关于哈希的澄清++

发布于 2024-11-17 04:10:53 字数 581 浏览 1 评论 0原文

我正在阅读 Damian Conway 的“Perl 最佳实践”，并发现以下代码片段：

$have_reconsidered{lc($name)}++;

我试图弄清楚哈希值到底发生了什么。我知道 ++ 在数字上下文中加一，但它对哈希有什么作用？

来自 perlop 文档：

undef 始终被视为数字，特别是更改为 0 在递增之前（这样a undef 值的后增量将返回 0 而不是 undef）。这自减运算符不是神奇的。

因此，在上面的示例中，键 lc($name) 的值被初始化为 0，然后通过 增加到 1 >++？

一般来说，我在哪里可以找到有关 ++、+= 等行为的更多信息...？

原文

I am reading Damian Conway's "Perl Best Practices" and found the following code snipet:

$have_reconsidered{lc($name)}++;

I am trying to figure out what is going on here with the hash. I know ++ increments by one in a numeric context, but what does it do to a hash?

From perlop documentation:

undef is always treated as numeric,
and in particular is changed to 0
before incrementing (so that a
post-increment of an undef value will
return 0 rather than undef). The
auto-decrement operator is not
magical.

So in the example above, is the value for the key lc($name) being initialized to 0 and then incremented to 1 by ++?

In general, where could I find out more about the behaviors of ++, +=, etc...?

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零時差 2024-11-24 04:10:53

%have_reconsidered 是您的哈希值。 $name 是一个字符串。 lc($name) 返回小写字符串。 $hash{$key} 将从使用键 $key 存储的哈希 %hash 返回标量值。所以：

// get scalar value from hash at key lc($name) and post-increment it
$have_reconsidered(lc($name)}++;

所以，您所做的就是在给定索引（即 lc($name)）处增加哈希值

测试用例：

#!/bin/env perl
my %hash = ( 'a' => '2' );
my $name = 'A';
print $hash{lc($name)}++; // prints 2 (incremented after statement)
print $hash{lc($name)};   // prints 3
print ++$hash{lc($name)}; // prints 4 (incremented before statement)

%have_reconsidered is your hash. $name is a string. lc($name) returns the lowercased string. $hash{$key} will return the scalar value from hash %hash stored with key $key. so:

// get scalar value from hash at key lc($name) and post-increment it
$have_reconsidered(lc($name)}++;

so, all you do is increment the value in a hash at a given index (namely lc($name))

test case:

#!/bin/env perl
my %hash = ( 'a' => '2' );
my $name = 'A';
print $hash{lc($name)}++; // prints 2 (incremented after statement)
print $hash{lc($name)};   // prints 3
print ++$hash{lc($name)}; // prints 4 (incremented before statement)

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妖妓 2024-11-24 04:10:53

哈希值是标量，因此 $have_reconsidered{lc($name)}++; 会递增标量 $have_reconsidered{lc($name)}。如果该标量之前未定义或不存在，++ 会将其设置为 1。

此代码的目标可能是删除重复项。

>perl -E"++$seen{$_} for qw( a a a b c a d ); say keys %seen;"
cabd

我更喜欢类似但不同的方法，因为它保留了秩序。

>perl -E"say grep !$seen{$_}++, qw( a a a b c a d );"
abcd

Hash values are scalars, so $have_reconsidered{lc($name)}++; increments the scalar $have_reconsidered{lc($name)}. If that scalar was previously undefined or didn't exist, ++ will set it to 1.

The goal of this code is probably to remove duplicates.

>perl -E"++$seen{$_} for qw( a a a b c a d ); say keys %seen;"
cabd

I prefer a similar but different approach because it preserves order.

>perl -E"say grep !$seen{$_}++, qw( a a a b c a d );"
abcd

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(り薆情海 2024-11-24 04:10:53

$have_reconsidered{lc($name)} 是属于键 lc($name) 的散列 %have_reconsidered 的值。该值肯定可以是数字，但即使它是字符串，它仍然可以自动递增（请参见http://perldoc.perl.org/perlop.html#Auto-increment-and-Auto-decrement)。

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~没有更多了~