buildMatrix

[Ducasse]

buildMatrix: aCollection
	"Answer a <Collection> of points that are in the same vs different groups.
	Different groups are 1, same groups are 0"

	^ aCollection collect: [ :index | 
		(index - aCollection) abs 
			collect: [ :each | each min: 1 ] ]

I do not get this code what means index - aCollection ? this code totally confuses me.
So we should add a comment about this implementation

[hernanmd]

This is a method to build matrix where each cell has 0 if the two input vectors classified a pair in the same cluster, and 1 if there is no match. IIRC the same can be achieved building a contingency matrix - and probably it would be more easier to read, but as this Rand Index seems to be more theoretical than practical (this is explained in the Chapter) I moved to Adjusted Rand Index.