# [GRASS-dev] [GRASS-user] compare a DCELL and FCELL question

ming han dustming at gmail.com
Sun Jan 24 08:05:52 PST 2021

```Hi Everyone

Many thanks for your help. Is if(fabs(map_A - map_B) <= 1.0e-15, ... )
approach may increase the runtime compare to '==' way?

Thanks
Ming

Markus Metz <markus.metz.giswork at gmail.com> 于2021年1月24日周日 上午10:57写道：

> Trying to answer the original question: with a DCELL map "cat1_acc_riv"
> and a FCELL map "cat1_minacc", why is "float(cat1_acc_riv) ==
> float(cat1_minacc)" not equal to "int(cat1_acc_riv) == int(cat1_minacc)" ?
>
> int truncates to integer while float converts to single precision floating
> point. E.g. with cat1_acc_riv = 1.1 and cat1_minacc = 1.9,
> "float(cat1_acc_riv) == float(cat1_minacc)" becomes "1.1 == 1.9" whereas
> "int(cat1_acc_riv) == int(cat1_minacc)" becomes "1 == 1", thus the results
> are different.
>
> Another reason for possible differences is that float can only represent
> max 7 decimal digits. E.g. float(194320567) becomes 194320560 but
> int(194320567) preserves the value 194320567.
>
> Thus the safest is to cast everything to the type with the highest
> precision. In this case with FCELL and DCELL, use "double(cat1_acc_riv) ==
> double(cat1_minacc)" or even better the suggestion of Markus N.
>
> Markus M
>
>
> On Sun, Jan 24, 2021 at 3:51 PM ming han <dustming at gmail.com> wrote:
> >
> > Hi Markus and Micha
> >
> >      I am just trying to find grids have the same values in these two
> rasters, I will try the threshold approach.
> >
> > Thanks
> > Ming
> >
> > Markus Neteler <neteler at osgeo.org> 于2021年1月24日周日 上午6:58写道：
> >>
> >> Hi Ming,
> >>
> >> On Sun, Jan 24, 2021 at 10:49 AM ming han <dustming at gmail.com> wrote:
> >> >
> >> > Hi Micha
> >> >
> >> >      Here is the command I am using:
> >> >
> >> >      if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())
> >> >
> >> >       The str_r is a CELL raster. the result is different when I
> change it to:
> >> >        if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())
> >>
> >> Note that numerical "equality" is better tested with a threshold test
> >> against the map pixel difference.
> >> As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.
> >>
> >> Hence the test needs to be implemented in a different way, i.e. by
> >> using an epsilon.
> >> Essentially something like this:
> >>
> >> if(fabs(map_A - map_B) <= 1.0e-15, ... )
> >>
> >> In your case (untested):
> >> r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r ,
> null())
> >>
> >> See related discussions here: [1], [2] and elsewhere.
> >>
> >> [1] Comment by Glynn:
> https://trac.osgeo.org/grass/ticket/2854#comment:9
> >> [2] Comment by Glynn:
> >> https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html
> >>
> >> Best,
> >> Markus
> >
> > _______________________________________________
> > grass-dev mailing list
> > grass-dev at lists.osgeo.org
> > https://lists.osgeo.org/mailman/listinfo/grass-dev
>
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