[Octave-bug-tracker] [bug #53311] Need a function to directly compute npr.

[Mike Miller]

Follow-up Comment #7, bug #53311 (project octave):

I am only offering my opinion as a member of the Octave community. I am
leaving this open for now for others to weigh in.

My opinion is that this function should not be added to Octave, mainly for
compatibility reasons.

There are two compatibility reasons to consider:

1. When Octave invents a new function that is not already in Matlab, there is
a potential for Matlab to create a function in the future that conflicts with
it.

The mitigation for this is that Octave chooses names that are not likely to be
used by Matlab. For this case, I would suggest a name like
"num_ordered_subsets" or "num_sequences_without_repetition".

2. Matlab does not provide the function that you are looking for, so anyone
familiar with Matlab or who wants to write scripts that are compatible with
Matlab already must express this in terms of factorial, nchoosek, or
pochhammer. Since Octave already supports all of those functions, we are fully
compatible. If Octave introduces a new function, users who want to remain
Matlab compatible will have to avoid using it.

Additionally, I am not a mathematician, but I gather there is some naming
confusion about permutations vs ordered subsets vs sequences without
repetition. If this function were added to Octave, it would probably be best
to avoid using the word permutation for this function, because permutations
are orderings of the complete set, not subsets, i.e. factorial(n).

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