README.md

OffsetArrays.jl

OffsetArrays provides Julia users with arrays that have arbitrary indices, similar to those found in some other programming languages like Fortran.

julia> using OffsetArrays

julia> y = OffsetArray{Float64}(undef, -1:1, -7:7, -128:512, -5:5, -1:1, -3:3, -2:2, -1:1);

julia> summary(y)
"OffsetArrays.OffsetArray{Float64,8,Array{Float64,8}} with indices -1:1×-7:7×-128:512×-5:5×-1:1×-3:3×-2:2×-1:1"

julia> y[-1,-7,-128,-5,-1,-3,-2,-1] = 14
14

julia> y[-1,-7,-128,-5,-1,-3,-2,-1] += 5
19.0

Example: Relativistic Notation

Suppose we have a position vector r = [:x, :y, :z] which is naturally one-based, ie. r[1] == :x, r[2] == :y, r[3] == :z and we also want to construct a relativistic position vector which includes time as the 0th component. This can be done with OffsetArrays like

julia> using OffsetArrays

julia> r = [:x, :y, :z];

julia> x = OffsetVector([:t, r...], 0:3)
OffsetArray(::Array{Symbol,1}, 0:3) with eltype Symbol with indices 0:3:
 :t
 :x
 :y
 :z

julia> x[0]
:t

julia> x[1:3]
3-element Array{Symbol,1}:
 :x
 :y
 :z

Example: Polynomials

Suppose one wants to represent the Laurent polynomial

6/x + 5 - 2*x + 3*x^2 + x^3

in julia. The coefficients of this polynomial are a naturally -1 based list, since the nth element of the list (counting from -1) 6, 5, -2, 3, 1 is the coefficient corresponding to the nth power of x. This Laurent polynomial can be evaluated at say x = 2 as follows.

julia> using OffsetArrays

julia> coeffs = OffsetVector([6, 5, -2, 3, 1], -1:3)
OffsetArray(::Array{Int64,1}, -1:3) with eltype Int64 with indices -1:3:
  6
  5
 -2
  3
  1

julia> polynomial(x, coeffs) = sum(coeffs[n]*x^n for n in eachindex(coeffs))
polynomial (generic function with 1 method)

julia> polynomial(2.0, coeffs)
24.0

Notice our use of the eachindex function which does not assume that the given array starts at 1.

Notes on supporting OffsetArrays

Julia supports generic programming with arrays that doesn't require you to assume that indices start with 1, see the documentation.