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structjulia
Mr.CG
Mr.CG

Reputation: 69

How to define a constant in a struct in Julia

I want to define a struct:

struct unit_SI_gen
    x::Float32
    const c = 2.99792458e8
    speed(x)=c*x
end

However, it raises an error:

syntax: "c = 2.99792e+08" inside type definition is reserved

I know I can't use struct as a class in Python, but I cannot find how to solve this problem.

How to define a constant in a struct?

Upvotes: 0

Views: 3334

Answers (2)

Bogumił Kamiński
Bogumił Kamiński

Reputation: 69949

Given I agree with what was said above about normal usage of struct in Julia, it is actually possible to define what was requested in the question using an inner constructor:

struct unit_SI_gen{F} # need a parametric type to make it fast
    x::Float32
    c::Float64 # it is a constant across all unit_SI_gen instances
    speed::F # it is a function

    function unit_SI_gen(x)
        c = 2.99792458e8
        si(x) = c*x
        new{typeof(si)}(x, c, si)
    end
end

Upvotes: 6

carstenbauer
carstenbauer

Reputation: 10157

I second @Tasos's comment, you should probably make yourself familiar with Julia's structs first. The relevant part of the documentation is probably here.

Since you declared your struct as struct (in contrast to mutable struct) it is immutable and hence all (immutable) fields of the struct are constants in the sense that they can't be changed.

julia> struct A
       x::Int
       end

julia> a = A(3)
A(3)

julia> a.x = 4
ERROR: type A is immutable
Stacktrace:
 [1] setproperty!(::A, ::Symbol, ::Int64) at .\sysimg.jl:19
 [2] top-level scope at none:0

Note, that they get their unchangeable value in the construction process and not in the struct definition.

Also, methods should generally live outside of the struct definition.

Upvotes: 3

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