in VB Why (1 = 1) is False

Question

I just came across this piece of code:

Dim d As Double

For i = 1 To 10
  d = d + 0.1
Next

MsgBox(d)
MsgBox(d = 1)
MsgBox(1 - d)

Can anyone explain me the reason for that? Why d is set to 1?

See http://stackoverflow.com/questions/1530069/comparing-floating-point-values — Darryl Braaten, Sep 16 '11 at 19:39

Oded · Accepted Answer · 2011-09-16T19:48:20.077

8

Floating point types and integer types cannot be compared directly, as their binary representations are different.

The result of adding 0.1 ten times as a floating point type may well be a value that is close to 1, but not exactly.

When comparing floating point values, you need to use a minimum value by which the values can differ and still be considered the same value (this value is normally known as the epsilon). This value depends on the application.

I suggest reading What Every Computer Scientist Should Know About Floating-Point Arithmetic for an in-depth discussion.

As for comaring 1 to 1.0 - these are different types so will not compare to each other.

edited Sep 16 '11 at 19:48

answered Sep 16 '11 at 19:43

Oded

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and why when I print it out its 1? and if so what might be the best way to compare it? thks! – Bartzilla Sep 16 '11 at 19:44
@Eduardo - How do you print it out? – Oded Sep 16 '11 at 19:45
I used MsgBox(d) or Console.WriteLine(d) – Bartzilla Sep 16 '11 at 19:47
@Eduardo - That's because `(1.0).ToString()` returns `"1"`. – Oded Sep 16 '11 at 20:20
is .toString implicitly been called? when using MsgBox or Console.WriteLine? – Bartzilla Sep 16 '11 at 20:22
@Eduardo - In both cases. The `double` needs to be converted to a string, so `ToString()` is called on it before outputting it. – Oded Sep 16 '11 at 20:23

score 3 · Answer 2 · answered Sep 16 '11 at 19:52

3

.1 (1/10th) is a repeating fraction when converted to binary:

.0001100110011001100110011001100110011.....

It would be like trying to show 1/3 as a decimal: you just can't do it accurately.

answered Sep 16 '11 at 19:52

Joel Coehoorn

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score 2 · Answer 3 · answered Sep 16 '11 at 19:48

This is because a double is always only an approximation of the value and not the exact value itself (like a floating point value). When you need an exact decimal value, instead use a Decimal.

Contrast with:

Dim d As Decimal

For i = 1 To 10
    d = d + 0.1
Next

MsgBox(1)
MsgBox(d = 1)
MsgBox(1 - d)

in VB Why (1 = 1) is False

3 Answers3

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