# How to determine when to broadcast

**URL:** <https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127>\
**Category:** New to Julia\
**Created:** [June 24, 2024, 12:02am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127 "2024-06-24T00:02:42Z")\
**Posts on this page:** 11\
**Page:** 1

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**Author:** ![lrennels](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lrennels/32/210171_2.png) [@lrennels](https://discourse.julialang.org/u/lrennels)\
**Post date:** [June 24, 2024, 12:02am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/1 "2024-06-24T00:02:42Z")

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I have some code that needs to perform a `setindex!(A::Array, X::Float64, Indices)` using `A[Indices...] = X` or broadcasting `A[Indices...] .= X`. Right now until that is called I don’t know if `Indices` will be a Vector of Ints or a Vector of Vector of Ints (when I need to broadcast).

I have a hack, but is there a good way to use Julia Base functions to determine whether broadcasting will be necessary?

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**Author:** ![lxvm](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lxvm/32/50010_2.png) [@lxvm](https://discourse.julialang.org/u/lxvm)\
**Post date:** [June 24, 2024, 12:53am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/2 "2024-06-24T00:53:23Z")

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You could try [`Iterators.flatten`](https://docs.julialang.org/en/v1/base/iterators/#Base.Iterators.flatten), which will unravel each element of your vector `X`. In Julia, numbers are also iterable, i.e. they return their value, so both cases of Vectors or Ints or Vector of Vector of Ints should work.

As for whether broadcasting is necessary, it should not be necessary if `X` is already computed. However, broadcasting can help avoid allocations if you would like to perform a calculation with `X`, such as `A .= X .* 2`

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**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [June 24, 2024, 12:56am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/3 "2024-06-24T00:56:46Z")

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Do you have an example of inputs and outputs in mind? Something seems off when you go from saying `X::Float64` to ~~saying `X::Vector{Int}` or `X::Vector{Vector{Int}}`.~~ scratch that after edit. But examples would still help.

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**Author:** ![lrennels](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lrennels/32/210171_2.png) [@lrennels](https://discourse.julialang.org/u/lrennels)\
**Post date:** [June 24, 2024, 12:59am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/4 "2024-06-24T00:59:37Z")

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Sorry, that was my mistake, it is `Indices` which varies. Admittedly it is not perfect code, I’m building on a code base that I didn’t fully write.

So the example would be this:

I could have

```julia
A = [1,2,3]
X = 3
Indices = [2]

A[Indices...] = 3

```

I could also have

```julia
A = [1,2,3]
X = 3
Indices = [[2]]

A[Indices...] = 3

```

which will throw

```julia

julia> A[Indices...] = 3
ERROR: ArgumentError: indexed assignment with a single value to possibly many locations is not supported; perhaps use broadcasting `.=` instead?
Stacktrace:
 [1] setindex_shape_check(::Int64, ::Int64)
   @ Base ./indices.jl:261
 [2] _unsafe_setindex!(::IndexLinear, A::Vector{Int64}, x::Int64, I::Vector{Int64})
   @ Base ./multidimensional.jl:953
 [3] _setindex!
   @ Base ./multidimensional.jl:944 [inlined]
 [4] setindex!(A::Vector{Int64}, v::Int64, I::Vector{Int64})
   @ Base ./abstractarray.jl:1393
 [5] top-level scope
   @ REPL[18]:1

```

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<div class="post-metadata">

**Author:** ![lrennels](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lrennels/32/210171_2.png) [@lrennels](https://discourse.julialang.org/u/lrennels)\
**Post date:** [June 24, 2024, 1:00am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/5 "2024-06-24T01:00:48Z")

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In the second case I need to broadcast, I’m looking for the best way possible to check if I will need to broadcast. I’ve done it but want it to be robust, and thought there may be some Julia base functions that would help instead of reinventing the wheel 🙂

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**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [June 24, 2024, 1:08am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/6 "2024-06-24T01:08:45Z")

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Oh, that is not what broadcasting is for, the error message is referring to something else. You’re trying to process improper `Indices` to proper indices. `A[Indices...]` works for `[2]` because it splats to the proper `A[2]`. You could accomplish the same thing with `A[Indices[begin]]`. `A[Indices[begin][begin]]` would work for `Indices = [[2]]`, but you really don’t want to do that.

I’m guessing that in the actual code, `Indices` does have a more apparent structure that we can process to proper indices in a more general manner. Is the destination array a matrix, it would be good to have \>1 dimension to distinguish linear and Cartesian indices, but not more where printing gets difficult. Saying the expected outcome (#=commented out=#) in addition to the inputs would also help a lot.

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<div class="post-metadata">

**Author:** ![lxvm](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lxvm/32/50010_2.png) [@lxvm](https://discourse.julialang.org/u/lxvm)\
**Post date:** [June 24, 2024, 2:10am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/7 "2024-06-24T02:10:24Z")

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This should get the job done

```julia
A[collect(Iterators.flatten(Indices))] .= 3

```

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<div class="post-metadata">

**Author:** ![lrennels](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lrennels/32/210171_2.png) [@lrennels](https://discourse.julialang.org/u/lrennels)\
**Post date:** [June 24, 2024, 2:20am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/8 "2024-06-24T02:20:31Z")

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Thank you both! @Benny agreed I should get to the bottom of the structure to do this more properly, I’m going to look into the preceding steps that generate the Indices and see what I can simply. @lxvm that works thank you!

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<div class="post-metadata">

**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [June 24, 2024, 2:31am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/9 "2024-06-24T02:31:01Z")

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[Single- and multi-dimensional Arrays · The Julia Language](https://docs.julialang.org/en/v1/manual/arrays/#man-indexed-assignment)  
Hopefully it helps to have the manual section visible as you mull over the code.

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**Author:** ![screw\_dog](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/screw_dog/32/48119_2.png) [@screw\_dog](https://discourse.julialang.org/u/screw_dog)\
**Post date:** [June 24, 2024, 2:32am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/10 "2024-06-24T02:32:11Z")

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In general, this kind of situation is well solved with multiple dispatch ie

```julia
setitems!(A, X, indices::Vector{Vector}) = A[indices...] .= X
setitems!(A, X, indices::Vector) = A[indices...] = X

...

setitems!(A, X, indices) # don't need to check what type indices is here

```

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<div class="post-metadata">

**Author:** ![lrennels](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lrennels/32/210171_2.png) [@lrennels](https://discourse.julialang.org/u/lrennels)\
**Post date:** [June 24, 2024, 2:55am UTC](https://discourse.julialang.org/t/how-to-determine-when-to-broadcast/116127/11 "2024-06-24T02:55:57Z")

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Great idea!