# Callbacks on parts of SharedArrays

**URL:** <https://discourse.julialang.org/t/callbacks-on-parts-of-sharedarrays/31624>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [November 28, 2019, 1:50pm UTC](https://discourse.julialang.org/t/callbacks-on-parts-of-sharedarrays/31624 "2019-11-28T13:50:39Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![hcarlsso](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hcarlsso/32/11563_2.png) [@hcarlsso](https://discourse.julialang.org/u/hcarlsso)\
**Post date:** [November 28, 2019, 1:50pm UTC](https://discourse.julialang.org/t/callbacks-on-parts-of-sharedarrays/31624/1 "2019-11-28T13:50:39Z")

</div>

The problem I have is how to define functions that modify parts of SharedArrays. What I want to achieve is the following:

```julia
using Distributed
addprocs(2)
@everywhere using SharedArrays
@everywhere function temp(trace)
    Nt = size(trace, 1)
    Fs = Vector{Any}(undef, Nt)
    for n = 1:Nt
        i = 1
        Fs[n] = res -> begin
            trace[n, i] = res
            i += 1
        end
    end
    return Fs
end

trace = SharedArray{Int}(3,4)
Fs = temp(trace)
for i = 1:4
    for n = 1:3
        # Large computation and save result through a callback
        res = n+i
        Fs[n](res) 
    end
end
show(stdout, "text/plain", trace)
println("\n")

```

which yields

```julia
3×4 SharedArray{Int64,2}:
 2 3 4 5
 3 4 5 6
 4 5 6 7

```

However, when trying the distributed version:

```julia
@everywhere trace = SharedArray{Int}(3,4)
@everywhere Fs = temp(trace)
for i = 1:4
    @distributed for n = 1:3
        Fs[n](n + i)
    end
end

show(stdout, "text/plain", trace)
println("\n")

```

I get

```julia
3×4 SharedArray{Int64,2}:
 0 0 0 0
 0 0 0 0
 0 0 0 0

```

Seems like `Fs` is not defined on the different processes. I think this could be solved using `RemoteChannels` but how would I do this using `SharedArrays`?

edit:

Tried with `pmap` as well

```julia
trace = SharedArray{Int}(3,4)
Fs = temp(trace)
for i = 1:4
    pmap(enumerate(Fs)) do (n, F)
        F(n + i)
    end
end

show(stdout, "text/plain", trace)
println("\n")

```

With the result

```julia
3×4 SharedArray{Int64,2}:
 5 0 0 0
 6 0 0 0
 7 0 0 0

```

Meaning that `i` does not increment in the loop where `Fs` is defined.