Struct with immutable arrays

New to Julia
1

I need to be able to keep certain arrays of a mixed type (e.g. ImmutableN.n::Array{Int,1}) from being modified. Here is an example:

struct ImmutableN
  n::Array{Int64,1}
  cond::Float64
  ImmutableN(n) = new(n,rand(1)[1])
end

check = ImmutableN([1,2])
check.n[1] = 2 # works (bad)

I googled “julia immutable numeric array” and found this post, but I don’t understand what is going on here. Can someone please clarify this or suggest areas of the documentation that I should check?

1 Like
2

You are not mutating check (which is immutable). You are mutating something referred to by check.n.

For example, you will get an error if you try to assign check.n to a new array:

check.n = [3,4,5]    # gives an error: check is immutable

In the C programming language, the analogous thing would be int * const n = ...... You can’t change the pointer n (e.g. you can’t do n = n + 1), but you can change the data that n points to (you can do n[0] = 3).

For a read-only array in Julia, see e.g. GitHub - bkamins/ReadOnlyArrays.jl: A wrapper type around AbstractArray that is read-only (heap-allocated, for big arrays) or https://github.com/JuliaArrays/StaticArrays.jl (non-heap-allocated, faster for small arrays).

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3

Thanks, this is helpful.