[GRASS-user] map algebra logical operators on DCELL rasters not admitted? maybe docs should outline it

[G. Allegri]

Thanks for the clear explanation Glynn. Maybe this behaviour could be
esplicited inside the docs...

giovanni
Il 22/ott/2015 23:35, "Glynn Clements" <glynn at gclements.plus.com> ha
scritto:

>
> G. Allegri wrote:
>
> > If logical operator is applied to, sat, Byte data it gives true/false
> > results, e.g. if(A && B,1, 0) behaves correctly even with cell values
> > values greater then 1.
> > That's why I expected it to work with floats operands too.
> > >From a user perspective if it works for a cell with value 2 it should
> also
> > work for a cell with value 2.0.
> > That'all.
>
> r.mapcalc's interpretation of booleans is borrowed from C. Booleans
> are just integers where zero is false and non-zero is true.
>
> But that doesn't necessarily make much sense for floats; the
> conventional wisdom is that comparing a float for equality with zero
> is usually a mistake, as results which should mathematically be equal
> to zero often aren't actually equal to zero due to intermediate
> rounding errors. In some cases, simply subtracting a number from
> itself won't yield zero (e.g. due to the x87 FPU using 80 bits for
> floating-point registers while values stored in memory use 32 or 64
> bits).
>
> You can obtain C-like behaviour by converting to an integer with the
> int() function then using that; the result is that any value between
> -1.0 and 1.0 (excluding both endpoints) will be considered false while
> anything else is true. Or you can convert to an integer with the
> round() function and use that, in which case any value between -0.5
> and 0.5 (including both endpoints) will be false while anything else
> is true. Or you can compare for equality with 0.0, in which case -0.0
> and 0.0 will be false while anything else is true. Or you can compare
> the magnitude (obtained using abs()) to an "epsilon" value so that
> anything smaller than the expsilon is false while anything larger is
> true.
>
> That's four different possibilities, none of which is any more
> "correct" than any of the others. Choosing one of them would just mean
> that r.mapcalc was probably using the "wrong" one, and doing so
> silently. The current behaviour forces the user to be explicit.
>
> --
> Glynn Clements <glynn at gclements.plus.com>
>
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