Hi Luís, čt 11. 4. 2024 v 8:26 odesílatel Luís Moreira de Sousa via grass-user <grass-user@lists.osgeo.org> napsal: > I am working with a raster with very large integers. That are stored as 64 > bit floats (imported from a GTiff). I wish to extract part of these values > with an integer division. The first approach was: > > r.mapcalc 'short_ints = int(long_ints)/1000000' > > However, this results in something that is not an integer division. In fact I > have no idea what the result is. Eventually, I succeeded with the following > formulation: > > r.mapcalc 'psu_ids = int(ssu_ids/1000000)' > > I have been going through the manual back and forth and can't understand why > these two expressions produce different results. Any insights?
This is probably due to the 32-bit size of `int()`. If your values are higher than `2 ** 31 -1` (i.e. 214783467; `31` because half of the numbers go to the negatives; `-1` because there is also NULL), the counter starts to rotate. It means that `2 ** 31` will be NULL and `2 ** 31 + 1` will be `- (2 ** 31 - 1)`. You can encounter this behaviour also in other software, such as in `numpy`. Indeed, there is no mention of that in the docs. I will update it today to include this information. It is true that it's easy to break your teeth on this. Cheers, Ondra _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org https://lists.osgeo.org/mailman/listinfo/grass-user
