Periodic or cyclic arrays possible?

[Ferran Mazzanti]

Hi folks,

I was wondering if it is possible to use in a simple way cyclic arrays in Julia? What I'm after is sometbing that understands that the next element in a[] after end is a[1], so a[end+1]=a[1], a[end+2]=a[2] etc... I know I can index the array with the remainder operator % to achieve this same result, but I wonder if one can declare the array directly in one way or another to achieve this directly.

Thanks in advance,

Ferran.

[Cedric St-Jean]

You can define our own datatype to do this. It's one of the most fundamental tasks in Julia!

immutable CircularArray{T}
    arr::Vector{T}
end

Base.getindex(ca::CircularArray, i) = ca.arr[(i-1) % length(ca.arr) + 1]
Base.setindex(...) = ...
...

a = CircularArray([1,2,3])
a[14] # yields 2

Cédric

[Erik Schnetter]

The expression `(i-1) % length + 1` is not correct for negative `i`.
You have to use `mod` instead of `%`.

Julia has a function `mod1(x,y)` that is essentially defined as
`mod(x-1, y) +1`, so that's what you probably want to use.

-erik

--
Erik Schnetter <schn...@gmail.com>
http://www.perimeterinstitute.ca/personal/eschnetter/

[Matt Bauman]

I highly recommend reading the Interfaces chapter[1] for a walk-through on creating your own array type.  Another great trick that you can use is `A[mod1(x, end)]`.

1. http://docs.julialang.org/en/release-0.4/manual/interfaces/