matrix | elegant Windows screensaver of the green Matrix | Graphics library

You do not need loops here. Just use the whole matrix in your call to x>5

Have you tried FLANN?

This code doesn't solve your problem completely. It simply finds the nearest 50 neighbors to each point in your 500000 point dataset:

You can use rrapply::rrapply:

This is essentially what singletons are there for. That's a value-level witness for a typeclass that gives you access to this (conceptually reduntant) information that every number can in fact be described in the standard form. A minimal implementation:

Currently (2022-02-05), CRAN builds R binaries for Apple silicon using Apple clang (from Command Line Tools for Xcode 12.4) and an experimental build of gfortran.

If you obtain R from CRAN (i.e., here), then you need to replicate CRAN's compiler setup on your system before building R packages that contain C/C++/Fortran code from their sources (and before using Rcpp, etc.). This requirement ensures that your package builds are compatible with R itself.

A further complication is the fact that Apple clang doesn't support OpenMP, so you need to do even more work to compile programs that make use of multithreading. You could circumvent the issue by building R itself and all R packages from sources with LLVM clang, which does support OpenMP, but this approach is onerous and "for experts only". There is another approach that has been tested by a few people, including Simon Urbanek, the maintainer of R for macOS. It is experimental and also "for experts only", but seems to work on my machine and is simpler than trying to build R yourself.

Warning: These instructions come with no warranty and could break at any time. They assume some level of familiarity with C/C++/Fortran program compilation, Makefile syntax, and Unix shells. As usual, sudo at your own risk.

I will try to address compilers and OpenMP support at the same time. I am going to assume that you are starting from nothing. Feel free to skip steps you've already taken, though you might find a fresh start helpful.

I've tested these instructions on a machine running Big Sur, and at least one person has tested them on a machine running Monterey. I would be glad to hear from others.

1. Download an R binary from CRAN here and install. Be sure to select the binary built for Apple silicon.

2. Run

Not a perfect solution, I must admit. But it's simple and comes pretty close.
I create 5 vectors that are gonna span the space of the matrix and create random linear combinations to fill the rest of the matrix. My initial thought was that a trivial solution will be to copy those vectors 20 times.
To improve that, I created linear combinations of them with weights drawn from a uniform distribution, but then the distribution of the entries in the matrix becomes normal because the weighted mean basically causes the central limit theorm to take effect.
A middle point between the trivial approach and the second approach that doesn't work is to use sets of weights that favor one of the vectors over the others. And you can generate these sorts of weight vectors by passing any vector through the softmax function with an appropriately high temperature parameter.
The distribution is almost uniform, but the vectors are still very close to the base vectors. You can play with the temperature parameter to find a sweet spot that suits your purpose.

I think you might be slightly overcomplicating things. Ideally, you'd just want a single key drawing method for the whole layer. However, because you're using a Stat to do the majority of calculations, this becomes hairy to implement. In my answer, I'm avoiding this.

Let's say I'd want to use a geom-only implementation of such a layer. I can make the following (simplified) class/constructor pair. Below, I haven't bothered width_scale or height_scale parameters, just for simplicity.

C++ Copy Constructors: must I spell out all member variables in the initializer list?

Yes, if you write a user defined copy constructor, then you must write an initialiser for every sub object - unless you wish to default initialise them, in which case you don't need any initialiser - or if you can use a default member initialiser.

the object contains some member variables which are pointers to its other member variables)

This is a design that should be avoided when possible. Not only does this force you to define custom copy and move assignment operators and constructors, but it is often unnecessarily inefficient.

But, in case that is necessary for some reason - or custom special member functions are needed for any other reason - you can achieve clean code by combining the normally copying parts into a separate dummy class. That way the the user defined constructor has only one sub object to initialise.

Like this:

One option could be: