Matrix rows notation
I'm working with a set of $M$ vectors $ \{\mathbf{w}_i \in \mathbb{R}^N, \, i = 1, \ldots, M \}$. Since single vectors are usually considered as column vectors, I'm defining a matrix
$$ \mathbf{W} = [\mathbf{w}_1, \ldots, \mathbf{w}_M] \in \mathbb{R}^{N \times M} $$
by placing the vectors as matrix columns.
However, for some descriptions, I need to refer to the matrix rows.
Is there an elegant notation to refer to this matrix rows (preferably with less notation overhead)?
Since $W_{nm}$ is unambiguous for the $n$th row, $m$th col entry of $W$, I quite like the following notation of using a star, dot, or colon as a placeholder for the other dimension (similar to how dot or star can be used as omitted entries in a matrix):
- For rows: $W_{n*}$ or $W_{n,*}$ (or $W_{n\cdot}$ or $W_{n:}$)
- For cols: $W_{*m}$ or $W_{*,m}$ (or other symbols likewise)
From a programming perspective, this is similar to how in R we simply leave off the other dimension, writing W[n,]
and W[,m]
, and in Numpy we use a colon to represent the entire other axis with W[n,:]
and W[m,:]
.
You can represent the rows of $\mathbf W$ by the $N$ - (column) vectors of $\mathbf W^{\mathsf T}$ $\mathbf w'_i :=\mathrm{col}_i\mathbf W^\mathsf T\;(i=1,\dots,N).$ There is no standard notation for this; I chose the prime notation for convenience. You would also need to state your chosen notation explicitly.