# How to prevent matrices decaying into vectors?

I have a function f that can take matrices of various sizes as inputs. When I try to call it with matrices of size d \times 1, produced by m[:, k] where k is an integer, they get converted to vectors of size d. How to prevent this? This must have been discussed many times but I could not find any solutions. Below is an MWE:

function f(x1, x2)
d, n1 = size(x1)
d, n2 = size(x2)

out = zeros(d, n1 * n2)
end

function test()
x1 = randn(2, 3)
x2 = randn(2, 5)

# Works fine
y = f(x1, x2[:, 1:2])

# Does not work
y = f(x1, x2[:, 1])

display(y)
end

test()

Like this:

julia> A = randn(2,2)
2Ă—2 Array{Float64,2}:
2.66127  0.568908
-1.40935  0.118752

julia> A[:,1]
2-element Array{Float64,1}:
2.6612671809730895
-1.409351022092897

julia> A[:,1:1]
2Ă—1 Array{Float64,2}:
2.6612671809730895
-1.409351022092897
3 Likes

Many routines in Base accept either a matrix or a vector, where the latter is treated as a matrix with one column. This is a pretty convenient pattern for linear-algebra code.

The way to do this is to simply pass dimension arguments to size, for example changing your code above to:

d, n1 = size(x1,1), size(x1,2)

The size function returns 1 for dimensions after the last one, so size(x1,2) will be 1 if x1 is a vector (1d array). Note if x1 is a vector, you are also allowed to index it as x1[i,1], i.e. you can pass 1 for â€śextraâ€ť dimensions, to treat it as a matrix with one column.

You can even declare this explicitly in your argument signature as function f(x1::AbstractVecOrMat, x2::AbstractVecOrMat), where AbstractVecOrMat{T} is a built-in type alias for Union{AbstractVector{T},AbstractMatrix{T}}.

I just tried this and was a little surprised that we throw an error:

julia> v = rand(5);

julia> d1, d2 = size(v)
ERROR: BoundsError: attempt to access (5,)
at index [2]

We could potentially allow it and just keep returning 1 as long as someoneâ€™s asking.

2 Likes

Specifically for size? How would that generalize?

1 Like

I have the same question as @tkoolen : size(v) is an expression that has a value regardless of the LHS, how could it know how long to run?

I consider automatic expansion by 1s a wart in the linear algebra design, but I am fine with it on demand. Eg expandedsize(v, n) could

1. expand with 1s whenever ndims(v) <= n,
2. error otherwise.

This would still allow combining code for vectors and matrices.

Yeah, itâ€™s probably a bad idea now that I think about it a bit more. It would require returning a special Dims object that acts like an infinite tuple.

We need to be able to dispatch on the number of return values.

As I understand it, multiple return values are basically returned as a Tuple - do you mean dispatching on the return type?

It was mostly a joke (https://se.mathworks.com/help/matlab/ref/nargout.html).

2 Likes