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Re: Printing line before matching expression

by jaredor (Priest)
on Sep 11, 2013 at 05:35 UTC ( #1053411=note: print w/replies, xml ) Need Help??


in reply to Printing line before matching expression

For fun, here's a version that uses a linked list. The "defined" ensures that there is something to be printed. If you change $lookback to 4 and rerun, you'll see it in action.

#!/usr/bin/env perl use strict; use warnings; my $lookback = 3; my $match = qr/\wiz/; my @lbuff = map { [] } 1..$lookback; $lbuff[$_-1][0] = $lbuff[($_) % $lookback] for 1..$lookback; my $curr = $lbuff[0]; while (<DATA>) { /$match/ and defined $$curr[1] and print "$$curr[1]"; ($$curr[1], $curr) = ($_, $$curr[0]); } __DATA__ foo bar baz biz buz goo car caz ciz cuz

P.S. If anyone knows how to reduce the two-line circular linked list definition into one line of idiomatic perl I would love to see it. I'm too tired to think of something clever at the moment.

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Re^2: Printing line before matching expression
by hdb (Monsignor) on Sep 11, 2013 at 06:43 UTC

    It seems a bit of overhead to me if you use an array of the required buffer size to store the linked list and the data. What you save compared to other proposals is the pushing and shifting which involves a lot of copying data around. This can be done more easily by cycling through the buffer using the modulo operator on the line number of the data.

    use strict; use warnings; my $lookback = 3; my $match = qr/\wiz/; my @lbuff; while (<DATA>) { /$match/ and defined $lbuff[ $. % $lookback ] and print "$lbuff[ $ +. % $lookback ]"; $lbuff[ $. % $lookback ] = $_; } __DATA__ foo bar baz biz buz goo car caz ciz cuz

      compared to other proposals is the pushing and shifting which involves a lot of copying data around.

      Sure it doesn't :)

      This can be done more easily by cycling through the buffer using the modulo operator on the line number of the data.

      If all the data you want is at the end

      my @data = ( undef, undef, qw/ bingo nameo / ); print @data[ -1,-2,-3 ]; __END__ nameobingo

      A small suggestion would be adding one more slot to the buffer and using some perl idiomcyncracies (sic) to reduce the need to manipulate the buffer in the inner loop:

      #!/usr/bin/env perl use Modern::Perl; my $lb = 3; # lookback my $match = qr/\wiz/; my @buff; while ($buff[$.%($lb+1)] = ($_ = <DATA>)) { my $lbi = ($.+1)%($lb+1); # lookback index print $buff[$lbi] if defined $buff[$lbi] and /$match/; } __DATA__ foo bar baz biz buz goo car caz ciz cuz

      P.S. Thanks for optimizing my two line circular linked list declaration to zero lines! ;-)

      P.P.S. Too bad: Yours is, IMHO, the best answer to the OP, but is buried in this subthread.

      EDIT Just reviewed this code and saw that I forgot defined! This opens the door for error if the lookback line is empty or a zero. While I was at it, I changed the emphasis from the lookback line itself to the index that finds it; plus, no more relying on side effects in the conditional.

      For the three of y'all who upvoted the earlier incarnation, the original code is below. One line was deleted, one line added, one line changed.

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