如何测试哈希中是否存在值? [英] How to test if a value exist in a hash?

问题描述

假设我有这个

#!/usr/bin/perl

%x = ('a' => 1, 'b' => 2, 'c' => 3);

我想知道值 2 是否是 %x 中的哈希值.

and I would like to know if the value 2 is a hash value in %x.

这是怎么做到的?

推荐答案

从根本上说，哈希是一种优化用于解决相反问题的数据结构，知道键 2 是否存在.但是在不知道的情况下很难判断，所以我们假设它不会改变.

Fundamentally, a hash is a data structure optimized for solving the converse question, knowing whether the key 2 is present. But it's hard to judge without knowing, so let's assume that won't change.

此处介绍的可能性取决于:

Possibilities presented here will depend on:

• 您需要多久做一次
• 哈希的动态程度

• grep $_==2, values %x(也拼写为 grep {$_==1} values %x)将返回一个包含多个 2 的列表出现在散列中，或者在标量上下文中，匹配的数量.在条件中评估为布尔值，它会产生您想要的结果.
  grep 适用于我记忆中最老的 Perl 版本.
• use List::Util qw(first);第一个 {$_==2} 值 %x 仅返回第一个匹配项，如果没有，则返回 undef.这使它更快，因为它一旦成功就会短路(停止检查元素).这对 2 来说不是问题，但要注意返回的元素不一定评估为布尔值 true.在这些情况下使用 defined.
  List::Util 是一个自 5.8 起成为 Perl 核心的一部分.
• use List::MoreUtils qw(any);任何 {$_==2} 值 %x 都准确地以布尔值形式返回您请求的信息，并表现出短路行为.
  List::MoreUtils 可从 CPAN 获得.立>
• 2 ~~ [values %x] 以布尔值形式准确返回您请求的信息，并表现出短路行为.
  智能匹配自 5.10 起在 Perl 中可用.

• grep $_==2, values %x (also spelled grep {$_==1} values %x) will return a list of as many 2s as are present in the hash, or, in scalar context, the number of matches. Evaluated as a boolean in a condition, it yields just what you want.
  grep works on versions of Perl as old as I can remember.
• use List::Util qw(first); first {$_==2} values %x returns only the first match, undef if none. That makes it faster, as it will short-circuit (stop examining elements) as soon as it succeeds. This isn't a problem for 2, but take care that the returned element doesn't necessarily evaluate to boolean true. Use defined in those cases.
  List::Util is a part of the Perl core since 5.8.
• use List::MoreUtils qw(any); any {$_==2} values %x returns exactly the information you requested as a boolean, and exhibits the short-circuiting behavior.
  List::MoreUtils is available from CPAN.
• 2 ~~ [values %x] returns exactly the information you requested as a boolean, and exhibits the short-circuiting behavior.
  Smart matching is available in Perl since 5.10.

构造一个将值映射到键的散列，并将其用作自然散列来测试键是否存在.

Construct a hash that maps values to keys, and use that one as a natural hash to test key existence.

my %r = reverse %x;
if ( exists $r{2} ) { ... }

重复操作，动态散列

使用如上所述的反向查找.您需要使其保持最新状态，这留给读者/编辑作为练习.(提示:值冲突很棘手)

Repeated op, dynamic hash

Use a reverse lookup as above. You'll need to keep it up to date, which is left as an exercise to the reader/editor. (hint: value collisions are tricky)

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