Perl 如何决定将标量视为字符串或数字？

Question

考虑以下代码及其输出：

代码

#!/usr/bin/perl -w
use strict;
use Data::Dumper;
my $HOURS_PER_DAY = 24.0 * 1.0;
my $BSA = 1.7 * 1.0;
my $MCG_PER_MG = 1000.0 * 1.0;
my $HOURS_DURATION    = 20.0 * $HOURS_PER_DAY;
my $dummy = $HOURS_PER_DAY * $BSA * $MCG_PER_MG * $HOURS_DURATION;
print Dumper($HOURS_PER_DAY);
print Dumper( $BSA);
print Dumper( $MCG_PER_MG);
print Dumper( $HOURS_DURATION );

输出

$VAR1 = 24;
$VAR1 = '1.7';
$VAR1 = 1000;
$VAR1 = 480;

---

正如您所看到的，第二个变量被视为字符串，而第一个和第四个变量被视为数字。 有人知道底层逻辑是什么吗？

添加的编辑算术计算并不能完全解决问题（请参阅 $BSA 变量）。

---

$ perl -v

This is perl, v5.10.0 built for cygwin-thread-multi-64int
(with 6 registered patches, see perl -V for more detail)

Copyright 1987-2007, Larry Wall

Answer 1

Data::Dumper 的工作是序列化数据，并且您无法根据其输出了解 perl 在内部对数据执行的操作。 Devel::Peek 模块可以转储存储在变量中的底层标志和值。 Devel::Peek POD 解释了标志的重要性。

#!/usr/bin/perl

use warnings;
use strict;
use Devel::Peek;

my $HOURS_PER_DAY = 24.0 * 1.0;
my $BSA = 1.7 * 1.0;
my $MCG_PER_MG = 1000.0 * 1.0;
my $HOURS_DURATION    = 20.0 * $HOURS_PER_DAY;
my $dummy = $HOURS_PER_DAY * $BSA * $MCG_PER_MG * $HOURS_DURATION;

Dump($HOURS_PER_DAY);
Dump($BSA);
Dump($MCG_PER_MG);
Dump($HOURS_DURATION);

__END__
SV = PVNV(0xd71ff0) at 0xd87f90
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,NOK,pIOK,pNOK)
  IV = 24
  NV = 24
  PV = 0
SV = PVNV(0xd71fc8) at 0xd87f60
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,pIOK,pNOK)
  IV = 1
  NV = 1.7
  PV = 0
SV = PVNV(0xd72040) at 0xd87f40
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,NOK,pIOK,pNOK)
  IV = 1000
  NV = 1000
  PV = 0
SV = IV(0xd8b408) at 0xd87f30
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 480
# compare the above output to output without the assignment to $dummy:
SV = IV(0x7b0eb8) at 0x7adf90
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 24
SV = NV(0x7c7c90) at 0x7adf60
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,pNOK)
  NV = 1.7
SV = IV(0x7b13d8) at 0x7adf40
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 1000
SV = IV(0x7b1408) at 0x7adf30
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 480

Answer 2

Perl 将变量视为字符串或数字的整个概念是有缺陷的。 当您需要时，Perl 会以正确的方式处理您的变量，例如，算术运算符总是将其参数视为数字（假设您没有滥用运算符重载或类似操作）。

您不应该担心这一点：Perl 知道它在做什么。

Answer 3

Data::Dumper 在变量的字符串表示形式上使用简单的模式来确定它是否是数字。 从源代码来看：

...
elsif ($val =~ /^(?:0|-?[1-9]\d{0,8})\z/) { # safe decimal number
  $out .= $val;
}
else {               # string
...

这与带有小数点的数字不匹配，这解释了您观察到的行为。

Answer 4

强制数字上下文的快速肮脏方法：

print Dumper( $HOURS_DURATION + 0.0 );

如果您关心的是数据将如何显示，那么干净的方法是：-

printf "%5.2d",$HOURS_DURATION;