I am reading a ML book it says all vectors are column vectors. Is this true in general in books (and especially in practice) that all vectors are implied to be column vectors. Or is it just for that book.
I see a lot of $X^TX$ in books, which would make sense if we assume X is a column vector.
Vectors are most often considered to be column vectors because if $v$ is a vector and $A$ is a matrix we want to write $Av$ instead of $vA$. This is simply a convention, and I'm sure at least one book reverses it and assumes all vectors are row vectors. I don't have an example handy though.