# Update vector without using a counter in a multithreaded loop

**URL:** <https://discourse.julialang.org/t/update-vector-without-using-a-counter-in-a-multithreaded-loop/61776>\
**Category:** General Usage\
**Tags:** multithreading, iterators\
**Created:** [May 25, 2021, 12:54pm UTC](https://discourse.julialang.org/t/update-vector-without-using-a-counter-in-a-multithreaded-loop/61776 "2021-05-25T12:54:54Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Iulian.Cioarca](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iulian.cioarca/32/30166_2.png) [@Iulian.Cioarca](https://discourse.julialang.org/u/Iulian.Cioarca)\
**Post date:** [May 25, 2021, 12:54pm UTC](https://discourse.julialang.org/t/update-vector-without-using-a-counter-in-a-multithreaded-loop/61776/1 "2021-05-25T12:54:54Z")

</div>

I would like to convert the following simplified example:

```julia
n=10
x = Vector{Float64}(undef,n)
y = 1:10
Threads.@threads for i=1:length(y)
    x[i] = y[i] * 2.0
end

```

into:

```julia
n=10
x = Vector{Float64}(undef,n)
y = 1:10
Threads.@threads for crt_y in y
    x[??] = crt_y * 2.0
end

```

How can I traverse and update `x` without using the `i` counter in a multithreaded loop?

---

<div class="post-metadata">

**Author:** ![pixel27](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pixel27/32/8902_2.png) [@pixel27](https://discourse.julialang.org/u/pixel27)\
**Post date:** [May 25, 2021, 1:21pm UTC](https://discourse.julialang.org/t/update-vector-without-using-a-counter-in-a-multithreaded-loop/61776/3 "2021-05-25T13:21:50Z")

</div>

I’m not 100% sure what you are trying to do, both examples the loop variable goes from 1 to 10 so you can use that for indexing. My best guess is you are asking for something like:

```julia
n=10
x = Vector{Float64}(undef,n)
y = 11:20
Threads.@threads for crt_y in y
    x[crt_y - y[1] + 1] = crt_y * 2.0
end

```

In which case as long as the range of y is 10 sequential numbers you will place the calculated values in the array.

---

<div class="post-metadata">

**Author:** ![FPGro](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fpgro/32/20822_2.png) [@FPGro](https://discourse.julialang.org/u/FPGro)\
**Post date:** [May 25, 2021, 1:21pm UTC](https://discourse.julialang.org/t/update-vector-without-using-a-counter-in-a-multithreaded-loop/61776/4 "2021-05-25T13:21:54Z")

</div>

Probably `axes`, so in the worst case something like `(ind, val) in zip(axes(x), y) `  
(not tested I’m not infront of a pc)

---

<div class="post-metadata">

**Author:** ![oheil](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oheil/32/220745_2.png) [@oheil](https://discourse.julialang.org/u/oheil)\
**Post date:** [May 25, 2021, 1:52pm UTC](https://discourse.julialang.org/t/update-vector-without-using-a-counter-in-a-multithreaded-loop/61776/5 "2021-05-25T13:52:04Z")

</div>

Another solution could be:

```julia
julia> Threads.@threads for crt_y in y
           index=indexin(crt_y,y)[1]
           x[index] = crt_y * 2.0
           end

```

But the task sounds a bit weird. Perhaps there is a better solution for your actual problem?  
This works only if elements in y are unique!

---

<div class="post-metadata">

**Author:** ![Iulian.Cioarca](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iulian.cioarca/32/30166_2.png) [@Iulian.Cioarca](https://discourse.julialang.org/u/Iulian.Cioarca)\
**Post date:** [May 25, 2021, 2:28pm UTC](https://discourse.julialang.org/t/update-vector-without-using-a-counter-in-a-multithreaded-loop/61776/6 "2021-05-25T14:28:24Z")

</div>

My usecase is a bit more complex. I have a dataframe `large_df`. One of its columns contains IDs (let’s say from 1 to 1e6, in a random order), the other columns contain data I need to process. I also have a second dataframe `ID_df` which contains some of those IDs(not necessarily all of them), but in a totally different order.  
The task is to identify each ID from the `ID_df` in the `large_df` ID column, and then get data from that specific row and store it somewhere (get first ID from `ID_df`, check if it exists in `large_df` ID column, if yes, go to that row and extract data from other columns and put it in a preallocated vector like `x`)

In order to make this more efficient, I use multithreading, since the search operations are independent.  
The code works using a dedicated counter.

I would like to do the modification just for styling: the code looks better with `Threads.@threads for col in eachcol(df)...`

```julia
using Random, DataFrames

n=10
ID_df = DataFrame(ID=shuffle(1:n))
large_df = DataFrame(ID=shuffle(1:n), data=randn(n))

x = Array{Float64,2}(undef,n,2)

Threads.@threads for i=1:size(ID_df)[1]
   # get current ID from ID_df
   crt_ID_row = ID_df[i,:]
   crt_ID = crt_ID_row.ID

   # find ID in large_df and extract data from that row
   df_row = filter([:ID] => x -> x == crt_ID, large_df)
   x[i,1] = crt_ID
   x[i,2] = df_row.data[1]
end

```

```julia
Threads.@threads for crt_ID_row in ID_df
   # get current ID from ID_df   
   crt_ID = crt_ID_row.ID

   # find ID in large_df and extract data from that row
   df_row = filter([:ID] => x -> x == crt_ID, large_df)
   x[i,1] = crt_ID
   x[i,2] = df_row.data[1]
end

```