Re: [julia-users] Help with a macro

[Mauro]

> Hello everyone,
>
> I am trying to write a macro that transforms an expression like:
>
> @foo v[1,2] + v[1,3]*v[2,4] -> v[I[1,2]] + v[I[1,3]]*v[I[2,4]]
>
> Basically, for each getindex I want to insert a lookup of the index in some
> other variable.
> The reason for this is that I am working with matrices encoded as vectors
> and I want to still call them like matrices. The "I" vector in the code
> is the lookup from two dimensional indexing to one dimensional.
>
> v is just a normal Vector so I cannot extend getindex which is why I
> thought a macro would be the cleanest solution.
>
> I am a bit lost how to start so any help would be appreciated.

Sounds like you may want to use
https://github.com/mbauman/RaggedArrays.jl (or my more primitive
Ragged.jl)

If not, this may work:

function sub!(e::Expr)
    if e.head==:ref
        for a in e.args
            sub!(a)
        end
        newe = :($(e.args[1])[I[$(e.args[2:end]...)]])
        e.args = newe.args
    else
        for a in e.args
            sub!(a)
        end
    end
    e
end
sub!(s) = s

e = :(v[1,2] + v[1,3]*v[2,4])
sub!(e)
macro foo(e)
    sub!(e)
end