VB.Net 一些添加后的双重比较

Question

在向双变量添加一些值后，我遇到了一个奇怪的情况。当多次将 (0.2) 添加到双变量时会出现问题 - 我认为它只发生在 (0.2) - 例如：考虑这段代码：

Dim i As Double = 2
i = i + 0.2
MsgBox(i) '2.2
MsgBox(i > 2.2) 'False >> No problem

但是如果我多次添加 (0.2)：

Dim i As Double = 2
i = i + 0.2
i = i + 0.2
MsgBox(i) '2.4
Msgbox(i > 2.4) 'True >> !!!!

还有

Dim i As Double = 2
For x As Integer = 1 to 5
    i = i + 0.2
Next
MsgBox(i) '3
Msgbox(i > 3) 'True >> !!!!

我用其他值尝试了相同的代码，我没有遇到这个问题：

Dim i As Double = 2
i = i + 0.5
i = i + 0.5
MsgBox(i) '3
Msgbox(i > 3) 'False >> No problem

有人对此有解释吗？？谢谢

Answer 1

如果您采用示例 3，您会发现结果实际上是 3.0000000000000009。

问题出在双精度数的舍入上。

如果您更改数据类型 decimal，问题就解决了：

Sub Main()

    Dim i As Decimal = 2

    For x As Integer = 1 To 5
        i = i + 0.2
    Next
    MsgBox(i) '3
    MsgBox(i > 3) 'False >> No problem

End Sub

This is 关于 C# 但是，我想，对于 vb.net.

也是一样的

Answer 2

此问题称为“Accuracy Problems (Wikipedia Link)”

The fact that floating-point numbers cannot precisely represent all real numbers, and that floating-point operations can not precisely represent true arithmetic operations, leads to many surprising situations. This is related to the finite precision with which computers generally represent numbers.

VB.Net 一些添加后的双重比较

VB.Net Double comparison after some additions

vb.net

comparison

double