How do I concretely type a field in a struct dynamically?

General Usage
1

Hello!

For performance reasons I have to concretely type this field in my struct;

struct Test{I,D}
    Stencil::Vector{Tuple{repeat([I],D)...}}
end

I’ve tried to do as follows but it errors out. “I” is the type I wish to use, in this case Int, and “D” is the dimensions, for example two or three. In this case I would like it to produce, dynamically:

Stencil::Vector{Tuple{I,I}}
# or
Stencil::Vector{Tuple{I,I,I}}

The type has to be concretely typed as this else I get a 100x performance penalty in my specific code.

Can one do this or do I need macro’s?

Kind regards

2

Might NTuple{D, I} be the type you are looking for?

1 Like
3

Indeed I was!

Thank you :slight_smile:

I found this out just a minute after your reply. I had to wrangle it out of ChatGPT haha. Here conversation is included for fun!

Me : Hello! For performance reasons I have to concretely type this field in my struct; struct Test{I,D} Stencil::Vector{Tuple{repeat([I],D)…}} end I’ve tried to do as follows but it errors out. “I” is the type I wish to use, in this case Int, and “D” is the dimensions, for example two or three. In this case I would like it to produce, dynamically: Stencil::Vector{Tuple{I,I}} # or Stencil::Vector{Tuple{I,I,I}} The type has to be concretely typed as this else I get a 100x performance penalty in my specific code. Can one do this or do I need macro’s? Kind regards

ChatGPT: Yes, you can achieve this in Julia by using type parameters and constructing the type inside the struct. Here’s an example:

struct Test{I, D}
    vec::Stencil.Vector{Tuple{repeat(I, D)...}}
end

julia> Test{Int, 2}
Test{Int64,2}

julia> Test{Int, 3}
Test{Int64,3}

In this example, I is the type parameter that you want to use (e.g., Int), and D is the number of dimensions (e.g., 2 or 3). The repeat function is used to repeat the type I D times, and the resulting type is Tuple{I, I} or Tuple{I, I, I}, depending on the value of D. This type is then used as the type parameter for the vec field of the Test struct.

Me: Stencil is the name of the field, not a function.

ChatGPT: My apologies, I misunderstood the name of the field. Here’s an updated version of the code that correctly uses Stencil as the field name:

struct Test{I, D}
    Stencil::Stencil.Vector{Tuple{repeat(I, D)...}}
end

julia> Test{Int, 2}
Test{Int64,2}

julia> Test{Int, 3}
Test{Int64,3}

Me: No you are still doing it the wrong. This is the correct version: struct Test{I,D} Stencil::Vector{Tuple{repeat([I],D)…}} end But this code gives an error

ChatGPT: Ah, I apologize for the mistake. The correct version is indeed as you wrote:

struct Test{I,D}
    Stencil::Vector{Tuple{repeat([I],D)...}}
end

In this case, repeat takes two arguments: the first one is [I], which creates an array with a single element I, and the second argument is D, which specifies the number of times to repeat [I]. The ... operator then spreads the elements of the repeated array into a tuple, creating Tuple{I,I,...,I} with D elements.

Me: Yes, but this cannot be called in a type declaration?

ChatGPT: Yes, you are correct. The repeat function cannot be used directly in a type declaration. To construct a type with repeated elements, you can use a combination of type parameters and type composition. Here’s an example that demonstrates how to achieve this:

struct Test{I,D}
    Stencil::Vector{NTuple{D, I}}
end

In this example, NTuple{D, I} is used to create a Tuple with D elements, all of type I. The type parameter D allows you to specify the number of dimensions, and I allows you to specify the type.

julia> Test{Int, 2}
Test{Int64,2}

julia> Test{Int, 3}
Test{Int64,3}

Just included for fun :slight_smile: