[1 2 [3 4] 5]
is short for
horzcat(1, 2, [3 4], 5)
which in turn would be short for
horzcat(1, 2, horzcat(3, 4), 5)
and there is no problem doing concatenation between a vector and a vector giving a longer vector result.
Consider that you could also have tried
A = [3 4]
[1 2 A 5]
This is valid in MATLAB as long as the number of rows generated by the part to the left of the A in [1 2 A 5] is the same as the number of rows in A, and the number of rows after [1 2 A] is the same as the number of rows in [5]
MATLAB [A B] syntax does not mean to set individual elements with individual scalars A and B: it means to drop in the contents of A and B and concatenate them together on the second dimension (by default), or on the first dimension if you use ; as the delimiter like [1 2;3 4] is vertcat(horzcat(1,2), horzcat(3,4))
How come I can do element-wise multiplication but not addition?
Addition and subtraction and logical and (&) and logical or (|) and relationship operators < and > and == and <= and >= and ~= are always element-by-element already and do not need a special element-by-element form.
