# Is it possible to do a \`mapreduce\` with multiple arrays while broadcasting over so
**URL:** https://discourse.julialang.org/t/is-it-possible-to-do-a-mapreduce-with-multiple-arrays-while-broadcasting-over-so/57433
**Category:** General Usage
**Created:** [March 18, 2021, 3:24am UTC](https://discourse.julialang.org/t/is-it-possible-to-do-a-mapreduce-with-multiple-arrays-while-broadcasting-over-so/57433 "2021-03-18T03:24:17Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@BridgeBot](https://discourse.julialang.org/u/BridgeBot)
#### Post date: [March 18, 2021, 3:24am UTC](https://discourse.julialang.org/t/is-it-possible-to-do-a-mapreduce-with-multiple-arrays-while-broadcasting-over-so/57433/1 "2021-03-18T03:24:17Z")
Is it possible to do a `mapreduce` with multiple arrays while broadcasting over some dimensions? Something like this
```julia
U = rand(5, 5, 5)
Δ = rand(1, 1, 5)
mapreduce((u, Δx) -> u / Δx, max, U, Δ)
```
where I really want to find the maximum of `U[i, j, k] / Δ[k]`.
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### Author:  [@PolarizedPoutine](https://discourse.julialang.org/u/PolarizedPoutine)
#### Post date: [March 18, 2021, 3:39am UTC](https://discourse.julialang.org/t/is-it-possible-to-do-a-mapreduce-with-multiple-arrays-while-broadcasting-over-so/57433/2 "2021-03-18T03:39:44Z")
Answering my own question lol: Came up a double `mapreduce` but maybe there’s a better solution out there
```julia
julia> A = rand(5, 5);
julia> B = rand(1, 5);
julia> extrema(A ./ B)
(0.00770023760419805, 13.798242177927923)
julia> mapreduce(i -> mapreduce((x, y) -> x / y, min, A[i, :], B), min, 1:5)
0.00770023760419805
julia> mapreduce(i -> mapreduce((x, y) -> x / y, max, A[i, :], B), max, 1:5)
13.798242177927923
```
---
### Author:  [@PolarizedPoutine](https://discourse.julialang.org/u/PolarizedPoutine)
#### Post date: [March 18, 2021, 3:54am UTC](https://discourse.julialang.org/t/is-it-possible-to-do-a-mapreduce-with-multiple-arrays-while-broadcasting-over-so/57433/3 "2021-03-18T03:54:01Z")
@Mason suggested a beautiful solution using Tullio.jl. Thank you!
```julia
julia> let U = rand(3, 3, 3), Δ = rand(3)
@tullio (max) out := U[i, j, k] / Δ[k]
end
4.490419859484241
```
---
### Author:  [@fabiangans](https://discourse.julialang.org/u/fabiangans)
#### Post date: [March 18, 2021, 7:00am UTC](https://discourse.julialang.org/t/is-it-possible-to-do-a-mapreduce-with-multiple-arrays-while-broadcasting-over-so/57433/4 "2021-03-18T07:00:29Z")
There is also [this workaround](https://discourse.julialang.org/t/is-there-something-like-broadcast-mapreduce/6076/15) where I made a non-allocating `broadcast_reduce`, but the tullio solution looks very elegant.
---
### Author:  [@PolarizedPoutine](https://discourse.julialang.org/u/PolarizedPoutine)
#### Post date: [March 18, 2021, 1:44pm UTC](https://discourse.julialang.org/t/is-it-possible-to-do-a-mapreduce-with-multiple-arrays-while-broadcasting-over-so/57433/5 "2021-03-18T13:44:57Z")
Ah that looks quite useful, thanks @fabiangans! Gonna experiment with what works best on large CuArrays so might end up needing `broadcast_reduce`.