Hi, what does the following line in Julia do? Thank you

The matrix H is being assigned a value based on a ternary operator, which takes the form a ? b : c. In this case, if H is nothing, it will made the same as h. Otherwise, it will be concatenated with h (as with hcat).

Note: When H === nothing, the code does not assign H the values of h, as has been suggested. Instead, H is a copy of h. For instance, if you change one of the elements in H, it will change an element in h, since they are the same matrix.

It may help to look at the documentation for arrays.

Suppose

H === nothing

evaluates to true, then H is assigned the value of h. If false, then H is assigned to a 1x2 matrix which has H as its first value and h as its second value.