为什么Perl认为-1是真的？

This is a piece of common example code:

这是一段常见的示例代码：

while (1) { 
    print "foo\n"; 
}

which prints 'foo' forever.

永远打印'foo'。

perl foo.pl
foo
foo
foo
...

and

和

while (0) { print "foo\n"; }

dies quietly as you expect:

像你期望的那样安静地死去：

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:

有人可以解释为什么这是一个有用的实现吗？这至少适用于5.10，Unix和MacOS X：

while (-1) { print "foo\n"; }

which gives

这使

foo
foo
foo
...

7 个解决方案

#1

If anything, one could say -1 is more likely to be true than 1 since -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.

如果有的话，可以说-1比1更可能是真的，因为-1（111..111b）是零（000..000b）的按位否定。 BASIC和GW-BASIC在需要返回真值时使用-1。

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.

无论如何，Perl认为那些意味着“空”或“无”的价值是错误的。大多数语言采用类似的观点。具体来说，整数零，浮点零，字符串零，空字符串和undef都是false。

This is documented, although the documentation is poorly worded. (It lists () as a value that's false, but there is no such value.)

这是记录在案的，尽管文档措辞不佳。（它将（）列为值为false的值，但没有这样的值。）

Aside from consistency, it's very useful to take this approach. For example, it allows one to use

除了一致性，采用这种方法非常有用。例如，它允许使用

if (@x)

instead of

代替

if (@x != 0)

#2

Every non-zero integer evaluates to true. And 0 is always false

每个非零整数的计算结果为true。 0总是假的

#3

From perldoc perlsyn (Truth and Falsehood):

来自perldoc perlsyn（真理和虚假）：

The number 0, the strings '0' and '' , the empty list () , and undef are all false in a boolean context. All other values are true.

在布尔上下文中，数字0，字符串'0'和''，空列表（）和undef都是false。所有其他值都是真的。

-1 is considered true.

-1被认为是真的。

#4

The question is 'why does perl think -1 is true?'.

问题是'为什么perl认为-1是真的？'。

The answer is when perl was developed it was decided that certain values would evaluate to false. These are:

答案是在开发perl时，决定将某些值评估为false。这些是：

0
0
undef
*基金
'' (empty string)
''（空字符串）

That is all I can think of a a suitable answer as to why. It was just designed that way.

这就是为什么我能想到一个合适的答案。它就是这样设计的。

#5

Only a 0 integer is considered false. Any other non-zero integer is considered true.

只有0整数被认为是假的。任何其他非零整数都被认为是真的。

#6

any integer <> 0 is true. 0 is always false.

任何整数<> 0都是真的。 0总是假的。

#7

Perl took this behavior from awk and C.

Perl从awk和C中采取了这种行为。

Why C does it is explained here.

为什么C会这样解释。

#1