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SKILL arithmetic comparisons

sri125

 SKILL is notorious for the way it stores numbers in memory. e.g variable w, which

should be 0.4 gets stored as 0.3999999999999 or something like that.

 

So when you use it in 'if-else' loop like:

if( w == 0.4 then 

    block good;

else

   block bad;

 )

 

You will end up in block bad unless you do something like

 

abs( w - 0.4 ) < 0.000001 

or something like above.

 

Is there an elegant way to handle this in SKILL ?

 

thanks

 

 

 

Parents
  • As Lawrence says, you get exactly the same kind of rounding errors in C and all languages using IEEE floating point numbers. In fact whenever this comes up with someone saying this is a bug in SKILL, I show a C program with the same behaviour.

    The trick is to write a function to do floating point "equality" checks. If you know the magnitude of the numbers, you can use a fixed size "epsilon" as in your example.

    Or you can do something like:

    procedure(myCompare(a b @key (reltol 1e-6) (abstol 1e-12))
    let((tol)
    tol=reltol*max(abs(a) abs(b))+abstol
    abs(a-b) < tol
    )
    )

    This has both a relative and absolute tolerance in the comparison.

    Regards,

    Andrew
Reply
  • As Lawrence says, you get exactly the same kind of rounding errors in C and all languages using IEEE floating point numbers. In fact whenever this comes up with someone saying this is a bug in SKILL, I show a C program with the same behaviour.

    The trick is to write a function to do floating point "equality" checks. If you know the magnitude of the numbers, you can use a fixed size "epsilon" as in your example.

    Or you can do something like:

    procedure(myCompare(a b @key (reltol 1e-6) (abstol 1e-12))
    let((tol)
    tol=reltol*max(abs(a) abs(b))+abstol
    abs(a-b) < tol
    )
    )

    This has both a relative and absolute tolerance in the comparison.

    Regards,

    Andrew
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