The mutation function changes all the values in a dictionary

General Usage
1

Hi everyone.
I am using a mutate function that changes the value of an array and then I save the array to a dictionary.
The problem is that the function changes all the values saved in the dictionary.

a = Dict()
b = ones(2,2)

function update_b!(b)
    b .= b .+1
end

for i in 1:2
    update_b!(b)
    a[i] = b
end

The result of a:

  2 => [3.0 3.0; 3.0 3.0]
  1 => [3.0 3.0; 3.0 3.0]

I would like something like:

  2 => [3.0 3.0; 3.0 3.0]
  1 => [2.0 2.0; 2.0 2.0]

Using the function with a return fixes the problem, but I would like to know if the problem can be fixed with a mutable function.

2

You are assigning the same object here to the left hand side. No copy is made implicitly. Changing b is thus reflected everywhere it is referenced.

Either assign a[i] = copy(b), or construct the dictionary like

a = Dict(i=>ones(2,2).+i for i in 1:2)
1 Like
3

Thank you very much, it worked perfectly.

4

Great! Then please mark my answer as the solution if you are happy with it and have no further questions, so others will know the topic needs no more immediate attention.