Extended array indexing

Given an array, it is possible to imagine a virtual array formed by padding the original array out in all directions. Valid subscripts for this virtual array range through all the negative and positive integers, without regard to the size of the original. This function implements this idea.

The padding out can be done in many ways. The following are implemented: using a constant value; replicating the border elements of the original; tiling the virtual array with copies of the original; tiling the virtual array with copies that have been flipped so that there is mirror symmetry along the borders of every copy. A different rule may be specified for each dimension.

The function is related to padarray, repmat and circshift, and padarray's option strings have been retained for consistency. However, exindex is more flexible than any of these; for example, the array can be cropped as well as extended, and indeed arbitrary indexing into the virtual array is allowed.

The function is easy to use:

exindex(arr, s1, s2, ...)

is equivalent to v(s1, s2, ...) where v stands for the virtual extended array. If one rule is to apply to all dimensions it can be specified with

exindex(arr, s1, s2, ..., rule)

or if different rules apply to different dimensions they can be specified with

exindex(arr, s1, r2, s2, r2, ...)