Perl等同于(Python-)列表理解

问题描述

我正在寻找在Perl中表达此Python代码段的方法:

I'm looking for ways to express this Python snippet in Perl:

data = {"A": None, "B": "yes", "C": None}
key_list = [k for k in data if data[k]]  
# in this case the same as filter(lambda k: data[k], data) but let's ignore that

所以从一种角度来看，我只想要键值为 None 或 undef 的键.用另一种方式来看，我想要的是与清单理解的简洁Perl等效项有条件的.

So looking at it one way, I just want the keys where the values are None or undef. Looking at it another way, what I want is the concise perl equivalent of a list comprehension with conditional.

推荐答案

我认为您想要 :

#!/usr/bin/env perl
use strict;
use warnings;

my %data = ( A => undef, B => 'yes', C => undef );

my @keys = grep { defined $data{$_} } keys %data;

print "Key: $_\n" for @keys;

我还认为输入速度太慢，因此在发布答案之前应该重新加载页面.顺便说一句，值0或undef可能是处理 null 值的好方法，但请确保您记住正在使用的值.错误值和未定义值在Perl中不是同一回事.需要说明的是:undef在布尔测试中返回false，但0也是.如果0是有效值，那么您要显式测试定义性，而不仅仅是真实性. (我之所以这样提，是因为James参加了0，而我却选择了另一种方式，所以您可能不知道这是否重要.)

I also think that I type too slowly, and that I should reload the page before posting answers. By the way, either a value of 0 or undef can be a good way to handle null values, but make sure you remember which you're using. A false value and and undefined value aren't the same thing in Perl. To clarify: undef returns false in a boolean test, but so does 0. If 0 is a valid value, then you want to explicitly test for definedness, not simply truth. (I mention it because James went for 0 and I went the other way, and you may or may not know if it matters.)

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