为什么 Perl 认为 -1 为真？
这是一段常见的示例代码：
while (1) {
print "foo\n";
}
它永远打印“foo”。
perl foo.pl
foo
foo
foo
...
并
while (0) { print "foo\n"; }
如您所愿静静地死去：
perl test.pl
有人能解释为什么这是 while 的一个有用的实现吗？这至少适用于 5.10、Unix 和 MacOS X
while (-1) { print "foo\n"; }
：
foo
foo
foo
...
This is a piece of common example code:
while (1) {
print "foo\n";
}
which prints 'foo' forever.
perl foo.pl
foo
foo
foo
...
and
while (0) { print "foo\n"; }
dies quietly as you expect:
perl test.pl
Can someone explain why this is a useful implementation of while? This works on 5.10 at least, Unix and MacOS X:
while (-1) { print "foo\n"; }
which gives
foo
foo
foo
...
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评论（7）
每个非零整数的计算结果都是
true。 0 总是假Every non-zero integer evaluates to
true. And 0 is alwaysfalse如果有的话，可以说
-1比1更有可能为真，因为-1(111..111bcode>) 是零的按位求反 (000..000b)。 BASIC 和 GW-BASIC 在需要返回真值时使用 -1。无论如何，Perl 认为表示“空”或“无”的值是错误的。大多数语言都持有类似的观点。具体来说，整数零、浮点零、字符串零、空字符串和 undef 为 false。
这是记录，尽管文档措辞不佳。 （它将
()列为 false 值，但没有这样的值。）除了一致性之外，采用这种方法非常有用。例如，它允许人们使用
而不是
If anything, one could say
-1is more likely to be true than1since-1(111..111b) is the bitwise negation of zero (000..000b). BASIC and GW-BASIC used -1 when they needed to return a true value.Regardless, Perl decided that values that mean "empty" or "nothing" are false. Most languages take a similar view. Specifically, integer zero, floating point zero, the string zero, the empty string and undef are false.
This is documented, although the documentation is poorly worded. (It lists
()as a value that's false, but there is no such value.)Aside from consistency, it's very useful to take this approach. For example, it allows one to use
instead of
来自perldoc perlsyn（真相与谎言）：
-1被认为是 true。From perldoc perlsyn (Truth and Falsehood):
-1is considered true.问题是“为什么 perl 认为 -1 是真的？”。。
答案是当 Perl 开发时，决定某些值将评估为 false。这些是：
这就是我能想到的关于原因的合适答案。它就是这样设计的。
The question is 'why does perl think -1 is true?'.
The answer is when perl was developed it was decided that certain values would evaluate to false. These are:
That is all I can think of a a suitable answer as to why. It was just designed that way.
只有 0 整数才被视为 false。任何其他非零整数都被视为 true。
Only a 0 integer is considered false. Any other non-zero integer is considered true.
任意整数<> 0 为真。
0 始终为假。
any integer <> 0 is true.
0 is always false.
Perl 从
awk和C继承了这种行为。这里解释了为什么
C这样做。Perl took this behavior from
awkandC.Why
Cdoes it is explained here.