# How to better initialize a dictionary containing multidimensional array?
**URL:** https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632
**Category:** Performance
**Tags:** performance, array, dictionaries
**Created:** [June 4, 2025, 7:59am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632 "2025-06-04T07:59:44Z")
**Posts on this page:** 11
**Page:** 1
### Author:  [@Harrykjg-physics](https://discourse.julialang.org/u/Harrykjg-physics)
#### Post date: [June 4, 2025, 7:59am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/1 "2025-06-04T07:59:45Z")
Hi everyone, I want to create a Dict() object where the keys are NTuple{6, Int} and the corresponding values are rank-6 arrays. I found the following function to create such Dict object with 64 items are much slower than I expected:
> function test\_time()
> a = Dict()
> for k in Iterators.product([0:1, 0:1, 0:1, 0:1, 0:1, 0:1]…)
> a[k] = Array{Float64}(undef, 16, 16, 16, 16, 16, 16)
> end
> return a
> end
julia\> @btime test\_time();
1.371 s (497 allocations: 8.00 GiB)
In the function above, I add 64 key-value pairs sequentially. But a single array initialization only cost little time :
julia\> @btime Array{Float64}(undef, 16, 16, 16, 16, 16, 16);
1.681 μs (2 allocations: 128.00 MiB)
And 1.371 s / 64 is much larger than 1.681 μs.
Can anybody help me with the problem, or provide better practices ?
Thanks in advance !
---
### Author:  [@oheil](https://discourse.julialang.org/u/oheil)
#### Post date: [June 4, 2025, 8:13am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/2 "2025-06-04T08:13:29Z")
Instead of using `Dict{Any, Any}` you can create the `Dict` with the proper types:
```julia
function test_time_new()
a = Dict{NTuple{6, Int64},Array{Float64}}()
for k in Iterators.product([0:1, 0:1, 0:1, 0:1, 0:1, 0:1]...)
a[k] = Array{Float64}(undef, 16, 16, 16, 16, 16, 16)
end
return a
end
```
```julia
julia> @btime test_time();
372.200 μs (564 allocations: 8.00 GiB)
julia> @btime test_time_new();
256.400 μs (447 allocations: 8.00 GiB)
```
But I suspect that you may be able to overthink your data representation to gain much more performance.
---
### Author:  [@barucden](https://discourse.julialang.org/u/barucden)
#### Post date: [June 4, 2025, 8:19am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/3 "2025-06-04T08:19:32Z")
Further type restriction might help for down-stream tasks:
```julia
a = Dict{NTuple{6, Int64}, Array{Float64, 6}}()
#^^^
```
---
### Author:  [@oheil](https://discourse.julialang.org/u/oheil)
#### Post date: [June 4, 2025, 8:25am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/4 "2025-06-04T08:25:20Z")
```julia
function test_time_new2()
a = Dict{NTuple{6, Int64},Array{Float64, 6}}()
for k in Iterators.product([0:1, 0:1, 0:1, 0:1, 0:1, 0:1]...)
a[k] = Array{Float64}(undef, 16, 16, 16, 16, 16, 16)
end
return a
end
```
```julia
julia> @btime test_time();
268.700 μs (564 allocations: 8.00 GiB)
julia> @btime test_time_new();
259.400 μs (447 allocations: 8.00 GiB)
julia> @btime test_time_new2();
257.900 μs (447 allocations: 8.00 GiB)
```
My first timings were wrong, the function are too volatile to window switches and mouse movings…
---
### Author:  [@Harrykjg-physics](https://discourse.julialang.org/u/Harrykjg-physics)
#### Post date: [June 4, 2025, 8:50am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/5 "2025-06-04T08:50:47Z")
Thanks for your comments, your timing results are much more reasonable compared with mine, Is this a Julia version problem ? My version is 1.10.2
---
### Author:  [@barucden](https://discourse.julialang.org/u/barucden)
#### Post date: [June 4, 2025, 9:10am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/6 "2025-06-04T09:10:17Z")
I observe similar timings on my laptop. Is there a chance that you don’t have enough available memory and your OS has to resort to the swap space? That might slow down the allocations.
---
### Author:  [@Harrykjg-physics](https://discourse.julialang.org/u/Harrykjg-physics)
#### Post date: [June 4, 2025, 9:17am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/7 "2025-06-04T09:17:35Z")
I am not quite sure but it seems to be a version problem. I switch to Julia 1.7.2 on the same machine and the timing results looks good as @oheil. How can different versions result in such a huge difference if it is so 🙃
---
### Author:  [@oheil](https://discourse.julialang.org/u/oheil)
#### Post date: [June 4, 2025, 9:48am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/8 "2025-06-04T09:48:19Z")
> [@Harrykjg-physics](#):
>
> How can different versions result in such a huge difference
This is possible in general, but I lack the skill to connect your code to the former changes in versions.
My Julia version is 1.11.5
---
### Author:  [@oheil](https://discourse.julialang.org/u/oheil)
#### Post date: [June 4, 2025, 9:49am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/9 "2025-06-04T09:49:42Z")
> [@barucden](#):
>
> Is there a chance that you don’t have enough available memory and your OS
Looking at the timing differences at OPs and mine (plenty of RAM) this seems to be reasonable.
---
### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [June 4, 2025, 11:00am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/10 "2025-06-04T11:00:14Z")
Can you explain what you want to do downstream with this dictionary?
---
### Author:  [@matthias314](https://discourse.julialang.org/u/matthias314)
#### Post date: [June 4, 2025, 11:52am UTC](https://discourse.julialang.org/t/how-to-better-initialize-a-dictionary-containing-multidimensional-array/129632/11 "2025-06-04T11:52:38Z")
I see the same slow behavior on a machine with plenty of RAM (48 GB) and Julia 1.11.5.
```julia
julia> @b test_time()
315.842 ms (204 allocs: 8.000 GiB, 99.55% gc time, without a warmup)
julia> @b Array{Float64}(undef, 2^6, 16, 16, 16, 16, 16, 16) # allocate all in one step
168.777 μs (3 allocs: 8.000 GiB, 81.04% gc time)
julia> @b begin GC.enable(false); test_time(); GC.enable(true) end
232.363 μs (204 allocs: 8.000 GiB)
```
Most of the time is spent on garbage collection. I wonder why.