extract every nth number - perl

Question

i want to extract every 3rd number ( 42.034 , 41.630 , 40.158 as so on ) from the file
see example-
42.034 13.749 28.463 41.630 12.627 28.412 40.158 12.173 30.831 26.823
12.596 32.191 26.366 13.332 32.938 25.289 12.810 32.419 23.949 13.329
Any suggestions using perl script ?
Thanks,
dac

Answer 1

You can split file's contents to separate numbers and use the modulo operator to extract every 3rd number:
my $contents = do { local $/; open my $fh, "file" or die $!; <$fh> };
my #numbers = split /\s+/, $contents;
for (0..$#numbers) {
$_ % 3 == 0 and print "$numbers[$_]\n";
}

Answer 2

use strict;
use warnings;
use 5.010; ## for say
use List::MoreUtils qw/natatime/;
my #vals = qw/42.034 13.749 28.463 41.630 12.627 28.412 40.158 12.173 30.831
26.823 12.596 32.191 26.366 13.332 32.938 25.289 12.810 32.419 23.949 13.329/;
my $it = natatime 3, #vals;
say while (($_) = $it->());

Answer 3

This is probably the shortest way to specify that. If #list is your list of numbers
#list[ grep { $_ % 3 == 0 } 0..$#list ]

Answer 4

It's a one-liner!
$ perl -lane 'print for grep {++$i % 3 == 1} #F' /path/to/your/input
-n gives you line-by-line processing, -a autosplitting for field processing, and $i (effectively initialized to zero for our purposes) keeps count of the number of fields processed...

Answer 5

This method avoids reading the entire file into memory at once:
use strict;
my #queue;
while (<>) {
push #queue, / ( \d+ (?: \. \d* ) ? ) /gx;
while (#queue >= 3) {
my $third = (splice #queue, 0, 3)[2];
print $third, "\n"; # Or do whatever with it.
}
}

Answer 6

If the file has 10 numbers in every line you can use this:
perl -pe 's/([\d.]+) [\d.]+ [\d.]+/$1/g;' file
It's not a clean solution but it should "do the job".

Answer 7

Looks like this post lacked a solution that didn't read the whole file and used grep.
#!/usr/bin/perl -w
use strict;
my $re = qr/-?\d+(?:\.\d*)/; # Insert a more precise regexp here
my $n = 3;
my $count = 0;
while (<>) {
my #res = grep { not $count++ % $n } m/($re)/go;
print "#res\n";
};

Answer 8

I believe you’ll find that this work per spec, behaves politely, and never reads in more than it needs to.
#!/usr/bin/env perl
use 5.010_001;
use strict;
use autodie;
use warnings qw[ FATAL all ];
use open qw[ :std IO :utf8 ];
END { close STDOUT }
use Regexp::Common;
my $real_num_rx = $RE{num}{real};
my $left_edge_rx = qr{
(?: (?<= \A ) # or use \b
| (?<= \p{White_Space} ) # or use \D
)
}x;
my $right_edge_rx = qr{
(?= \z # or use \b
| \p{White_Space} # or use \D
)
}x;
my $a_number_rx = $left_edge_rx
. $real_num_rx
. $right_edge_rx
;
if (-t STDIN && #ARGV == 0) {
warn "$0: reading numbers from stdin,"
. " type ^D to end, ^C to kill\n";
}
$/ = " ";
my $count = 0;
while (<>) {
while (/($a_number_rx)/g) {
say $1 if $count++ % 3 == 0;
}
}