D - my apologies, "ptrdiff_t" was a C joke: it's just the type that a C compiler would assign to the difference between two pd array variables (as pointers).  it's neither a pd internal nor external object, and its only meaning is in terms of memory addresses; so it's probably not what you're looking for.  if you're talking not just about arrays of numeric values but also symbols, you might have to turn to python or lua to compute something like the Levenshtein distance (aka "string edit distance").  if your argument arrays are always the same length, you can probably twiddle together a fairly simple Pd patch to compute the Hamming distance.

marmosets,

  Bryan