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Incorrect behavior of a comparison
Posted: Thu Mar 26, 2009 11:58 am
by QuimV
Anyone have idea why this happens?
Code: Select all
;Code
UVa.f = 5.1
UV.f = 5
BM.f = 0.1
Dif.f= (UVa-UV)
If Dif<0
Dif * (-1)
EndIf
;Incorrect behavior of the comparison
If Dif >= BM
Debug "major"
EndIf
;Correct behavior of the comparison
If StrF(Dif,3) >= StrF(BM,3)
Debug "major"
EndIf
Thanks in advanced.
Posted: Thu Mar 26, 2009 12:06 pm
by srod
It's all in the rounding of decimal values when stored internally in floating point form.
5.1, when stored as a float value, equates to 5.09999990463257 and thus you can see why your first comparison fails.
Your StrF() comparison is adding additional rounding errors (because you are limiting the number of decimal places) and thus you are getting a positive result this time (a false false positive!)
Posted: Thu Mar 26, 2009 12:16 pm
by Goos E
It has something to do with the way a Floating point variable is stored. I am not sure how it works. The example you wrote it shows that the variable UVa = 5.099999990463257 and UV=5.0 so when you subtract UV from UVa the result=0.09999999046325 that is smaller then BM wich is actualy 0.1000000014012.
The use off StrF() is logical becauce it's actualy rounded, wich is for both UVa and BM "0.100"
Posted: Thu Mar 26, 2009 12:57 pm
by QuimV
so, if I need to make comparations using floats, I should to use the expresion:
If StrF(Dif,3) >= StrF(BM,3)
Debug "major"
EndIf
Is that correct?
Posted: Thu Mar 26, 2009 1:21 pm
by srod
Messy and slow!
Relying on a string comparison following additional rounding of the floating point values is not really an ideal solution.
The following results in an incorrect comparison because of the number of decimal places being used by StrF() in this case. A spurious example I know and one that can be countered by using more places of decimals, but it still serves to show how problematic this all is.
Code: Select all
a.f = 1.6500002
b.f = 1.6500001
If StrF(a) > StrF(b)
Debug "This should be debugged!"
EndIf
What is the exact context of the floating point comparisons? I ask because there may be a better way of proceeding depending on eactly what it is you are trying to achieve? For example, if you are comparing currency values then you stop right there!
Posted: Thu Mar 26, 2009 1:24 pm
by Kaeru Gaman
I got another funny example... o.o/
Code: Select all
a.f = 1.6500000
b.f = 1.6500000
If StrF(a,2) = StrF(b,3)
Debug "This should be debugged!"
Else
Debug "But this gets debugged!"
EndIf
Posted: Thu Mar 26, 2009 1:57 pm
by srod
Code: Select all
a.f = 1.6500000
b.f = 1.6500000
If StrF(a,2) = StrF(b,3)
Debug "This should NOT be debugged!"
Else
Debug "But this gets debugged correctly!"
EndIf
You're comparing two strings "1.65" and "1.650" which of course are not equal!
Posted: Thu Mar 26, 2009 3:56 pm
by Kaeru Gaman
well, I know that....
but I use a value-to-string convesion to eliminate incertaincies of floating-point notation,
and overlook 'by accident' that both conversions use different numbers of decimals....
someone who uses strings to compare floats, and process the conversion somewhere-out-there-in-the-attic
would surely wonder why they are not equal, where the bug is.
.... it's mofo frustrating today that I seem to need to explain each and every lousefart to you... are you having your cycle?
Posted: Thu Mar 26, 2009 7:08 pm
by srod
Well I was just responding to your 'funny example' so 'scuse me for trying to explain where you had gone wrong!
I don't think anyone who uses strings to compare floats would wonder why StrF(a, 2) and StrF(a, 3) differed?
Whatever............. if you don't want people replying to your posts then don't post!
Posted: Thu Mar 26, 2009 7:41 pm
by QuimV
Just I need to compare currency values.
How could I do that and avoid errors?
Thanks
Posted: Thu Mar 26, 2009 7:53 pm
by srod
First, don't store the currencies as floating point values - no sir!
You really want to store your currencies in an exact form. I released a currency library some time ago in the tips and tricks forum. Should be easy enough to find.
**EDIT :
http://www.purebasic.fr/english/viewtop ... t=currency
Posted: Thu Mar 26, 2009 9:52 pm
by QuimV
srod,
Thanks a lot for your help