我可以复制散列而不重置其“每个"散列吗?迭代器? [英] Can I copy a hash without resetting its "each" iterator?

问题描述

我正在使用each遍历Perl哈希:

I am using each to iterate through a Perl hash:

while (my ($key,$val) = each %hash) {
   ...
}

然后发生了一些有趣的事情，我想打印出哈希值.起初，我考虑类似的东西:

Then something interesting happens and I want to print out the hash. At first I consider something like:

while (my ($key,$val) = each %hash) {
   if (something_interesting_happens()) {
      foreach my $k (keys %hash) { print "$k => $hash{$k}\n" }
   }
}

但是那是行不通的，因为每个人都知道在哈希上调用keys(或values)会重置用于each的内部迭代器，我们可能会遇到无限循环.例如，这些脚本将永远运行:

But that won't work, because everyone knows that calling keys (or values) on a hash resets the internal iterator used for each, and we may get an infinite loop. For example, these scripts will run forever:

perl -e '%a=(foo=>1); while(each %a){keys %a}'
perl -e '%a=(foo=>1); while(each %a){values %a}'

没问题，我想.我可以复制哈希，然后打印出来.

No problem, I thought. I could make a copy of the hash, and print out the copy.

   if (something_interesting_happens()) {
      %hash2 = %hash;
      foreach my $k (keys %hash2) { print "$k => $hash2{$k}\n" }
   }

但这也不起作用.这也会重置each迭代器.实际上，在列表上下文中使用%hash似乎都会重置其each迭代器.因此，它们也将永远运行:

But that doesn't work, either. This also resets the each iterator. In fact, any use of %hash in a list context seems to reset its each iterator. So these run forever, too:

perl -e '%a=(foo=>1); while(each %a){%b = %a}'
perl -e '%a=(foo=>1); while(each %a){@b = %a}'
perl -e '%a=(foo=>1); while(each %a){print %a}'

此文件记录在任何地方吗? Perl可能需要使用相同的内部迭代器来将哈希的内容压入返回堆栈，这是有道理的，但是我也可以想象不需要这样做的哈希实现.

Is this documented anywhere? It makes sense that perl might need to use the same internal iterator to push a hash's contents onto a return stack, but I can also imagine hash implementations that didn't need to do that.

更重要的是，有什么方法可以做我想要的吗?要获得哈希的所有元素而无需重置each迭代器?

More importantly, is there any way to do what I want? To get to all the elements of a hash without resetting the each iterator?

这也建议您也不能在each迭代中调试哈希.考虑在以下位置运行调试器:

This also suggests you can't debug a hash inside an each iteration, either. Consider running the debugger on:

%a = (foo => 123, bar => 456);
while ( ($k,$v) = each %a ) {
    $DB::single = 1;
    $o .= "$k,$v;";
}
print $o;

只需检查调试器停止处的哈希(例如，键入p %a或x %a)，就可以更改程序的输出.

Just by inspecting the hash where the debugger stops (say, typing p %a or x %a), you will change the output of the program.

更新:我上传了 Hash::SafeKeys 作为这个问题的一般解决方案.感谢@gpojd为我指出正确的方向，并感谢@cjm的建议使解决方案更加简单.

Update: I uploaded Hash::SafeKeys as a general solution to this problem. Thanks @gpojd for pointing me in the right direction and @cjm for a suggestion that made the solution much simpler.

推荐答案

您是否尝试过可存储的 dclone要复制吗?可能是这样的:

Have you tried Storable's dclone to copy it? It would probably be something like this:

use Storable qw(dclone);
my %hash_copy = %{ dclone( \%hash ) };

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