Rounding of doubles!

[srod]

Came across this earlier which surprised me a little. I am sure it has been discussed before, but I probably missed the relevant posts.

Of course I am aware of the limitations of the floating point representation of real numbers, but I did expect the following to produce the same results.

Code: Select all

y.d = 0.333333333333333
Debug y
Debug "======="

a$ = "0.333333333333333"
x.d = ValD(a$)
Debug x

The 2 outputs differ only very slightly, but enough for me to raise an eyebrow or six!

I came across it because I coded my own version of ValD() which does a little more than PB's function. It does produce identical results to ValD() however. I've looked at the ASM generated by PB and can see that the difference is in the lower 32-bits of the 64-bit value.

Does this inconsistency equate to a bug or is it something I just have to accept?

Thanks.

[IdeasVacuum]

It is a fault, but does anyone need an accuracy beyond ten decimal places?

[srod]

If they did then they wouldn't use floating point that's for sure!

Just found it a bit odd that's all.

[jack]

the difference is probably due to the first expression being evaluated at compile time and the second evaluated at run-time, and obviously use different routines.

[skywalk]

Use StrD()

Code: Select all

Debug "-- StrD(y.d) --"
y.d = 0.333333333333333
Debug StrD(y,16)
Debug "-- StrD(a$,15digits) --"
a$ = "0.333333333333333"
x.d = ValD(a$)
Debug StrD(x,16)
Debug "-- StrD(a$,16digits) --"
b$ = "0.3333333333333333"
z.d = ValD(b$)
Debug StrD(z,16)

[srod]

Well, yes, but that is somewhat beside the point.

@Jack : yes I agree, but still, the inconsistency...

The compiler probably calls some function from the standard c lib to make the conversion or Fred coded his own c routine?

It's no big deal what with floating point being what it is anyhow. Just caught me by surprise.

[Danilo]

The compiler probably calls some function from the standard c lib to make the conversion or Fred coded his own c routine?

sscanf and sprintf give the same result on MS Windows.

Code: Select all

ImportC "msvcrt.lib"
    CompilerIf #PB_Compiler_Unicode
        sscanf(*input_string,*format,*result) As "_swscanf"
        sprintf(*buffer,*format,arg1.d)       As "_swprintf"
    CompilerElse
        sscanf(*input_string,*format,*result) As "_sscanf"
        sprintf(*buffer,*format,arg1.d)       As "_sprintf"
    CompilerEndIf
EndImport

Procedure.d ValD2(string.s)
    Protected result.d
    sscanf(@string,@"%lf",@result)
    ProcedureReturn result
EndProcedure

Procedure.s StrD2(arg1.d)
    Protected string.s = Space(100)
    sprintf(@string,@"%.17lf",arg1)
    ProcedureReturn string
EndProcedure


Debug ""
Debug "PureBasic"
Debug ""

y.d = 0.333333333333333
Debug y
Debug "======="

a$ = "0.333333333333333"
x.d = ValD(a$)
Debug x

Debug ""
Debug "C Runtime Library"
Debug ""

y.d = 0.333333333333333
Debug StrD2(y)
Debug "======="

x.d = ValD2(a$)
Debug x

[srod]

Still comes down to the way the PB compiler interprets y.d = 0.333333333333333 though. Your ValD2() routine suggests that PB doesn't actually use a c-runtime function then to make this interpretation.