3

我试图比较两个Double变量Powershell。当变量超过 2 位（不计算精度）时，相等性测试意外失败。

我在这里遗漏了一些明显的东西吗？

这是测试脚本输出：

PS C:\test> .\test.ps1
==========
TEST ONE
==========
Value of Double1: 336.1
Type of Double1: System.Double
-----
Value of Double2: 336.2
Type of Double2: System.Double
-----
Value of Double1+.1: 336.2
Type of Double1+.1: System.Double
-----
Does Double1 not equal Double2: True
Does Double1+.1 not equal Double2: True

==========
TEST TWO
==========

Value of Double3: 36.1
Type of Double3: System.Double
-----
Value of Double4: 36.2
Type of Double4: System.Double
-----
Value of Double3+.1: 36.2
Type of Double3+.1: System.Double
-----
Does Double3 not equal Double4: True
Does Double3+.1 not equal Double4: False

这是测试脚本，请注意第一个测试未通过我将 .1 添加到变量的测试，但第二个测试通过。

##################################
# START TEST
##################################

$Double1 = 336.1
$Double2 = 336.2

write-host "=========="
write-host "TEST ONE"
write-host "=========="
write-host "Value of Double1: $Double1"
write-host "Type of Double1:" $Double1.GetType().FullName
write-host "-----"
write-host "Value of Double2: $Double2"
write-host "Type of Double2:" $Double2.GetType().FullName
write-host "-----"
write-host "Value of Double1+.1:" ($Double1+.1)
write-host "-----"
write-host "Does Double1 not equal Double2:" ($Double1 -ne $Double2)    
write-host "Does Double1+.1 not equal Double2:" (($Double1+.1) -ne $Double2)
write-host ""

write-host "=========="
write-host "TEST TWO"
write-host "=========="

$Double3 = 36.1
$Double4 = 36.2

write-host ""
write-host "Value of Double3: $Double3"
write-host "Type of Double3:" $Double3.GetType().FullName
write-host "-----"
write-host "Value of Double4: $Double4"
write-host "Type of Double4:" $Double4.GetType().FullName
write-host "-----"
write-host "Value of Double3+.1:" ($Double3+.1)
write-host "-----"
write-host "Does Double3 not equal Double4:" ($Double3 -ne $Double4)    
write-host "Does Double3+.1 not equal Double4:" (($Double3+.1) -ne $Double4)
write-host ""
4

2 回答 2

7

请注意，这不是 Powershell 的问题，它只是浮​​点运算存在的问题。您可以在 C# 中重现相同的行为

double d = 336.1;
Console.WriteLine((d + .1) == 336.2); // False

问题是336.1 + .1实际上并不等于336.2。您可以通过转储 2 个值的原始字节来证明这一点

 unsafe
 {
    double d1 = 336.1;
    double d2 = d + .1d;
    double d3 = 336.2;
    Console.WriteLine(*(long*)(&d2));
    Console.WriteLine(*(long*)(&d3));
 }

印刷

4644622109139743540
4644622109139743539

请注意，最后 2 个字节偏移 1 个值。至于为什么会出现这种情况，更多处理浮点运算的人不得不说。一般来说，在比较浮点值时应该小心。与其严格相等，不如检查它们是否在特定范围内

if (Math.Abs(d2 - d3) <= .01)) {
  ...
}
于 2013-04-01T20:31:15.467 回答
3

尝试将变量类型转换为 [System.Decimal] 如何正确比较 powershell 中的双精度值？ http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

于 2013-04-01T20:53:50.523 回答