# Creating Sparse NTuple
**URL:** https://discourse.julialang.org/t/creating-sparse-ntuple/103350
**Category:** General Usage
**Tags:** question, ntuple
**Created:** [August 30, 2023, 3:17am UTC](https://discourse.julialang.org/t/creating-sparse-ntuple/103350 "2023-08-30T03:17:34Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@bmit](https://discourse.julialang.org/u/bmit)
#### Post date: [August 30, 2023, 3:17am UTC](https://discourse.julialang.org/t/creating-sparse-ntuple/103350/1 "2023-08-30T03:17:34Z")
I have a problem where I want to make an `NTuple` of length `N` where I’m given indices and values for the non-zero components.
The equivalent function for creating a `Vector` would look something like
```julia
function foo(N, nzindices, nzvalues)
bar = zeros(N)
bar[nzindices] .= nzvalues
return bar
end
```
How would I make an equivalent function that returns a `Tuple` of length `N` without allocating a `Vector`?
---
### Author:  [@jishnub](https://discourse.julialang.org/u/jishnub)
#### Post date: [August 30, 2023, 4:13am UTC](https://discourse.julialang.org/t/creating-sparse-ntuple/103350/2 "2023-08-30T04:13:33Z")
You may broadcast `Base.setindex`
---
### Author:  [@longemen3000](https://discourse.julialang.org/u/longemen3000)
#### Post date: [August 30, 2023, 4:20am UTC](https://discourse.julialang.org/t/creating-sparse-ntuple/103350/3 "2023-08-30T04:20:58Z")
```julia
function foo(::Val{N}, nzindices::Vector, nzvalues::Vector) where N
function f(i)
idx = findfirst(isequal(i),nzindices)
if isnothing(idx)
return zero(eltype(nzvalues))
else
return nzvalues[idx]
end
end
return ntuple(f,Val(N))
end
```
that works like this?
```julia
julia> foo(Val{8}(),[1,4],[10,40])
(10, 0, 0, 40, 0, 0, 0, 0)
```
another option, with `Base.setindex`:
```julia
function foo2(::Val{N}, nzindices, nzvalues) where N
res = ntuple(i -> zero(eltype(nzvalues)),Val{N}())
for i in 1:length(nzindices)
res = Base.setindex(res,nzvalues[i],nzindices[i])
end
return res
end
```
---
### Author:  [@bmit](https://discourse.julialang.org/u/bmit)
#### Post date: [August 30, 2023, 1:39pm UTC](https://discourse.julialang.org/t/creating-sparse-ntuple/103350/4 "2023-08-30T13:39:36Z")
Quick question: what is the difference between `Val(N)` and `Val{N}()` in your example above?
Also, I did a little benchmarking of the two that I thought I would share:
```julia
N = 10
Nnz = 4
nzindices = rand(1:N, Nnz)
nzvalues = rand(Nnz)
@btime foo($(Val(N)), $nzindices, $nzvalues) # 21.606 ns (0 allocations: 0 bytes)
@btime foo2($(Val(N)), $nzindices, $nzvalues) # 8.583 ns (0 allocations: 0 bytes)
N = 100
Nnz = 27
nzindices = rand(1:N, Nnz)
nzvalues = rand(Nnz)
@btime foo($(Val(N)), $nzindices, $nzvalues) # 867.103 ns (0 allocations: 0 bytes)
@btime foo2($(Val(N)), $nzindices, $nzvalues) # 37.958 μs (2808 allocations: 86.48 KiB)
```
So it appears that the first method _might_ be faster, at least until `N` gets large and starts allocating. Do you know why this would be the case?
---
### Author:  [@longemen3000](https://discourse.julialang.org/u/longemen3000)
#### Post date: [August 30, 2023, 10:06pm UTC](https://discourse.julialang.org/t/creating-sparse-ntuple/103350/5 "2023-08-30T22:06:51Z")
I suppose that the first method creates an `NTuple` once, but calls a function with conditionals. at low `N`, the cost of the function might be dominated by that call.
The second method creates `nz` NTuples, that may be fast at low `N`, but i suppose there is a limit on that. if you want the best of both worlds, you can combine both, if you find the `N` threshold that most suits your enviroment