# Parellel arrays

**URL:** <https://discourse.julialang.org/t/parellel-arrays/35645>\
**Category:** General Usage\
**Tags:** multithreading\
**Created:** [March 6, 2020, 3:45pm UTC](https://discourse.julialang.org/t/parellel-arrays/35645 "2020-03-06T15:45:16Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![eaubanel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/eaubanel/32/13327_2.png) [@eaubanel](https://discourse.julialang.org/u/eaubanel)\
**Post date:** [March 6, 2020, 3:45pm UTC](https://discourse.julialang.org/t/parellel-arrays/35645/1 "2020-03-06T15:45:16Z")

</div>

I need to have multiple threads write values to a shared array. I’m thinking of having each thread write to a private array, and then have the threads each write their private values into a shared array. In the following example I’ve chosen values that assume 2 threads, and I’m assuming thread 1 has written the values 1 and 2, and thread 2 has written the values 3, 4, 5. A prefix sum is used to let each thread know where to start writing in the shared array (`b`).

```julia
using Base.Threads

function copyVerts!(b, subarray, start, nper, id)
    for i in 1:nper[id]
        b[start[id]+i] = subarray[id][i]
    end
end

# assuming nthreads() is 2
b = Array{Int64,1}(undef,5)
subarray = [similar(b, 0) for i = 1:Threads.nthreads()]
subarray[1] = [1,2]
subarray[2] = [3,4,5]
nper = [2,3] # thread 1 has 2 values and thread 2 has 3 values
start = cumsum(nper) - nper # exclusive prefix sum
for i = 1:nthreads()
    Threads.@spawn copyVerts!(b, subarray, start, nper, i)
end

```

This gives:

```julia
julia> b
5-element Array{Int64,1}:
 1
 2
 3
 4
 5

```

Is this a good solution, and how do I wait for all tasks to complete? Feedback would be appreciated.

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<div class="post-metadata">

**Author:** ![pbayer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pbayer/32/11675_2.png) [@pbayer](https://discourse.julialang.org/u/pbayer)\
**Post date:** [March 6, 2020, 7:53pm UTC](https://discourse.julialang.org/t/parellel-arrays/35645/2 "2020-03-06T19:53:48Z")

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Just playing around with your idea I came up with the following:

```julia
using .Threads

function copyVerts!(b, sub, start)
    for i in 1:length(sub)
        b[start + i - 1] = sub[i]
    end
end

s1 = 1:nthreads()
s2 = s1 .+ 1
c1 = cumsum(s1)
c2 = cumsum(s2)
sa = [c1[i]:c2[i] for i in s1]
b = zeros(Int,maximum(c2))

@sync for i = 1:nthreads()
    Threads.@spawn copyVerts!(b, sa[i], c1[i])
end

```

then with

```julia
julia> nthreads()
8

julia> show(sa)
UnitRange{Int64}[1:2, 3:5, 6:9, 10:14, 15:20, 21:27, 28:35, 36:44]
julia> show(b)
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44]

```

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<div class="post-metadata">

**Author:** ![eaubanel](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/eaubanel/32/13327_2.png) [@eaubanel](https://discourse.julialang.org/u/eaubanel)\
**Post date:** [March 6, 2020, 8:32pm UTC](https://discourse.julialang.org/t/parellel-arrays/35645/3 "2020-03-06T20:32:11Z")

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Thanks! It looks basically the same as mine, although I think your `copyVerts` function is better than mine since you’re taking out reference to the thread index, so you don’t have my indirect indexing (`b[start[id]+i]`). I used an array of subarrays since I don’t know how long each subarray will be and may have to resize. My example happened to have consecutive integers as the final result, but the final result could have any values.

If the code were to continue past the `for` loop, how can one wait until all the spawned tasks have completed?

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<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [March 6, 2020, 8:33pm UTC](https://discourse.julialang.org/t/parellel-arrays/35645/4 "2020-03-06T20:33:16Z")

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> If the code were to continue past the `for` loop, how can one wait until all the spawned tasks have completed?

`@sync`.

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<div class="post-metadata">

**Author:** ![pbayer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pbayer/32/11675_2.png) [@pbayer](https://discourse.julialang.org/u/pbayer)\
**Post date:** [March 6, 2020, 8:33pm UTC](https://discourse.julialang.org/t/parellel-arrays/35645/5 "2020-03-06T20:33:43Z")

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the `@sync` does this.