<div dir="ltr">Hi Everyone <div><br></div><div> Many thanks for your help. Is if(fabs(map_A - map_B) <= 1.0e-15, ... ) approach may increase the runtime compare to '==' way? </div><div><br></div><div>Thanks</div><div>Ming </div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Markus Metz <<a href="mailto:markus.metz.giswork@gmail.com">markus.metz.giswork@gmail.com</a>> 于2021年1月24日周日 上午10:57写道：<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>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)" ?<br></div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>Markus M<br></div><div><br></div><div><br>On Sun, Jan 24, 2021 at 3:51 PM ming han <<a href="mailto:dustming@gmail.com" target="_blank">dustming@gmail.com</a>> wrote:<br>><br>> Hi Markus and Micha<br>><br>> I am just trying to find grids have the same values in these two rasters, I will try the threshold approach.<br>><br>> Thanks<br>> Ming <br>><br>> Markus Neteler <<a href="mailto:neteler@osgeo.org" target="_blank">neteler@osgeo.org</a>> 于2021年1月24日周日 上午6:58写道：<br>>><br>>> Hi Ming,<br>>><br>>> On Sun, Jan 24, 2021 at 10:49 AM ming han <<a href="mailto:dustming@gmail.com" target="_blank">dustming@gmail.com</a>> wrote:<br>>> ><br>>> > Hi Micha<br>>> ><br>>> > Many thanks for your reply.<br>>> > Here is the command I am using:<br>>> ><br>>> > if(float(cat1_acc_riv) == float(cat1_minacc), str_r, null())<br>>> ><br>>> > The str_r is a CELL raster. the result is different when I change it to:<br>>> > if(int(cat1_acc_riv) == int(cat1_minacc), str_r, null())<br>>><br>>> Note that numerical "equality" is better tested with a threshold test<br>>> against the map pixel difference.<br>>> As the threshold, we use GRASS_EPSILON which is defined as 1.0e-15.<br>>><br>>> Hence the test needs to be implemented in a different way, i.e. by<br>>> using an epsilon.<br>>> Essentially something like this:<br>>><br>>> if(fabs(map_A - map_B) <= 1.0e-15, ... )<br>>><br>>> In your case (untested):<br>>> r.mapcalc diffepsilon = if( abs( map_A - map_B) <= 1.0e-15, str_r , null())<br>>><br>>> See related discussions here: [1], [2] and elsewhere.<br>>><br>>> [1] Comment by Glynn: <a href="https://trac.osgeo.org/grass/ticket/2854#comment:9" target="_blank">https://trac.osgeo.org/grass/ticket/2854#comment:9</a><br>>> [2] Comment by Glynn:<br>>> <a href="https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html" target="_blank">https://lists.osgeo.org/pipermail/grass-user/2015-October/073200.html</a><br>>><br>>> Best,<br>>> Markus<br>><br>> _______________________________________________<br>> grass-dev mailing list<br>> <a href="mailto:grass-dev@lists.osgeo.org" target="_blank">grass-dev@lists.osgeo.org</a><br>> <a href="https://lists.osgeo.org/mailman/listinfo/grass-dev" target="_blank">https://lists.osgeo.org/mailman/listinfo/grass-dev</a></div></div>
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