dictionary,equality,julia-lang , Julia's dictionary method `haskey` returning false when key is present


Julia's dictionary method `haskey` returning false when key is present

Question:

Tag: dictionary,equality,julia-lang

I am new to Julia and I am not sure why the last line evaluates to false:

type Point{T}
  x::T
  y::T
end

D = [Point(1.,2.) => 42]
haskey(D, Point(1., 2.))  #False!

Clearly the key exists so what's going on here!?

Edit.

If I don't use a class Point, it works fine:

D = [(1.,2.) => 42]
haskey(D, (1., 2.))  #True!

Can we not use classes for dictionary key types?


Answer:

But look at this:

type Point{T}
  x::T
  y::T
end
P = Point(1., 2.)
D = [P => 42]
haskey(D, P)

evaluates to true.

It does work if you use the same object, but it does not work if you use 2 objects with the same field values. Note that objects defined using type are mutable, so even after you used a Point as a key, you could have still changed the values of its fields without the dictionary knowing. The dictionary needs to hash on something that cannot change, like a type-object's identity, and not it's field values of the moment. Since tuples are immutable their values can be safely used as keys. You could also use

immutable Point{T}
 x::T
 y::T
end    

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