Assigning a UnitRange to an array

General Usage
1

Hi,

I have a matrix with elements of type UnitRange

a = Array{UnitRange}(undef ,2,2)

I would like to assign a unit range like (200:375) to a range of elements using broadcast.

a[1,:] .= (200:375)

but I get this error

ERROR: DimensionMismatch: array could not be broadcast to match destination

Clearly Julia wants to interpret the UnitRange rather than just assign the array elements to the UnitRange.

Is there a way to achieve what I need?

Thanks
Steve

2

Have you tried this?

a[1,:] .= Ref(200:375)
3 Likes
3

Perfect, thanks.

4

Also, to maintain type stability, you might consider setting a’s element type to a concrete type. UnitRange is a UnionAll, and to specify a concrete type you need to parameterize it with the type of its elements—in this case, UnitRange{Int}:

a = Array{UnitRange{Int}}(undef, 2, 2)
5

The reason it won’t broadcast is because the range has a length and so will try to broadcast over that length. The Ref works because this has a length one.

As an alternative to the Ref which may in some situations be allocating, you can also wrap it as a 1-tuple or in the StaticArrays Scalar made specifically for this reason:

# Tuple
a[1, :] .= (200:375,)

# Scalar
using StaticArrays
a[1, :] .= Scalar(200:375)

I prefer Scalar as this is more semantic - the next person looking at your code doesn’t have to work out why you used a Ref or a tuple, the reason is right there in the documentation for Scalar.

2 Likes
6

Huh, didn’t know about Scalar. Definitely feels like something that should be in Base; the Ref trick to cause scalar broadcasting always felt like an ugly hack, especially since you aren’t using it for its mutability.

7

Just to add maybe part of your confusion stems from the fact that the brackets arround the range don’t do anything here, the key is this difference:

julia> (200:375)
200:375

julia> (200:375,)
(200:375,)

this is a difference between arrays and tuples:

julia> a[1,:] .= [200:375]
2-element view(::Matrix{UnitRange}, 1, :) with eltype UnitRange:
 200:375
 200:375

for tuples just doing (x) isn’t possible because parens are used to group expressions as well, while [x] will create a 1-element vector. You normally don’t want to use [x] though because that has to allocate, hence why people go for (x,), Ref, or Scalar as discussed.

8

Thanks for all the help here. Scalar looks like the best solution for me.