# \`break\` with a threaded loop?

**URL:** https://discourse.julialang.org/t/break-with-a-threaded-loop/78487
**Category:** General Usage
**Created:** [March 25, 2022, 6:36pm UTC](https://discourse.julialang.org/t/break-with-a-threaded-loop/78487 "2022-03-25T18:36:53Z")
**Posts on this page:** 1
**Showing post:** 10

<div class="post-metadata">

### Author: ![tkf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkf/32/17635_2.png) [@tkf](https://discourse.julialang.org/u/tkf)
#### Post date: [March 28, 2022, 2:13am UTC](https://discourse.julialang.org/t/break-with-a-threaded-loop/78487/10 "2022-03-28T02:13:40Z")

</div>

> [@Mason](#):
>
> ```julia
> julia> foldxt(1:10^9, init=Int[]) do r, _
> sleep(0.01)
> r = append!!(r, SingletonVector(Threads.threadid()))
> if length(r) >= 100
> reduced(r)
> else
> r
> end
> end
> 
> ```

There are two problems in this code.

First, `init = Int[]` means that this vector is shared across multiple tasks (and threads if `Threads.nthreads() > 1`). It means that the same vector is mutated by multiple tasks that can run on different threads. So, this code is not thread-safe (in particular can have data races). You need to pass `OnInit(() -> Int[])` or `EmptyVector{Int}()` etc.

Second, the binary function passed to `foldxt` is not “good” in the sense that it probably is not doing what you want it to do. The binary function passed to `foldxt` have to combine two intermediate results. But, your function ignores the second argument. It means that it only process the first basecase. It can be observed by something like

```julia
julia> foldxt(1:100, init=EmptyVector{Int}(), basesize=10) do r, _
           # sleep(0.01)
           r = append!!(r, SingletonVector(Threads.threadid()))
           if length(r) >= 100
               reduced(r)
           else
               r
           end
       end
10-element Vector{Int64}:
 1
 1
 1
 1
 1
 1
 1
 1
 1
 1

```

You can see that the output has 10 elements even if the input has 100 elements.

Note that I use `init=EmptyVector{Int}()`. If you use `init=Int[]`, it may look like doing what you want it to do. However, as I said, it was not a valid Julia program.

Perhaps what you wanted to write was something like this:

```julia
1:1000 |>
Map() do _
    sleep(0.01)
    SingletonVector(Threads.threadid())
end |>
foldxt(init = OnInit(() -> Int[]), basesize = 1) do a, b
    r = append!!(a, b)
    if length(r) >= 100
        reduced(r)
    else
        r
    end
end

```

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