Perl - 以相反的顺序逐行读取文件 [英] Perl - reading a file line by line in reverse order

问题描述

可能的重复:
如何从末尾读取行Perl 中的文件?

先读取最后一行，然后再读取最后一行，依此类推.文件太大，无法放入内存.

First read the last line, and then the last but one, etc. The file is too big to fit into memory.

推荐答案

逆序阅读:

use File::ReadBackwards;

my $back = File::ReadBackwards->new(shift @ARGV) or die $!;
print while defined( $_ = $back->readline );

我最初误读了这个问题，并认为您想以交替方式向后和向前阅读——这似乎更有趣.:)

I misread the question initially and thought you wanted to read backward and forward, in alternating fashion -- which seems more interesting. :)

use strict;
use warnings;
use File::ReadBackwards ;

sub read_forward_and_backward {
    # Takes a file name and a true/false value.
    # If true, the first line returned will be from end of file.
    my ($file_name, $read_from_tail) = @_;

    # Get our file handles.
    my $back = File::ReadBackwards->new($file_name) or die $!;
    open my $forw, '<', $file_name or die $!;

    # Return an iterator.
    my $line;    
    return sub {
        return if $back->tell <= tell($forw);
        $line = $read_from_tail ? $back->readline : <$forw>;
        $read_from_tail = not $read_from_tail;
        return $line;
    }

}

# Usage.    
my $iter = read_forward_and_backward(@ARGV);
print while defined( $_ = $iter->() );

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