# Speeding up mapslices

**URL:** https://discourse.julialang.org/t/speeding-up-mapslices/61907
**Category:** Performance
**Tags:** performance, arrays, mapslices
**Created:** [May 27, 2021, 9:01am UTC](https://discourse.julialang.org/t/speeding-up-mapslices/61907 "2021-05-27T09:01:28Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![nandoconde](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nandoconde/32/19497_2.png) [@nandoconde](https://discourse.julialang.org/u/nandoconde)
#### Post date: [May 27, 2021, 9:01am UTC](https://discourse.julialang.org/t/speeding-up-mapslices/61907/1 "2021-05-27T09:01:28Z")

</div>

Hello!

I was trying to perform a matrix inversion in a 3-D array along the third dimension (more info [here](https://discourse.julialang.org/t/broadcast-over-a-specific-dimension-of-array/43158/4)) and reached two possible solutions.

# First solution – Manual inversion

I am not worrying too much about strange types (the inverse of `Int` are `Float64` and so on, but it could be easily solved).

```julia
# Create example array
S = (rand(4,4,1000) + 1im .* rand(4,4,1000)) ./ sqrt(2);

# Manual inversion
function f(A)
       B = Array{eltype(A),3}(undef,size(A))
       for i in 1:size(A,3)
           B[:,:,i] = inv(A[:,:,i])
       end
       return B
end

```

```julia-auto
julia> @btime f($S);
    1.650 ms (5002 allocations: 5.05 MiB)

```

# Second solution – `mapslices`

```julia-auto
julia> @btime mapslices(inv,$S; dims=(1,2));
    2.192 ms (11504 allocations: 4.91 MiB)

```

# Problem

When I use `@code_warntype` I see there is an `Any` in the second solution, which may slow things down. Apart from it, there are twice as many allocations.

Is there a way to fix this in Julia internals? Is it on purpose?

Thanks!