Hello, I’m a very early beginner. I wanted to have an array of dictionaries. Simple example below. It seems that when I push a dictionary which does not exactly match the type of array, instead of, say, getting a complaint from the compiler, the dictionary is copied. Fine, but naturally one wo…

Please quote your code (you can edit your post), and perhaps don’t make all text bold, it is a distraction. [image] PSA: how to quote code with backticks Meta Discussion This is a short post on how to use backticks (` and ```) to quote code, so it is easy to r…

I’ve gone ahead and quoted @jbaxter ’s code since otherwise it was impossible to cut and paste into a REPL to try to help answer the question.

The key issue is that Dict{String, Union{Int, Nothing}} and Dict{String, Int} are different, disjoint types. In order to put a Dict{String, Int} value into an array with element type Dict{String, Union{Int, Nothing}} it must be converted to a value of the correct type, which means copying it. If you…

A tangential point: debug prints like these [image] jbaxter: println("x == a[1] is now "* string(x == a[1])) can be easier with the @show macro, which saves you the trouble of writing the expression again inside a string literal.

thanks for all these points! Fewer explicit types seems to be best for me.

I’m trying now to take a general lesson from this example, to help write correct programs in the future. I know Julia uses pass by sharing, but something else is evidently involved here. Is there a general rule about when Julia makes a copy of a mutable value instead of passing a reference, or issui…

[image] jbaxter: I know Julia uses pass by sharing No. For immutables, it is actually undefined what happens under the hood, when the user can’t tell the difference. This gives the compiler room for optimizations, see eg a recent discussion . [image] jbaxter: Is there a general rule about…

That is very helpful. I read some of the linked manual section, especially “conversion versus construction”. For me the key point was not whether the operation is called “copying”, but whether a new object is created, as in my example it seems to be. In the manual it says that since convert can…

Implicit conversion of mutable types is a bit dangerous since it amounts to implicit copying. Can’t be changed now but worth keeping in mind for Julia 2.0. Then again people love the convenience of implicit conversion based on types locations, especially since Julia is very careful about conversions…

Unexpected copy when pushing

New to Julia

Tamas_Papp March 5, 2019, 3:14pm 2

Please quote your code (you can edit your post), and perhaps don’t make all text bold, it is a distraction.

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Unexpected copy when pushing

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