# How to access the localpart of a distributed array?
**URL:** https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711
**Category:** Julia at Scale
**Created:** [October 26, 2017, 9:44pm UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711 "2017-10-26T21:44:37Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@xiaodai](https://discourse.julialang.org/u/xiaodai)
#### Post date: [October 26, 2017, 9:44pm UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711/1 "2017-10-26T21:44:37Z")
I was trying to learn [distributed array](https://github.com/JuliaParallel/DistributedArrays.jl/blob/master/README.md). And I can distribute the arrays using `distribute`, however I can’t work out how to get the localparts of arrays.
As a MWE here what I am trying to do
```julia
addprocs()
@everywhere using DistributedArrays
a = rand(1:5, 1_000_000)
b= rand(1_000_000)
@time da= distribute(a)
@time db= distribute(a)
sum(da) #these work fine
sum(db)
# I want to something like
@everywhere res = localpart(da) .* localpart(db)
```
But it says `da` and `db` are not defined on the workers, which makes sense, so how can define `da` and `db` in the workers?
My actual use cases uses much more complicated algorithms, but I would lilke to work with two or more distributed vectors.
Also just realised, wouldn’t it cause an issue where the vectors are distributed differently on different workers? I need finer control over how I distribute the vectors too then.
---
### Author:  [@raminammour](https://discourse.julialang.org/u/raminammour)
#### Post date: [October 26, 2017, 10:41pm UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711/2 "2017-10-26T22:41:24Z")
Two things:
1- if you just do `res=da.*db`, then `res` is a distributed array with its localparts corresponding to what I think you want.
2- the message that you are seeing has to do with `@everywhere`. from the help:
```julia
@everywhere bar=1
```
will define Main.bar on all processes.
Unlike `@spawn` and `@spawnat`,` @everywhere` does not capture any local variables. Prefixing `@everywhere` with `@eval` allows us to broadcast local variables using interpolation :
```julia
foo = 1
@eval @everywhere bar=$foo
```
So if you really wanted `res` to be the same variable name everywhere, this will do:
```julia
@eval @everywhere res=localpart($da).*localpart($db)
```
---
### Author:  [@xiaodai](https://discourse.julialang.org/u/xiaodai)
#### Post date: [October 27, 2017, 9:15pm UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711/3 "2017-10-27T21:15:42Z")
The reason why distributed arrays are useful is because we can parallelize operations? Does `res=da.*db` auto-parallelize? The solution on how to do this is not clear.
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [October 27, 2017, 9:58pm UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711/4 "2017-10-27T21:58:09Z")
> [@xiaodai](#):
>
> The reason why distributed array are useful is because we can parallelize operation? Does res=da.\*db auto-parallelize? The solution on how to do this is not clear.
Yes and yes, but it needs help.
>
---
### Author:  [@raminammour](https://discourse.julialang.org/u/raminammour)
#### Post date: [October 27, 2017, 10:21pm UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711/5 "2017-10-27T22:21:20Z")
Yes, as Chris said. Not everything is implemented or optimal but for simple operations it is.
The easiest way to check
```julia
@which da.*db
```
If it is not implemented, falls back on AbstractArray implementation (which can be slow).
In this case you will see that it is implemented and does what you want, efficiently.
---
### Author:  [@xiaodai](https://discourse.julialang.org/u/xiaodai)
#### Post date: [October 27, 2017, 10:22pm UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711/6 "2017-10-27T22:22:46Z")
Actually my use-case was alot more complicated than `.*`. I guess I have to learn alot more about distributed arrays before I can do what I want with it.
---
### Author:  [@raminammour](https://discourse.julialang.org/u/raminammour)
#### Post date: [October 27, 2017, 11:57pm UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711/7 "2017-10-27T23:57:55Z")
A more general want to code with them is as follows:
```julia
da=DArray(...)
for ip in procs(da)
@spawnat ip begin
Do things with local parts or access other parts (entail communication)
End
End
```
---
### Author:  [@xiaodai](https://discourse.julialang.org/u/xiaodai)
#### Post date: [October 27, 2017, 11:59pm UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711/8 "2017-10-27T23:59:28Z")
> [@raminammour](#):
>
> @spawnat ip begin
> Do things with local parts or access other parts (entail communication)
How do I refer to `da` in each of the subprocesses?
---
### Author:  [@raminammour](https://discourse.julialang.org/u/raminammour)
#### Post date: [October 28, 2017, 5:37am UTC](https://discourse.julialang.org/t/how-to-access-the-localpart-of-a-distributed-array/6711/9 "2017-10-28T05:37:40Z")
If you use the spawnat macro, then just `da`.
`@spawnat procs(da)[2] sum(localpart(da))`
Would work, for example. `@spawnat` interpolates, unlike `@everywhere`.