Concatenation arguments

I do not understand the difference between :

# applying hcat to a vector
hcat( [hcat(Gray.(Float64.(rand(10)))) for _ in 1:10] )

# applying hcat to a list of arguments, using the splat operator
vecs = [hcat(Gray.(Float64.(rand(10)))) for _ in 1:10]

Hcat takes its arguments and concatenates them horizontally. When you pass multiple vectors to hcat it will create a matrix with each vector as its columns. It does exactly this when you only pass a single vector, creating a Nx1 Matrix.

When you use the splat operator it separates the vector into its components and concatenates them creating I.e a row vector.

julia> hcat(rand(10))
10×1 Matrix{Float64}

julia> hcat(rand(10)...)
1×10 Matrix{Float64}

Since your vector is made of 10x1 matrices it creates a 10x10 matrix.

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