Scientific computing

(for the rest of us)

Dispatch on element types

In this lesson, we will see how we can dispatch on parametric types, in order to have a fine-grained control on what method is used for different types of data collections. This is a core design pattern in Julia, and we will illustrate it by building some functions related to measuring the distances between points.

In the previous modules, we have learned that (i) collections have a type, that is often a parameter of the type of the elements they contain, and (ii) methods are dispatched based on the type of their arguments. From this, we can build functions that are specialized to the content of a collection:

function whatsinthebox(x::Array{T, N}) where {T, N}
    return "This collection has $(N) dimension$(N>1 ? "s" : "") and stores $(T) elements"
whatsinthebox (generic function with 1 method)

Let us first see whether this function works, and then we will take a bit of time to explain what is going on.

whatsinthebox(rand(Float64, 4))
"This collection has 1 dimension and stores Float64 elements"
whatsinthebox(rand(Float64, 4, 3))
"This collection has 2 dimensions and stores Float64 elements"
whatsinthebox(rand(Float64, 4, 3, 5))
"This collection has 3 dimensions and stores Float64 elements"

The part of this function signature that makes all of this happens is the where statement: it states that the type of the argument is Array{T, N}, but that we care about T and N (essentially). But we can do something more interesting with this, because we can add conditions to the types of T and N.

There is a simple example we can build here, related to pairwise distances. Assuming we have a number of points, represented by a vector of numbers giving their position in some space, we might want to calculate the distance between each consecutive points. This is, in fact, the basis for the travelling salesperson problem!

But the nature of how the information is represented should give use some type of clue as to what the proper distance function is. For example, integer positions might represent intersections between streets (and so we care about the taxicab, Manhattan, snake, city-block, … distance); floating point positions are likely to represent the position on a plane, and the Euclidean distance is enough.

Let us generate a matrix giving the series of successive positions in a 3d space:

M = rand(Float64, (3, 8))
3×8 Matrix{Float64}:
 0.798833   0.722977   0.457948  0.0276315  0.198074  0.391967  0.563174  0.158679
 0.0090282  0.0913964  0.84911   0.630704   0.480115  0.884138  0.796374  0.872747
 0.543308   0.951015   0.537437  0.0824887  0.608526  0.371304  0.376383  0.464377

And let us also generate a matrix giving the positions in a 3d space made of street intersections:

P = rand(1:10, (3, 8))
3×8 Matrix{Int64}:
 3  5  4  8  3  5  9  9
 3  7  7  9  9  8  7  1
 8  3  3  8  5  6  2  3

Perfect! Now, what would a distance function for such data look like? It would take two arrays of numbers as arguments, and return a number of the same type.

function distance(p1::Vector{T}, p2::Vector{T})::T where {T <: Real}
    return sqrt(sum((p1 .- p2).^(2.0)))
distance (generic function with 1 method)

We can try this on the matrix of continuous positions:

distance(M[:,1], M[:,2])

This would not work on the matrix of discrete positions. The first reason is that this is not the distance measure we care about (but the code has no way to know about that), and the second reason is that the return point would not be an integer! And so, we need to define a method for distance that would specifically return the taxicab distance:

function distance(p1::Vector{T}, p2::Vector{T})::T where {T <: Integer}
    return sum(abs.(p1 .- p2))
distance (generic function with 2 methods)

If we call this function on vectors from the matrix with discrete positions:

distance(P[:,1], P[:,2])

Now that we have defined this function, we can wrap everything up to return the total travel length corresponding to an array of successive positions:

function travel(m::Array{T, N})::T where {T <: Number, N}
    travel_length = zero(T)
    for i in axes(m, 2)[2:end]
        travel_length += distance(m[:,(i-1)], m[:,i])
    return travel_length
travel (generic function with 1 method)

And there we have it! This module is scratching the surface of what is possible when dispatching on parametric types. We will make use of these capacities in the later modules.