core

It doesn't rearrange the array, it constructs a new one. While I'm not 100% sure about the J implementation* , K and related languages usually modify the array in place if there's only a single reference to it (so that mutation is still referentially transparent), to avoid extra allocation & copying. * It's now open-source at https://github.com/openj/core, but I have a hard time following their C style -- it's legal C, but written like J. APLs have memory allocators that are designed with arrays in mind, making this less expensive than you might expect. See this post I wrote about the memory management for Kona (https://github.com/kevinlawler/kona/), an open-source implementation of K: https://groups.google.com/forum/#!topic/kona-dev/fs5GoSBtF3Y... .