Two variables with the same value are NOT equal 1

[ClickHere]

Is there a logical explanation why the following code will not work?
I came across this problem in an app I'm working on where I have to increment a variable ('A') by .1 inside of a loop and If 'A'='B' then 'do something'. It works fine when 'B' is 1.1 or 1.2, but when it's 1.3 and above, it skips what it's suppose to do. According to Debug 'A'=1.3 and 'B'=1.3 but 'A' does not equal 'B'.

To try this simple test you'll need a Form with 2 Labels (Label1 & Label2) and a Command Button (Command1). Then click the button to add .1 to 'A'.

Option Explicit
Private A As Single
Private B As Single

Private Sub Command1_Click()

A = A + 0.1
Label1 = A
If A = B Then Beep

End Sub

Private Sub Form_Load()

A = 1
B = 1.3

Label1 = A
Label2 = B

End Sub

Any thoughts?
Thanks

 

[johnwm]

Singles are stored as 'floating point numbers' - that means they are only stored approximately, not exactly. There will be times when an 'equals' test will not make your 1.1 + .1 + .1 exactly the same as 1.3

To get round this you can use an exact variable type (Integer, Long, Decimal) or (depending on your requirements) use something like:

If Abs(A - B) < .000001 Then

Look up Variable Types in VBHelp (or MSDN) for more details


________________________________________________________________
If you want to get the best response to a question, please check out FAQ222-2244 first

'People who live in windowed environments shouldn't cast pointers.'

 

[ClickHere]

Thanks alot johnwm. The Decimal data type has done the trick. Although using the name Decimal (Private A as Decimal) gives a compile error. I can however use the literal char for Decimal (Private A@). Seems to work great.

A star for you.
Thanks again.

 

[ADoozer]

i think johnwm meant double and not decimal

good luck

If somethings hard to do, its not worth doing - Homer Simpson
------------------------------------------------------------------------
A General Guide To Excel in VB FAQ222-3383
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http://www.wotsit.org

 

[ADoozer]

oops.... or maybe not!!!



If somethings hard to do, its not worth doing - Homer Simpson
------------------------------------------------------------------------
A General Guide To Excel in VB FAQ222-3383
File Formats Galore @

http://www.wotsit.org

 

[johnwm]

ADoozer

I know johnwm meant Decimal and not Double - Double is still a floating point representation (just with a higher accuracy than Single)

I still wouldn't use an 'Equals' comparison with any of the FP types

as I said - Look up Variable Types in VBHelp (or MSDN) for more details!


________________________________________________________________
If you want to get the best response to a question, please check out FAQ222-2244 first

'People who live in windowed environments shouldn't cast pointers.'

 

[ADoozer]

sorry for the tripple post but:-

Note At this time the Decimal data type can only be used within a Variant, that is, you cannot declare a variable to be of type Decimal. You can, however, create a Variant whose subtype is Decimal using the CDec function

i think that was the point!

If somethings hard to do, its not worth doing - Homer Simpson
------------------------------------------------------------------------
A General Guide To Excel in VB FAQ222-3383
File Formats Galore @

http://www.wotsit.org

 

[johnwm]

Talk about collisions - We're falling all over each other here!


________________________________________________________________
If you want to get the best response to a question, please check out FAQ222-2244 first

'People who live in windowed environments shouldn't cast pointers.'

 

[ADoozer]

LOL it'll teach me to stay in my box with matters that dont concern me

If somethings hard to do, its not worth doing - Homer Simpson
------------------------------------------------------------------------
A General Guide To Excel in VB FAQ222-3383
File Formats Galore @

http://www.wotsit.org

 

[strongm]

>the Decimal data

it's good to see that this is getting more famous...

 

[chiph]

Or, you could use the Currency/Money datatype. It's fixed-point.

Johnwm is correct - 1.1, when converted to binary, is stored as an approximation (you can't accurately represent 0.1 in binary). So any attempt to do equality comparisons on these values wouldn't work.

Chip H.

 

[Robse]

Or you could use Format:

If Format(a, &quot;0.0&quot = Format(b, &quot;0.0&quot Then

 

[ClickHere]

Thanks chiph and Robse. Those are good work-around ideas also. And I probably would have gone one of those routes had johnwm not opened my eyes to the Decimal data type. So far it's working slicker-than-cat-slobber. And he answered my main question 'WHY'. I mean, even a simple calculator application needs to handle .1's. It just stumped me when it wouldn't work. Live and learn.

Thanks

 

[CCLINT]

The Format function will just round the number.
It will still at times still result in differences.