Comparing a Single to a Double in VB.Net.

Question

Here's my test code:

Dim testSingle As Single = 7.2
Dim testSingleF As Single = 7.2F
Dim testDouble As Double = 7.2

If testSingle = testDouble Then ' this is false
    Label1.Text = "true"
Else
    Label1.Text = "false"
End If

If testSingleF = testDouble Then ' this is false
    Label2.Text = "true"
Else
    Label2.Text = "false"
End If

If testSingle = 7.2F Then ' this is true
    Label3.Text = "true"
Else
    Label3.Text = "false"
End If

As you can see by my comments, the first two statements are false, and the third is true. Why is that? The precision shouldn't matter as it's such a small number.

What's going on here?

Answer 1

Yes, precision matters even though it is a small number. Why? Because 7.2 has an infinite number of digits in binary notation: 7.2 (dec) = 111.001100110011... (bin) -- just like 10/3 = 3.333... in decimal notation.

Thus, Single and Double are bad choices if you need to accurately represent non-integer numbers. You have the following options:

• Use Double, but never compare numbers for absolute equality a = b. Instead, check if Abs(a-b) is smaller than some small threshold.
• Use the Decimal data type, which has been created exactly for this purpose. In a nutshell, it stores 72 (which can be represented exactly in binary) plus the information that there is one digit at the right of the decimal point. Thus, these problems don't occur here (at least not for numbers which can be represented accurately in decimal notation. 10/3 is still a problem here...)

Further information can be found, for example, on Wikipedia.

Answer 2

See http://stackoverflow.com/questions/566958/double-precision-problems-on-net/566983#566983