[jodrellblank]

> “What would the APL code look like that had a similar off-by-one error? Would it be easy to spot by reading the code?”

It kinda does have an off by one error; (Dyalog) APL defaults to indexing from one and the code assumes indexing from zero. The blog author isa well known APL-er and other “modern APL” users often assume indexing from zero. And the change/fix is the quad-io in my other comment so it’s not easily visible in the code because it’s not in the code at all but it is an easy fix.

At least, that’s one kind of off-by-one error; APL doing whole array transforms in each operation tends not to have as much room for off-by-one errors when you aren’t counting through things in the typical imperative way.