Error message in array indexing

New to Julia
1

Hi!

I’m on Julia 1.5.3

The last expression is the result of a typing error that I made. The previous expression is something I was aiming to type. Probably this is something that I should know but here goes. In REPL ;

julia> k = [1, 2, 3]
3-element Array{Int64,1}:
 1
 2
 3

julia> m = 1
1

julia> n = 2
2

julia> k[m + n]
3

julia> k[m +n]
ERROR: MethodError: no method matching typed_hcat(::Array{Int64,1}, ::Int64, ::Int64)
Closest candidates are:
  typed_hcat(::Type{T}, ::Number...) where T at abstractarray.jl:1365
  typed_hcat(::Type{T}, ::Union{Number, LinearAlgebra.Adjoint{T,var"#s828"} where var"#s828"<:(AbstractArray{T,1} where T) where T}...) where T at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\LinearAlgebra\src\adjtrans.jl:236
  typed_hcat(::Type{T}, ::Union{Number, LinearAlgebra.Transpose{T,var"#s828"} where var"#s828"<:(AbstractArray{T,1} where T) where T}...) where T at C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.5\LinearAlgebra\src\adjtrans.jl:237
  ...
Stacktrace:
 [1] top-level scope at REPL[25]:1

I’m sorry if I have bothered you for nothing but I really would like to know what’s going on.

Jussi Salmela

1 Like
2

This is one of the handful of places where whitespace matters. In short, it’s the same as the difference between [1 + 2] and [1 +2]:

julia> [1 + 2]
1-element Array{Int64,1}:
 3

julia> [1 +2]
1×2 Array{Int64,2}:
 1  2

In the latter case, the + becomes a unary operator and you get a horizontal concatenation (that’s why hcat is showing up in the error).

9 Likes
3

Thanks for your answer. I understand what is happening but I can’t find this in the documentation. Has this feature a name I could search for? And, being a noob, I can’t understand what is the use case for this i.e. what is the rationale of this feature. I’m asking this because with the values m==1 & n==2 these
[m n]
[m +n]
produce the same result. So I’m wondering if there are more complicated situations where this feature is essential or makes coding essentially easier.

4

Probably the implementation of that is essentially easier. In that context spaces are used to indicate a row-major array instead of a column vector which you would get with commas. Of course [ 1 -2 ] should parse to

julia> [ 1 -2 ]
1×2 Array{Int64,2}:
 1  -2

Thus, for [ 1 +2 ] to be parsed otherwise there should be a specific implementation for that case. And things could get strange. For example, to what [ 1 +2 -5 ] should be parsed to?

5

Yeah, it’s the same as [1 -2] or [3 :end]. It’s not really about the ease of implementation, it’s just an unfortunate ambiguity that comes with the combination of:

  • space-delimiting elements in a horizontal concatenation
  • allowing optional spaces around infix operators like + and - and :
  • parsing expressions like 1 + 2 and 3 - 4 to be a single element
  • supporting + and - as both infix and unary operators
  • unary operators take precedence over infix ones

Change any one of those things and the behavior here would change.

As far as why + is a unary operator in the first place, I can see two reasons:

  • Symmetry with -. This is why C introduced it. Besides just following that tradition, it can be handy for some interop; some programs always output numbers with an explicit prefixed + or -.
  • While it is a no-op for all builtin types, it could be extended to do something different. Not sure what, and it doesn’t seem like the ecosystem has done that in my quick scan.
3 Likes
6

Thanks to both for answering although this is a very minor detail in the Julia environment. The incidence was caused by my mistyping of leaving the space out after the plus sign. Now I’m familiar with this so I don’t have to ask again in case I do it again which is quite probable.

1 Like