Reading nth line from a file

perlthirst has asked for the wisdom of the Perl Monks concerning the following question:

I am having one program, which read a line by line from a file, if the line matches for the particular pattern, then i should read nth line from a current line.

some simple example is:

open(FH,"test.txt");

while ( <FH> ) {

        ## If the line matches for this pattern, I should read 4th lin
+e from current line
        if ( $_ =~/you have to read the fourth line/ ){
                $i=0;
                while ($i<4)
                {
                        $line = <FH>;
                        $i++;
                }
                print $line;
        }
        ## If the line matches for this pattern, I should read 2nd lin
+e from current line
        elsif ( $_ =~/you have to read the second line/ ){
                $i=0;
                while ($i<2)
                {
                        $line = <FH>;
                        $i++;
                }
                print $line;
        }

}
[download]

Input:
first line
second line
third line
you have to read the fourth line
one
two
three
four
five
six
you have to read the second line
one
two
three

output:
four
two

The above coding works for me for my requirement

but i felt that the way that i have handled to read nth line from a file, is inefficient.. kindly advice some efficient way to do the above.

Comment on Reading nth line from a file Download Code

Replies are listed 'Best First'.
Re: Reading nth line from a file by jwkrahn (Abbot) on Dec 19, 2008 at 05:53 UTC
`$ echo "first line second line third line you have to read the fourth line one two three four five six you have to read the second line one two three" \| perl -e' my %lookup = qw/ fourth 4 second 2 /; while ( <> ) { if ( /you have to read the (\w+) line/ && exists $lookup{ $1 } ) { $lines{ $. + $lookup{ $1 } }++; } if ( $lines{ $. } ) { print; } } ' four two` [download]	[reply] [d/l]
Re: Reading nth line from a file by brsaravan (Scribe) on Dec 19, 2008 at 06:15 UTC
Here is another simple way. `my @arrvar = <FH>; map { if ($arrvar[$_] =~ /you have to read the fourth line/) { print "$arrvar[$_+4]"; } elsif ($arrvar[$_] =~ /you have to read the second line/) { print "$arrvar[$_+2]"; } }0..$#arrvar;` [download]	[reply] [d/l]
Re^2: Reading nth line from a file by perlthirst (Scribe) on Dec 19, 2008 at 06:44 UTC
Thanks for your reply. But reading the whole file into an array may not be efficient. If the file size is too large, it will take so much memory i think.	[reply]
Re: Reading nth line from a file by NetWallah (Canon) on Dec 19, 2008 at 06:32 UTC
Something extensible .. more coding efficient, but not necessarily less execution time ... `use strict; my %actionlist=( "you have to read the fourth line" => 4, "you have to read the second line" => 2 ); my $re = "(" . join ("\|", keys %actionlist) . ")"; $re= qr\|$re\|; my $printthis = -1; while (<DATA>){ if (my ($match) = /$re/){ $printthis = $actionlist{$match}; } $printthis-- == 0 and print; } __DATA__ first line second line third line you have to read the fourth line one two three four five six you have to read the second line one two three` [download] Update: On reflection, this solution is very similar to jwkrahn's. ..to maintain is to slowly feel your soul, sanity and sentience ebb away as you become one with the Evil.	[reply] [d/l]
Re: Reading nth line from a file by imrags (Monk) on Dec 19, 2008 at 07:25 UTC
Use "$." to keep a tab on you line number. It's better than looping thru the whole file. Else you can use a counter which increments after each line. Raghu	[reply]
Re: Reading nth line from a file by Bloodnok (Vicar) on Dec 19, 2008 at 12:47 UTC
Methinx recipe 8.8 Reading a Particular Line in a File, from the perl Cookbook would be of use to you. Update Ooops, my apologies to O'Reilly ... seemingly genuine URL removed - apparently referenced an unauthorised Ukrainian website - many thanx to Narveson. A user level that continues to overstate my experience :-))	[reply]
Re: Reading nth line from a file by gmontema (Initiate) on Dec 19, 2008 at 18:41 UTC
open(FH,"test.txt"); $i=0; $j1=0; $j2=0; while ( <FH> ) { ## If the line matches for this pattern, I should read 4th line from current line if ( $_ =~/you have to read the fourth line/ ){ $j1=-4; } elsif ( $j1<0 ){ $j1++; print $_ if $j1==0; } ## If the line matches for this pattern, I should read 2nd line from current line elsif ( $_ =~/you have to read the second line/ ){ $j2=-2; } elsif ( $j2<0 ){ $j2++; print $_ if $j2==0; } print $i++,"\n"; }	[reply]
Re^2: Reading nth line from a file by gmontema (Initiate) on Dec 19, 2008 at 18:42 UTC
sorry disregard the previous message.	[reply]
Re^3: Reading nth line from a file by gmontema (Initiate) on Dec 22, 2008 at 23:31 UTC
open(FH,"test.txt"); my $i=0; my @p =(); while ( <FH> ) { ## If the line matches for this pattern, I should read 4th line from current line if ( $_ =~/you have to read the fourth line/ ){ push @p,$i+4; } ## If the line matches for this pattern, I should read 2nd line from current line elsif ( $_ =~/you have to read the second line/ ){ push @p,$i+2; } @p=sort(@p); foreach my $j(@p){print $_ if $j==$i; shift @p if $j<=$i;} $i++; } Input: first line 1 second line 2 third line 3 you have to read the fourth line one 5 two three 7 four five 9 six 6 you have to read the second line one 12 two three 13 first line 14 second line 15 third line 16 you have to read the fourth line you have to read the second line one 20 two with in a four four with two inside five 23 six 24 three 25 you have to read the second line you have to read the fourth line two with a for two 29 three 30 four in a two ten 32 one 33 Output: four two two with in a four four with two inside two with a for four in a two	[reply]