Question

请问这两种哈希初始化方法有什么区别？

第一种方法：

$items{"food"} = "4.4";
$items{"water"} = "5.0";
$items{"shelter"} = "1.1";

foreach $item (keys $items) {
    print "$item\n";
}

输出为：

food
water
shelter

第二种方法：

%items = { 
    'food' => '4.4', 
    'water' => '5.0', 
    'shelter' => '1.1' 
};

foreach $item (keys %items) {
    print "$item\n";
}

输出是哈希引用：

HASH(0x8cc41bc)

为什么第二种方法返回引用而不是实际值？

Answer 1

因为你误解了{}的作用。

它创建一个匿名哈希，返回一个引用。

您刚刚完成的功能类似于：

my %stuff = (
   'food' => '4.4', 
    'water' => '5.0', 
    'shelter' => '1.1' 
);


my %items = \%stuff;

这没有多大意义。

使用()初始化哈希，它可以正常工作。

Answer 2

这是一个很好的示例，说明为什么始终打开Perl代码中的警告。

$ perl -Mwarnings -E'%h = {}; say keys %h'
Reference found where even-sized list expected at -e line 1.
HASH(0xbb6d48)

有关更详细的说明，请同时使用“诊断”。

$ perl -Mwarnings -Mdiagnostics -E'%h = {}; say keys %h'
Reference found where even-sized list expected at -e line 1 (#1)
    (W misc) You gave a single reference where Perl was expecting a list
    with an even number of elements (for assignment to a hash). This usually
    means that you used the anon hash constructor when you meant to use
    parens. In any case, a hash requires key/value pairs.

        %hash = { one => 1, two => 2, };        # WRONG
        %hash = [ qw/ an anon array / ];        # WRONG
        %hash = ( one => 1, two => 2, );        # right
        %hash = qw( one 1 two 2 );              # also fine

HASH(0x253ad48)

在Perl中打印哈希对

2 个答案: