local具体用途

pony2001mx

在DNA序列文件中每个序列的ID以>开头，见如下文件：
>seq1
ATGCTAGCTAGCTACGATCGATGCATCGATCGATCGATCG
>seq2
GCGCCGCGGCTATTCGCGTTGACTGCTGACTGCATGACTG

有些PERL脚本有

1. local $/ = '>'

复制代码

的语句，即以>而不以\n做分割。我的问题是：local 的具体作用是什么？ 如果我想在一个while循环以>做分割，转到下一个while循环（或foreach循环）又以\n做分割，如何实现？

非常谢谢！

jason680

回复 1# pony2001mx

please refer the perldata and perlvar

$ perldoc perlvar
    perlvar - Perl predefined variables
    ...

    A few of these variables are considered "read-only". This means that if
    you try to assign to this variable, either directly or indirectly through
    a reference, you'll raise a run-time exception.

    You should be very careful when modifying the default values of most
    special variables described in this document. In most cases you want to
    localize these variables before changing them, since if you don't, the
    change may affect other modules which rely on the default values of the
    special variables that you have changed. This is one of the correct ways
    to read the whole file at once:

            open my $fh, "<", "foo" or die $!;
            local $/; # enable localized slurp mode
            my $content = <$fh>;
            close $fh;

    But the following code is quite bad:

            open my $fh, "<", "foo" or die $!;
            undef $/; # enable slurp mode
            my $content = <$fh>;
            close $fh;

    since some other module, may want to read data from some file in the
    default "line mode", so if the code we have just presented has been
    executed, the global value of $/ is now changed for any other code running
    inside the same Perl interpreter.

    Usually when a variable is localized you want to make sure that this
    change affects the shortest scope possible. So unless you are already
    inside some short "{}" block, you should create one yourself. For example:

            my $content = '';
            open my $fh, "<", "foo" or die $!;
            {
                    local $/;
                    $content = <$fh>;
            }
            close $fh;

    Here is an example of how your own code can go broken:

            for ( 1..3 ){
                   $\ = "\r\n";
                    nasty_break();
                    print "$_";
            }
            sub nasty_break {
            $\ = "\f";
            # do something with $_
            }

    You probably expect this code to print the equivalent of

        "1\r\n2\r\n3\r\n"

    but instead you get:

        "1\f2\f3\f"

    Why? Because "nasty_break()" modifies $\ without localizing it first. The
    value you set in "nasty_break()" is still there when you return. The fix
    is to add "local()" so the value doesn't leak out of "nasty_break()":

            local $\ = "\f";

    It's easy to notice the problem in such a short example, but in more
    complicated code you are looking for trouble if you don't localize changes
    to the special variables.