Assigning a value to an element of a tuple in Julia

Question

I am facing a problem with tuples in Julia. Here is what I am trying to do

julia> temp = [1 2 3]
julia> A = tuple(temp, temp)
julia> B = tuple(A,A,A)

B looks like this :

julia> B
((
[1 2 3],

[1 2 3]),(
[1 2 3],

[1 2 3]),(
[1 2 3],

[1 2 3]))

I understand I can access the 'very' first element in B by B[1][1][1] which returns 1 (as expected)

But, if I try to assign a particular value to B[1][1][1], say if I do, julia> B[1][1][1] = 20, this is what I get,

julia> B
((
[20 2 3],

[20 2 3]),(
[20 2 3],

[20 2 3]),(
[20 2 3],

[20 2 3]))

The first elements of all the sub-tuples have been changed. Is there a way to change the value of B[1][1][1] without affecting the other sub-tuples ??

Thanks in advance.

PS : I'm using Julia 0.5.0 on Ubuntu 16.04 (64-bit)

Answer 1

This is not quite a problem with tuples. Exactly the same thing will happen if you use arrays.

A = (temp, temp) will reference the same address in memory twice. So you need to do A = (copy(temp), copy(temp))

But ...

copy(x)

Create a shallow copy of x: the outer structure is copied, but not all internal values. For example, copying an array produces a new array with identically-same elements as the original.

So for B = (A, A, A) we need to use deepcopy in order to get the values (rather than the references) of each variable. i.e. B = (deepcopy(A), deep copy(A), deepcopy(A))

Check out the docs for copy and deepcopy here: http://docs.julialang.org/en/release-0.4/stdlib/base/#Base.copy

Answer 2

A = tuple(temp, temp)

You make this a tuple of all the same arrays (note you can just write A = (temp,temp)). If you want to use a copy of the array, use A = (copy(temp),copy(temp)). The value of an array is its reference, and thus when you do A = (temp,temp), you just have two references to the same slab of memory, which is why when you change one, the other changes.