Perl do...while 和 last 命令

问题描述

我刚刚遇到了一些我真的无法解释的非常奇怪的行为:

I've just encountered some very weird behavior that I really can't explain:

do {
    my $qry = $self->getHTMLQuery(undef, $mech->content());
    next if (!defined($qry));

    push(
        @prods,
        map { 'http://www.XXXXYYYX.com'.$_->attr('href') }
            $qry->query('div.prodInfo div.prodInfoBox a.prodLink.GridItemLink')
    );

    $qry->delete();
    $TEST++;

    last if ($TEST >= 10);

} while(eval { $mech->follow_link(class => 'jump next') });

print "WHILE ENDED\n";

上面的代码从不打印WHILE ENDED"，即使当 $TEST >= 10 时它似乎确实退出了 while 循环.

The code above never prints "WHILE ENDED" even though it does seem to go out of the while loop when $TEST >= 10.

但以下代码确实打印了WHILE ENDED":

But the following code does print "WHILE ENDED":

do {
    my $qry = $self->getHTMLQuery(undef, $mech->content());
    next if (!defined($qry));

    push(
        @prods,
        map { 'http://www.XXXXYYYX.com'.$_->attr('href') }
            $qry->query('div.prodInfo div.prodInfoBox a.prodLink.GridItemLink')
    );

    $qry->delete();
    $TEST++;

} while(eval { $mech->follow_link(class => 'jump next') } && $TEST <= 10);

print "WHILE ENDED\n";

在两个测试中，$TEST的初始值都是0.

In both tests, the initial value of $TEST is 0.

do...while 中 last 的行为是否不同于 for 和 while {...}?

Is the behavior of last in do...while different than in for and while {...}?

推荐答案

带有循环修饰符的 do 块在 next 范围内不算作真正的循环、last 和 redo 是相关的.perlsyn 中提到了这一点，您可以在其中找到 Schwern 提到的关于用裸块围绕它的提示使 last 工作.但这不适用于 next，因为一个裸块只执行一次，所以 next 的作用类似于 last.要使 next 工作，您可以将裸块 inside 放在 do 中，但是 last 将像 .

A do block with a looping modifier doesn't count as a real loop as far as next, last, and redo are concerned. This is mentioned in perlsyn, where you'll find the tip Schwern mentioned about surrounding it with a bare block to make last work. But that won't work with next, because a bare block is only executed once, so next acts like last. To make next work, you can put the bare block inside the do, but then last will act like next.

如果您需要 next 和 last 与 do ... while 一起工作，最简单的方法是使用无限循环在 continue 块中使用真实条件.这两个循环是等效的，除了第二个是真正的循环，因此它与 next & 一起使用.最后:

If you need both next and last to work with a do ... while, the easiest way is to use an infinite loop with the real condition in a continue block. These 2 loops are equivalent, except that the second is a real loop, so it works with next & last:

do { ... } while condition;
while (1) { ... } continue { last unless condition };

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