有没有办法在Perl中本地更改输入记录分隔符？

问题描述

通过$x将变量my $x的范围限制为特定的代码块或子例程，从而避免了由“全局变量”引起的混乱的编码器。

但是，当涉及到输入记录分隔符$/时，显然不能限制其范围。 我对吗？

因此，如果我忘记在循环结束时或在子例程内部重置输入记录分隔符，则调用该子例程的下面代码可能会产生意外的结果。 以下示例对此进行了演示。

#!/usr/bin/perl
use strict; use warnings;
my $count_records; my $infile = $ARGV[0]; my $HANDLEinfile;

open $HANDLEinfile,'<',$infile or die "cannot open $infile for reading";
$count_records = 0;
while(<$HANDLEinfile>)
{
    $count_records++; 
    print "$count_records:\n";
    print;
}
close $HANDLEinfile;

look_through_other_file();

print "\nNOW,after invoking look_through_other_file:\n";
open $HANDLEinfile,$infile or die "cannot open $infile for reading";
$count_records = 0;
while(<$HANDLEinfile>)
{
    $count_records++; 
    print "$count_records:\n";
    print;
}
close $HANDLEinfile;

sub look_through_other_file
{
    $/ = undef;
    # here,look through some other file with a while loop
    return;
}

这是输入文件上的行为：

> z.pl junk
1:
All work
2:
and
3:
no play
4:
makes Jack a dull boy.

NOW,after invoking look_through_other_file:
1:
All work
and
no play
makes Jack a dull boy.
> 

请注意，如果尝试更改为

my $/ = undef;

在子例程中，这会生成错误。

顺便说一句，在stackoverflow标签中，为什么没有用于“输入记录分隔符”的标签？

解决方法

my $/ = undef;问题的答案是将其更改为local $/ = undef;。然后修改后的代码如下。

#!/usr/bin/perl
use strict; use warnings;
my $count_records; my $infile = $ARGV[0]; my $HANDLEinfile;

open $HANDLEinfile,'<',$infile or die "cannot open $infile for reading";
$count_records = 0;
while(<$HANDLEinfile>)
{
    $count_records++; 
    print "$count_records:\n";
    print;
}
close $HANDLEinfile;

look_through_other_file();

print "\nNOW,after invoking look_through_other_file:\n";
open $HANDLEinfile,$infile or die "cannot open $infile for reading";
$count_records = 0;
while(<$HANDLEinfile>)
{
    $count_records++; 
    print "$count_records:\n";
    print;
}
close $HANDLEinfile;

sub look_through_other_file
{
    local $/ = undef;
    # here,look through some other file with a while loop
    return;
}

然后，无需手动将输入记录分隔符返回到另一个值或默认值$/ = "\n";。

,

您可以使用local来临时更新全局变量的值，包括$/。

sub look_through_other_file {
    local $/ = undef;
    # here,look through some other file with a while loop
    return;
}

只要$/子例程在调用堆栈中，

将使用未定义的look_through_other_file。

您可能会在这种常见用法中遇到这种构造，即将文件的整个内容包含到变量中，而无需更改程序其余部分的$/的值：

open my $fh,"<","/some/file";
my $o = do { local $/; <$fh> };