u_vect = rand(5,2)
for i in eachindex(u_vect)
println(i)
a = u_vect[i,2]
end

And get the error: “BoundsError: attempt to access 5×2 Matrix{Float64} at index [6, 2].” Why does my 5x2 vector have 6 index positions? I know that arrays are considered one dimensional, but that still doesn’t explain the error?

eachindex(u_vect) gives you the linear indices of every element in the matrix:

julia> eachindex(u_vect)
Base.OneTo(10)

So by doing u_vect[i, 2], you’re trying to access u_vect[1, 2], u_vect[2, 2], ..., u_vect[10, 2], which doesn’t work (it errors as soon as you get to u_vect[6, 2]).

If you’re using eachindex, then you want to index with just i, as in:

for i in eachindex(u_vect)
a = u_vect[i]
...
end

or if you just want to iterate over the indices in the first dimension, then you want axes: