AsFloat vs. AsCurrency

[Alberto Pilat]

Hello,
Using D7, DOA 4.0.5.0, Oracle 9.01.

A TOracleDataSet (ODS) read 2 columns (COL1, COL2) of NUMBER type. The corrispondent TField is a TFloatField.
Suppose COL1 and COL2 contains the same decimal value : the following test
ODS.FieldByName('COL1').AsFloat = ODS.FieldByName('COL2').AsFloat
return TRUE, correctly.
Then, I edit and post a record, replacing the same value in COL2.
After it, testing COL1 and COl2 as above, the result is FALSE.
I verified that the incorrect behavior happens after the TOracleDataset has been posted, while after a refresh it works fine.
I verified too that if I do the same test using AsCurrency instead AsFloat, it works properly:
(ODS.FieldByName('COL1').AsCurrency = ODS.FieldByName('COL2').AsCurrency)

It seems a bug.... any idea?

 

[Marco Kalter]

Floating point precision is not 100%, so you cannot compare floating point numbers like this. There is no exact representation of (for example) 1.42, only an approximation that is up to 15 digits accurate (for example 1.41999999999997).

 

[Alberto Pilat]

sorry for reading so later...
OK, I known the limits of digital rappresentation, but the case show 2 different behaviors on the same datas, this is the problem...
Is it true that AsCurrency has less problems then AsFloat?

Thanks

 

[Andrey Maximenko. Ukraine]

if abs(ODS.FieldByName('COL1').AsFloat-ODS.FieldByName('COL2').AsFloat) < 0.000001 then ...

 

[Marco Kalter]

The AsCurrency property returns the rounded floating point value of a TFloatField. It does not have any of the floating point accuracy problems, because it is a fixed point data type. It is represented internally by a 64 bit integer.

In short: comparing AsCurrency values for equality is safe.