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Hi Ben, On Fri, 15 May 2020 at 18:07 Benjamin Root <ben.v.root@xxxxxxxxx> wrote: > The documentation for the strided access to variables specify a pointer to > ptrdiff_t types, which I infer means that I could specify a negative value > to read/write in a dimension in reverse. However, when I gave this a shot, > specifying a -1 for the latitude dimension of one of my datasets, the C > api came back and said I had an illegal stride. > > Is there any way to read/write dimensions in reverse order, much like > python's [::-1] with the C-API, or am I going to have to flip the data > around myself (I'd like to avoid the copy, as well as all of the fun code > to do so for an arbitrary dimension). At the very least, are there helper > functions for this? That's not supported by the C API: https://github.com/Unidata/netcdf-c/blob/64d821b/libdispatch/dvarget.c#L245-L248 https://github.com/Unidata/netcdf-c/blob/b0e0d81/libhdf5/hdf5var.c#L1845-L1847 Flipping the data around doesn't necessarily require a copy; it can be done in-place with a complexity of N/2 swaps (`std::reverse` in C++). But yes doing it for an arbitrary dimension rather than the full array will require some index calculations. I think something like this might do the job, albeit not the most efficient approach (untested): 1. Get the variable values along that dimension, and reverse. 2. Flatten the N-D indexes into 1-D indexes by calculating an inner product with the strides. In NumPy this is `ravel_multi_index`; in C++ I use `std::inner_product` in <numeric>. 3. Copy the reversed elements back into the original array using the 1-D indexes. I have some code here in a simple `get_var1` iterator which does the reverse operation, unraveling a 1-D index into an N-D index given the shape (`unravel_index` in NumPy). https://github.com/jbuonagurio/ncpp/blob/717bb1e/include/ncpp/variable.hpp#L421-L495 I'm sure there are more clever ways. I would use a multidimensional 'array view' library if you can and your data are stored contiguously. I mostly work in C++ so not sure about other options, but I know that xtensor and the reference implementation of the `std::mdspan` proposal support arbitrary strides. - John Buonagurio
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