Can we be neighbors? / by Siobhán Cronin

When I interviewed for my recent gig with Uncountable, I was asked to talk about something I was working on. Given my now public swarm intelligence obsession, I talked about particle swarms.

I'm sure my eyes lit up as I talked about the stochastic initial positions, the velocity calculation, the difference between local and global best in the original algorithm, the position updating, and how convergence to optima compares to other stochastic search methods. But I was asked a question that stumped me that I resolved to dedicate some time to - in local best, when calculating neighbors, what would govern our choice of one distance measurement over another?

Here are four contenders for measuring the nearness of any two points in an n-dimensional real vector space with fixed cartesian coordinates:

As the name suggests, this is the square root of the sum of squares for each corresponding input pair of our points. 

The sum of the lengths of the projections of the line segment between the points onto the coordinate axes. Or, how many "city blocks" (i.e. coordinate units) lie between the two points. 

The minimum number of substitutions required to change one vector into another. Simply tally up how many input position pairs differ.

This generalizes the Euclidean and Manhattan distance metrics, allowing us to to set the sum of distance units (exponent of 1 on differences, with 1/1' exponent to norm the sum) or the triangular distance (exponent of 2 on differences, with 1/2 exponent to norm the sum). So you can toggle between the two to your heart's content. Would we ever want higher dimensionality?

This is all well and good, but what contexts would prompt us to use one distance measurement over another? In particular, given the context here is particle swarms, which should be used when implementing PSOs?

This work suggests we consider the dimensionality of our data, and make decisions accordingly. Is this being done in practice? There are so many parameters to tune with PSOs, is there a straightforward guide to making informed decisions for each? Hmm....I feel some sketchnoting coming on.