0

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;

如您所见，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。有人知道基本逻辑是什么吗？

编辑添加的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
4

4 回答 4

7

Data::Dumper 的工作是对数据进行序列化，您无法根据其输出告诉 perl 在内部对数据做了什么。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
于 2008-12-01T16:01:41.033 回答
6

Perl 将变量视为字符串或数字的整个概念是有缺陷的。Perl 会在您需要时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等）。

你不应该担心这个：Perl 知道它在做什么。

于 2008-12-01T14:57:46.373 回答
5

Data::Dumper 在变量的字符串表示上使用一个简单的模式来确定它是否是一个数字。从源代码：

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

这与具有解释您观察到的行为的小数点的数字不匹配。

于 2008-12-01T13:47:01.803 回答
0

强制数字上下文的快速肮脏方法：

打印自卸车（$HOURS_DURATION + 0.0）；

如果您担心数据将如何显示，那么干净的方法是：-

printf "%5.2d",$HOURS_DURATION;
于 2008-12-01T13:51:51.763 回答