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

Hi all: I am new to Perl and having some problems... I have a txt file that contains data like this. 
A         B        C       D
1.0E-1  2.0E-2  3.1E-3  3.0E-9
1.0E-1  2.0E-2  3.2E-3  3.0E-9
1.0E-1  2.0E-2  3.3E-3  3.0E-9
1.0E-1  2.0E-2  3.4E-3  1.0E-1

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A B C D are separated by tab. I would like Perl to capture the C value right before D is larger than 1e-3 (capture Cn-1 when Dn > 1e-3; in this case that would be 3.3E-3), and my code right now looks like this: 
open (linedata, "/test.txt");
while  (<linedata>) {    
    ($A, $B, $C, $D) = split(/\t/, $_);
    print "$A, $B, $C, $D, \n"
    }
close (linedata);

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I don't really know how to process the "capturing Cn-1" part... Would someone please give me some insight? And if I add chomp to my code, the program whined about doesn't return a chomp value, which I don't know the reason. Appreciate to have some insight on chomp, too. Thank you guys!

Replies are listed 'Best First'.
Re: Perl starter with questions about processing data from a txt file
by choroba (Cardinal) on Jan 17, 2013 at 16:50 UTC
    Just remember the C value everytime, but only report it when needed: 
    #!/usr/bin/perl
use warnings;
use strict;

open my $IN, '<', 'test.txt' or die $!;

my $remember;
while (<$IN>) {
    my @columns = split /\t/;
    print "$remember\n" if $columns[3] > 1e-3;
    $remember = $columns[2];
}

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    لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
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[d/l]

      Thanks! it worked but I am now confused. The code really listed the C value before D > 1e-3. But if I am understanding the code right, shouldn't Perl only report C values when D is > 1e-3 (that being said, I get Cn value instead of Cn-1)?

      And if I have multiple D values larger than 1e-3, can I ask perl to report only the first Cn-1 value to me, instead of listing all?

      Thanks again for any insights or answers!
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        The code reports the remembered value when D > 1e-3, to the actual C value. As it is, it will report all the values. If you want to report only the first one, just exit the loop via last after printing a value. If you want to skip values that are on the neighbouring lines, just set a flag once you output a value, clear it otherwise, and only report a value if the flag is not set.
        لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
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