# How to create SMatrix from SVectors efficiently

**URL:** <https://discourse.julialang.org/t/how-to-create-smatrix-from-svectors-efficiently/76969>\
**Category:** Performance\
**Tags:** staticarrays\
**Created:** [February 23, 2022, 12:35pm UTC](https://discourse.julialang.org/t/how-to-create-smatrix-from-svectors-efficiently/76969 "2022-02-23T12:35:08Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![ncxst](https://avatars.discourse-cdn.com/v4/letter/n/e47774/32.png) [@ncxst](https://discourse.julialang.org/u/ncxst)\
**Post date:** [February 23, 2022, 12:35pm UTC](https://discourse.julialang.org/t/how-to-create-smatrix-from-svectors-efficiently/76969/1 "2022-02-23T12:35:08Z")

</div>

Hi,  
I have a function `f(i) = SA[i,2i]` that generates SVectors. I want to concatenate a small, but variable amount of them into a SMatrix. For a fixed amount I can do:

```julia
julia> [f(1) f(2) f(3)]'
3×2 SMatrix{3, 2, Int64, 6} with indices SOneTo(3)×SOneTo(2):
 1 2
 2 4
 3 6

```

which is fast:

```julia
julia> @benchmark [f(1) f(2) f(3)]'
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
 Range (min … max): 0.023 ns … 0.082 ns ┊ GC (min … max): 0.00% … 0.00%
 Time (median): 0.027 ns ┊ GC (median): 0.00%
 Time (mean ± σ): 0.027 ns ± 0.003 ns ┊ GC (mean ± σ): 0.00% ± 0.00%
 Memory estimate: 0 bytes, allocs estimate: 0.

```

Is there a fast way to do this with a variable amount of calls to `f`, like `[f(1) f(2) ... f(N)]`?  
I know I can do

```julia
julia> hcat( [f(i) for i in 1:3]...)'
3×2 SMatrix{3, 2, Int64, 6} with indices SOneTo(3)×SOneTo(2):
 1 2
 2 4
 3 6

```

but this approach is very slow:

```julia
julia> @benchmark hcat( [f(i) for i in 1:3]...)'
BenchmarkTools.Trial: 10000 samples with 705 evaluations.
 Range (min … max): 166.813 ns … 5.976 μs ┊ GC (min … max): 0.00% … 95.94%
 Time (median): 176.813 ns ┊ GC (median): 0.00%
 Time (mean ± σ): 206.796 ns ± 361.318 ns ┊ GC (mean ± σ): 12.38% ± 6.82%
 Memory estimate: 336 bytes, allocs estimate: 6.

```

Thank you,  
Nico

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<div class="post-metadata">

**Author:** ![DNF](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dnf/32/10191_2.png) [@DNF](https://discourse.julialang.org/u/DNF)\
**Post date:** [February 23, 2022, 12:48pm UTC](https://discourse.julialang.org/t/how-to-create-smatrix-from-svectors-efficiently/76969/2 "2022-02-23T12:48:31Z")

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> [@ncxst](#):
>
> `hcat( [f(i) for i in 1:3]...)`

You could do

```julia
hcat( (f(i) for i in 1:4)...)
# or
hcat(ntuple(i->f(i), 4 )...)

```

They both appear to get constant folded. Remember that this only works well if the length (e.g. 4) is a compile-time constant. If it can change at runtime, you should probably not be using StaticMatrix for this.