. My question is how would I get the
actual expression.
I tested my code with a small input file of 2 records. The first one is
in the @mhsarray the second one is not.
I am trying to get my output file to have "CISCSTT6....line from
@mhsarray that it matched".

$ cat mhs.pl
#!/usr/bin/perl
use warnings;

$file="/adsm/nodes";
$mhs="/adsm/mhs_alloc";
$file_out="/adsm/in_mhs";

open (INFILE, "<", "$file") or
die "$file could not be opened: $!";
open (MHSFILE, "<", "$mhs") or
die "$mhs could not be opened: $!";
open (OUTFILE, ">", "$file_out") or
die "$file_out could not be opened: $!";

while (<MHSFILE> ) {
chomp($_);
push (@mhsArray, $_);
}

foreach $line (<INFILE> ) {
chomp($line);
print "this is line $line\n";
@inmhs = grep(/$line/,@mhsArray);
print "size of inmhs array is $#inmhs\n";
if ($#inmhs > -1) {
print (OUTFILE "$line\n"); # would like to be
$line...line from @mhsArray
# print "$_\n";
}
} #end foreach
close(INFILE);
close(MHSFILE);
close(OUTFILE);
cicstt6 () [ /a
Reggie Johnson
TSM Admin
904.218.4620
Jacksonville, Fl
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Yitzle

2008-03-26, 7:11 pm

Have you tried it?
Grep does return the result lines when used in a list context, ie how
you used it
> @inmhs = grep(/$line/,@mhsArray);
print "$_\n" for (@inmhs);

From Perldoc:
"Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value consisting of those
elements for which the expression evaluated to true. In scalar
context, returns the number of times the expression was true."

John W. Krahn

2008-03-26, 7:11 pm

Johnson, Reginald (GTI) wrote:
> I am trying to grep an array return the lines that match. From my
> reading I see that grep returns the number of times an expression was
> true not the actual expression. My question is how would I get the
> actual expression.
> I tested my code with a small input file of 2 records. The first one is
> in the @mhsarray the second one is not.
> I am trying to get my output file to have "CISCSTT6....line from
> @mhsarray that it matched".
>
> $ cat mhs.pl
> #!/usr/bin/perl
> use warnings;

use strict;

> $file="/adsm/nodes";
> $mhs="/adsm/mhs_alloc";
> $file_out="/adsm/in_mhs";

my $file = '/adsm/nodes';
my $mhs = '/adsm/mhs_alloc';
my $file_out = '/adsm/in_mhs';

> open (INFILE, "<", "$file") or
> die "$file could not be opened: $!";
> open (MHSFILE, "<", "$mhs") or
> die "$mhs could not be opened: $!";
> open (OUTFILE, ">", "$file_out") or
> die "$file_out could not be opened: $!";

perldoc -q quoting

> while (<MHSFILE> ) {
> chomp($_);
> push (@mhsArray, $_);
> }

Or simply:

chomp( my @mhsArray = <MHSFILE> );

> foreach $line (<INFILE> ) {

while ( my $line = <INFILE> ) {

> chomp($line);
> print "this is line $line\n";
> @inmhs = grep(/$line/,@mhsArray);

$line may contain regexp meta-characters so you should quotemeta() it:

my @inmhs = grep /\Q$line/, @mhsArray;

Or did you really want to check for equality:

my @inmhs = grep $_ eq $line, @mhsArray;

If you just want the number of times that $line matches @mhsArray then
use a scalar:

my $inmhs_count = grep /\Q$line/, @mhsArray;

> print "size of inmhs array is $#inmhs\n";

$#inmhs is *not* the size of @inmhs, $#inmhs is the index of the last
element of @inmhs. @inmhs in scalar context is the size of @inmhs.

print "size of inmhs array is ", scalar @inmhs, "\n";

> if ($#inmhs > -1) {

Just use @inmhs in scalar context:

if ( @inmhs > 0 ) {

Or simply:

if ( @inmhs ) {

> print (OUTFILE "$line\n"); # would like to be $line...line from @mhsArray
> # print "$_\n";
> }
> } #end foreach
> close(INFILE);
> close(MHSFILE);
> close(OUTFILE);

John
--
Perl isn't a toolbox, but a small machine shop where you
can special-order certain sorts of tools at low cost and
in short order. -- Larry Wall

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