# UndefVarError when initializing variable inside function with @everywhere

**URL:** <https://discourse.julialang.org/t/undefvarerror-when-initializing-variable-inside-function-with-everywhere/25183>\
**Category:** Julia at Scale\
**Created:** [June 12, 2019, 11:46am UTC](https://discourse.julialang.org/t/undefvarerror-when-initializing-variable-inside-function-with-everywhere/25183 "2019-06-12T11:46:54Z")\
**Posts on this page:** 1\
**Showing post:** 12

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**Author:** ![Bruno\_Amorim](https://avatars.discourse-cdn.com/v4/letter/b/f6c823/32.png) [@Bruno\_Amorim](https://discourse.julialang.org/u/Bruno_Amorim)\
**Post date:** [June 13, 2019, 2:01pm UTC](https://discourse.julialang.org/t/undefvarerror-when-initializing-variable-inside-function-with-everywhere/25183/12 "2019-06-13T14:01:31Z")

</div>

Thank you very much! You are correct, indeed the matrices created by @spawant are never used by pmap. Furthermore, and this is something I was missing before, both @everywhere, @spawnat and sendto from ParallelDataTransfer.jl, when called from inside a function, define A not in the function local scope, but in Main.

This example shows this. I defined a module:

```julia
module run_h
using Distributed, Random, LinearAlgebra, ParallelDataTransfer

export run_h_check

function h!(mat, x) 
    
    println("Matrix inside pmap ", myid(), " A=", mat)
    return eigvals(rand!(MersenneTwister(x), mat))
    
end

function run_h_check(n, iter)
    A = zeros(n, n) # initialized A
    println("original A")
    println(A)
    sendto(workers(), A = A) # make A available to other processes, this actually exports A to Main
    @everywhere workers() A[1, 1] = myid()
    println("Check that A was modified in the workers")
    @everywhere workers() println(A)
    println("Pmap...")
    return pmap(x->h!(A, x), iter)
end

end

```

And then I run fro IJulia:

```julia
using Distributed
addprocs(4)

@everywhere include("run_h.jl")
@everywhere using .run_h

run_h_check(2, 1:5)

```

which returns:

```julia
original A
[0.0 0.0; 0.0 0.0]
Check that A was modified in the workers
      From worker 3:	[3.0 0.0; 0.0 0.0]
      From worker 5:	[5.0 0.0; 0.0 0.0]
      From worker 2:	[2.0 0.0; 0.0 0.0]
      From worker 4:	[4.0 0.0; 0.0 0.0]
Pmap...
      From worker 4:	Matrix inside pmap 4 A=[0.0 0.0; 0.0 0.0]
      From worker 5:	Matrix inside pmap 5 A=[0.0 0.0; 0.0 0.0]
      From worker 3:	Matrix inside pmap 3 A=[0.0 0.0; 0.0 0.0]
      From worker 2:	Matrix inside pmap 2 A=[0.0 0.0; 0.0 0.0]
      From worker 4:	Matrix inside pmap 4 A=[0.0 0.0; 0.0 0.0]
5-element Array{Array{Float64,1},1}:
 [0.470351, -0.226408] 
 [0.191387, 0.994748]  
 [-0.00607352, 1.78786]
 [-0.102802, 1.71222]  
 [0.191155, 1.22056]

```

Indeed, inside the pmap, only the original A=[0.0 0.0; 0.0 0.0] is used. I can then check that `sendto` actually moved the matrix A to Main. If I run:  
`@everywhere workers() println(Main.A)`  
I get:

```julia
      From worker 4:	A= [4.0 0.0; 0.0 0.0]
      From worker 2:	A= [2.0 0.0; 0.0 0.0]
      From worker 3:	A= [3.0 0.0; 0.0 0.0]
      From worker 5:	A= [5.0 0.0; 0.0 0.0]

```

But if I simply run:  
`println(Main.A)`, I obtain the error `UndefVarError: A not defined`, since `A` was not exported to Main of process 1.

I wasn’t expecting for the matrix A to be automatially made available to all processes in pmap. This is probably the same behaviour that is discussed in [@everywhere and pmap inside of a function?](https://discourse.julialang.org/t/everywhere-and-pmap-inside-of-a-function/5405)

Once again, thank you very much [tanhevg](https://discourse.julialang.org/u/tanhevg)! This was really useful!

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