# Error message in array indexing

**URL:** <https://discourse.julialang.org/t/error-message-in-array-indexing/57322>\
**Category:** New to Julia\
**Created:** [March 16, 2021, 6:20pm UTC](https://discourse.julialang.org/t/error-message-in-array-indexing/57322 "2021-03-16T18:20:52Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Jussi\_Salmela](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jussi_salmela/32/22992_2.png) [@Jussi\_Salmela](https://discourse.julialang.org/u/Jussi_Salmela)\
**Post date:** [March 16, 2021, 6:20pm UTC](https://discourse.julialang.org/t/error-message-in-array-indexing/57322/1 "2021-03-16T18:20:52Z")

</div>

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
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

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<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [March 16, 2021, 6:24pm UTC](https://discourse.julialang.org/t/error-message-in-array-indexing/57322/2 "2021-03-16T18:24:06Z")

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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
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).

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<div class="post-metadata">

**Author:** ![Jussi\_Salmela](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jussi_salmela/32/22992_2.png) [@Jussi\_Salmela](https://discourse.julialang.org/u/Jussi_Salmela)\
**Post date:** [March 16, 2021, 8:06pm UTC](https://discourse.julialang.org/t/error-message-in-array-indexing/57322/3 "2021-03-16T20:06:30Z")

</div>

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.

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<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [March 16, 2021, 8:45pm UTC](https://discourse.julialang.org/t/error-message-in-array-indexing/57322/4 "2021-03-16T20:45:21Z")

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> [@Jussi\_Salmela](#):
>
> makes coding essentially easier

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
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?

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<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [March 16, 2021, 10:15pm UTC](https://discourse.julialang.org/t/error-message-in-array-indexing/57322/5 "2021-03-16T22:15:24Z")

</div>

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](https://stackoverflow.com/a/7611586/176071). 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](https://juliahub.com/ui/CodeSearch?q=%2B&u=define&t=function) in my quick scan.

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<div class="post-metadata">

**Author:** ![Jussi\_Salmela](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jussi_salmela/32/22992_2.png) [@Jussi\_Salmela](https://discourse.julialang.org/u/Jussi_Salmela)\
**Post date:** [March 17, 2021, 11:57am UTC](https://discourse.julialang.org/t/error-message-in-array-indexing/57322/6 "2021-03-17T11:57:20Z")

</div>

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.