Select elements in a list of tuples of quaternions that satisfy a condition

Question

I defined 24 quaternions using the Quaternions package and i create the list of all possible 4-tuples using

ListQuad = Tuples[ListQuaternions, 4]

where ListQuaternions is the list of the 24 elements I defined. Now I would like to select all the 4-tuples such that the product of the four quaternions is equal to Quaternion[1, 0, 0, 0].

I tried to use this:

Select[ListQuad,{x_, y_, z_, w_} /; x ** y ** z ** w == Quaternion[1, 0, 0, 0] &]

but this gives me an empty list, which I'm sure it can't be true because I know that some 4-tuples satisfy that condition.

I don't understand what I'm doing wrong. Probably the condition I put in the Select command?

Answer 1

Henrik has already mentioned Select[] and Cases[]. For this answer, I'll show how to use Pick[]:

<< Quaternions`

ListQuaternions = UnitQuaternions;
ListQuad = Tuples[ListQuaternions, {4}];

prods = NonCommutativeMultiply @@@ ListQuad;

sel = Pick[quads, Map[Norm, prods - Quaternion[1, 0, 0, 0]], 0];

{Length[ListQuad], Length[sel]}
   {331776, 13824}

The slightly complicated condition in Pick[] is necessitated by the fact that Equal (==) cannot handle quaternions; e.g. Quaternion[1, 0, 0, 0] == Quaternion[-1, 0, 0, 0] remains unevaluated.