# How does julia prioritize operations and array concatenation?
**URL:** https://discourse.julialang.org/t/how-does-julia-prioritize-operations-and-array-concatenation/96447
**Category:** General Usage
**Tags:** question
**Created:** [March 22, 2023, 2:40pm UTC](https://discourse.julialang.org/t/how-does-julia-prioritize-operations-and-array-concatenation/96447 "2023-03-22T14:40:00Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@ovt](https://discourse.julialang.org/u/ovt)
#### Post date: [March 22, 2023, 2:40pm UTC](https://discourse.julialang.org/t/how-does-julia-prioritize-operations-and-array-concatenation/96447/1 "2023-03-22T14:40:00Z")
Why is it ok to write `1 .-A` but not `[A;1 .-A]` ? Somehow, it only works with a space between `-` and `A`.
Is it because Julia prioritize the `A;1` part over the `1 .-A` part ? Is there any documentation on how and why julia prioritize one over the other ?
```julia
julia> A = rand(3,3)
3×3 Matrix{Float64}:
0.740468 0.317305 0.247191
0.734218 0.903331 0.647368
0.115604 0.461894 0.854236
julia> [A;1 .-A]
ERROR: DimensionMismatch: mismatch in dimension 1 (expected 1 got 3)
Stacktrace:
[1] _cs
@ ./abstractarray.jl:1717 [inlined]
[2] _cshp
@ ./abstractarray.jl:1713 [inlined]
[3] _cat_size_shape
@ ./abstractarray.jl:1693 [inlined]
[4] cat_size_shape
@ ./abstractarray.jl:1691 [inlined]
[5] _cat_t
@ ./abstractarray.jl:1732 [inlined]
[6] typed_hcat(::Type{Float64}, ::Int64, ::Matrix{Float64})
@ Base ./abstractarray.jl:1881
[7] typed_hvcat(::Type{Float64}, ::Tuple{Int64, Int64}, ::Matrix{Float64}, ::Vararg{Any})
@ Base ./abstractarray.jl:2094
[8] hvcat(::Tuple{Int64, Int64}, ::Matrix{Float64}, ::Vararg{Union{Number, LinearAlgebra.AbstractTriangular{T, A} where {T, A<:(Matrix)}, Adjoint{<:Any, <:Vector}, Bidiagonal, Diagonal, Hermitian{T, A} where {T, A<:(Matrix)}, SymTridiagonal, Symmetric{T, A} where {T, A<:(Matrix)}, Transpose{<:Any, <:Vector}, Tridiagonal, Matrix, Vector}})
@ LinearAlgebra ~/julia/julia-1.8.3/share/julia/stdlib/v1.8/LinearAlgebra/src/special.jl:422
[9] top-level scope
@ REPL[2]:1
julia> [A;1 .- A]
6×3 Matrix{Float64}:
0.740468 0.317305 0.247191
0.734218 0.903331 0.647368
0.115604 0.461894 0.854236
0.259532 0.682695 0.752809
0.265782 0.0966693 0.352632
0.884396 0.538106 0.145764
julia> 1 .-A
3×3 Matrix{Float64}:
0.259532 0.682695 0.752809
0.265782 0.0966693 0.352632
0.884396 0.538106 0.145764
```
---
### Author:  [@nilshg](https://discourse.julialang.org/u/nilshg)
#### Post date: [March 22, 2023, 3:03pm UTC](https://discourse.julialang.org/t/how-does-julia-prioritize-operations-and-array-concatenation/96447/2 "2023-03-22T15:03:21Z")
Julia is whitespace sensitive within `[]` brackets as this is used for matrix construction, e.g.
```julia
[1 2; 3 -4]
```
is a valid matrix with a negative element. If in doubt it’s best to just use brackets to clarify intent, i.e.
```julia
[A;(1 .-A)]
```
will work fine.
---
### Author:  [@mcabbott](https://discourse.julialang.org/u/mcabbott)
#### Post date: [March 22, 2023, 3:38pm UTC](https://discourse.julialang.org/t/how-does-julia-prioritize-operations-and-array-concatenation/96447/3 "2023-03-22T15:38:14Z")
IMO using the bracket syntax for concatenation is usually a bad idea for anything other than writing a literal matrix of numbers. `vcat(A, 1 .-A)` works fine, no special parsing rules, no need later to squint at whether you have a semicolon or a comma there later.
---
### Author:  [@tbeason](https://discourse.julialang.org/u/tbeason)
#### Post date: [March 22, 2023, 3:38pm UTC](https://discourse.julialang.org/t/how-does-julia-prioritize-operations-and-array-concatenation/96447/4 "2023-03-22T15:38:47Z")
Also note that while
```julia
[A; 1 .-A]
```
fails because of whitespace, you can also just remove the whitespace
```julia
[A; 1.0.-A]
```