在上一个answer to a question about hash slices评论,有人想知道如何使用箭头语法通过散列引用来访问一个哈希片,心想也许是
@$ref->{@keys}
会这样做。
是的,正确的语法是@$ref{@keys}
或@{$ref}{@keys}
,但这不在这个问题的重点。
我试图算出与一个箭头的表达需要的数据结构:
#! /usr/bin/env perl
use strict;
use warnings;
my $ref = {"a" => 1, "b" => 2, "c" => 3};
my @keys = qw/ a b c /;
#$ref = [ { a => 9, b => 8, c => 7 } ];
#$ref = [ { a => {}, b => {}, c => {} } ];
print @$ref->{@keys}, "\n";
作为写入时,代码失败
Not an ARRAY reference at ./prog line 12.
有道理:@$ref
想要的参考一个数组,所以我尝试在一个匿名数组的引用内包装哈希引用。这些尝试失败,
Can't use an undefined value as a HASH reference at ./prog line 12.
跟踪输出是
$ debugperl -Dt prog [...] (prog:12) pushmark (prog:12) padsv($ref) (prog:12) rv2av (prog:12) rv2hv Can't use an undefined value as a HASH reference at prog line 12.
为print
行的语法转储
$ debugperl -Dx prog [...] { 484 TYPE = print ===> 2 FLAGS = (VOID,KIDS) { 485 TYPE = pushmark ===> 486 FLAGS = (SCALAR) } { 372 TYPE = helem ===> 371 FLAGS = (SCALAR,KIDS) { 487 TYPE = rv2hv ===> 361 TARG = 5 FLAGS = (SCALAR,KIDS,REF) PRIVATE = (STRICT_REFS) { 373 TYPE = rv2av ===> 487 TARG = 4 FLAGS = (SCALAR,KIDS,REF) PRIVATE = (STRICT_REFS) { 486 TYPE = padsv ===> 373 TARG = 1 FLAGS = (SCALAR,MOD) } } } { 361 TYPE = padav ===> 372 TARG = 2 FLAGS = (SCALAR) } } { 371 TYPE = const ===> 484 TARG = 19 FLAGS = (SCALAR) } } [...]
哪里是未定义的值是从哪里来的?程序的$ref
的值是否正常终止?
它是有效的(即使没有意义),因此它不应该是语法错误。不过,这应该是一个严格的错误。 – ikegami