Hi Often I would like to initialize an array without specifying an eltype in advance, even though I know all elements of the array will have the same concrete type. By way of example, function makesomearray() arr = [] for i in 1:10 element = some_computation(i) #some type stable computation push!(arr, element) end return arr #Array{Any, 1} end where element can have a fairly nontrivial type that is hard to know in advance. Of course, the above code is not performant, since arr will have type Array{Any, 1}. In order to make it type stable, I often write something like this: function makesomearray2() element = some_computation(1) arr = [element] for i in 2:10 element = some_computation(i) push!(arr, element) end return arr #Array{eltype(element), 1} end which is ugly. What I much prefer of is some kind of “lazy empty array”, which does not fix its type until a first element is pushed into it, such that the following code, almost identical to makesomearray will return the correct type. function makesomearray3() arr = LazyEmptyArray[] for i in 1:10 element = some_computation(i) push!(arr, element) # the first invocation of this should tell the compiler what type arr should be end return arr #Array{eltype(element), 1} end My questions are Is there already some macro or package for handling this kind of logic? If not, is there any obstacle to implementing LazyEmptyArray? Thank you!

Or map: it also produces a collection with an element type that contains what actually goes in the array. For example: julia> a = [rand(Bool) ? rand(1:10) : randstring() for _ = 1:10] 10-element Vector{Any}: 9 "UUa9X3To" 6 "5NAPmkJw" 9 9 7 1 2 "aCmCF7vc" julia> map(length, a) 10-element Vector{Int64}: 1 8 1 8 1 1 1 1 1 8

if you know the type (not that you have to hard-code manually): T[] If you see an input array and like to have an empty array with same elements types: eltype(input_array)[] (also checkout useful functions like similar)

Thanks for your comment! However, those strategies do not work for the situation I have in mind. I am thinking of a scenario where there is no easy way to write down the type of element in advance, and the only practical way of knowing the type of element is via typeof(element). A example might be the solution type for DifferentialEquations.jl. If you run this tutorial code, the return type is something like typeof(sol) = ODESolution{Float64,1,Array{Float64,1},Nothing,Nothing,Array{Float64,1},Array{Array{Float64,1},1},ODEProblem{Float64,Tuple{Float64,Float64},false,DiffEqBase.NullParameters,ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}},DiffEqBase.StandardODEProblem},Tsit5,OrdinaryDiffEq.InterpolationData{ODEFunction{false,typeof(f),LinearAlgebra.UniformScaling{Bool},Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing,Nothing},Array{Float64,1},Array{Float64,1},Array{Array{Float64,1},1},OrdinaryDiffEq.Tsit5ConstantCache{Float64,Float64}},DiffEqBase.DEStats} which is practically impossible to know in advance.

then just use eltype or tyoepf. But mind in these cases, you can probably just push to Any[]. I don’t see a huge complicated Array of Struct being super performant anyways. (and you’re probably not using this array with such needs)

You have a good point that Array of complicated Struct might not be required for performance. You suggest to use eltype or typeof. Do you mind providing a code for doing it concisely? If I trye to use typeof(element) to initialize an empty array, , I need to first obtain element somehow, and the code I write up end up looking as ugly as makesomearray2().

It all depends on how your loop works. For example, I don’t see the point of repeatedly calling solve and push results somewhere. Notice, if you map(solve, array), the array probably will be typed

You could use a comprehension. Comprehensions determine the appropriate array type as elements are added.

@benninkrs @StefanKarpinski Thank you for your reply! I did not think of making a clever use of map for this purpose. That seems to work well if what I want is a single array. Sometimes, however, I’d like to create multiple arrays from a single for loop. I am trying to figure out what is the best idiomatic way of writing that in Julia. What I can think of is something like the following: using StructArray using Unpack # pattern 1 : anonymous function USVs = map(1:L) do x # do something with x return (U=U, S=S, V=V) end @unpack Us, Ss, Vs = StructArray(USVs) # pattern 2 : named function # somefunc(i) returns a vanilla tuple (U, S, V) Us, Ss, Vs = map(somefunc, 1:L) |> StructArray |> fieldarrays While these are reasonably concise, It relies on StructArray. I am wondering if this is the best way to accomplish this, or if there is a more idiomatic way of doing this with Julia Base. Do you guys have any advice?

[image] tomohiro_soejima: It relies on StructArray . I am wondering if this is the best way to accomplish this Currently, use StructArray. The same widening strategy that collect uses in Base could be ported easily to functions returning a tuple and building up a tuple of arrays, but AFAIK so far no one has bothered to do it.

@tkf has implemented a couple of these things. There’s Empty in BangBang.jl, which is pretty much the idea you described, and InitialValues.jl for a more general idea.

A way to initialize an array without specifying its eltype in advance?

General Usage

mschauer September 28, 2020, 9:02am 12

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A way to initialize an array without specifying its eltype in advance?

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