如何检查Perl标量变量是否已初始化?

Perl没有提供检查变量是否已初始化的方法。

undef 默认情况下。你说得对 defined 是检查变量值是否为的正确方法 未定义

还有其他几种方法。如果你想分配给变量 未定义 ,您的示例代码似乎表明了这一点,例如,您可以使用perl的定义or运算符:

$var //= 'a default value';

这取决于你想做什么。适当的 C Perl is not C ,因此您可能需要以下选项之一:

  1)   $var = "foo" unless defined $var;      # set default after the fact
  2)   $var = defined $var? $var : {...};     # ternary operation
  3)   {...} if !(defined $var);              # another way to write 1)
  4)   $var = $var || "foo";                  # set to $var unless it's falsy, in which case set to 'foo'
  5)   $var ||= "foo";                        # retain value of $var unless it's falsy, in which case set to 'foo' (same as previous line)
  6)   $var = $var // "foo";                  # set to $var unless it's undefined, in which case set to 'foo'
  7)   $var //= "foo";                        # 5.10+ ; retain value of $var unless it's undefined, in which case set to 'foo' (same as previous line)

C做事的方式( 未推荐的 ):

# initialize the variable to a default value during declaration
#   then test against that value when you want to see if it's been changed
my $var = "foo";
{...}
if ($var eq "foo"){
   ... # do something
} else {
   ... # do something else
}

如果你不在乎它是否是空的,它是空的。否则你可以查一下

if ( length( $str || '' )) {}

perl51000delta ):

 $a // $b

仅仅等同于

defined $a ? $a : $b

以及声明

$c //= $d;

现在可以用来代替

$c = $d unless defined $c;

另一方面,在散列中,这可能是真的:

if(exists $h{$e} && !defined $h{$e})