-$ |在Perl工作? [英] How does --$| work in Perl?

问题描述

最近，我遇到了这种方法，用于过滤列表的第二个值:

Recently I came across this way to filter out every second value of a list:

perl -E 'say grep --$|, 1..10'
13579

它如何工作?

推荐答案

$|是一个特殊的Perl变量，只能具有值0和1. 对$|的 true 非零值的任何赋值，例如

$| is a special Perl variable that can only have the values 0 and 1. Any assignment to $| of a true non-zero value, like

$| = 1;
$| = 'foo';
$| = "4asdf";          # 0 + "4asdf" is 4
$| = \@a;

将具有将$|设置为1的效果. false 零值的任何赋值

will have the effect of setting $| to 1. Any assignment of a false zero value

$| = 0;
$| = "";
$| = undef;
$| = "erk";            # 0 + "erk" is 0

会将$|设置为0.

将--$|扩展到$| = $| - 1，现在您可以看到发生了什么. 如果$|最初为1，则--$|会将值更改为0. 如果$|最初为0，则--$|会尝试将值设置为-1，但实际上会将值设置为1.

Expand --$| to $| = $| - 1, and now you can see what is going on. If $| was originally 1, then --$| will change the value to 0. If $| was originally 0, then --$| will try to set the value to -1 but will actually set the value to 1.

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