\begin{eqnarray*} A = \begin{bmatrix} 2 & 6 & 3 \\ 4 & 1 & 5 \\ \end{bmatrix} \end{eqnarray*}

A <- c(2, 4, 6, 1, 3, 5)
A
dim(A) <- c(2, 3)
A
class(A)

A <- c(2, 4, 6, 1, 3, 5)
A
AbyCol <- matrix(nrow=2, ncol=3, A)
AbyCol
AbyRow <- matrix(nrow=2, ncol=3, A, byrow=T)
AbyRow

A <- c(2, 4, 6, 1, 3, 5)
A
AbyCol <- matrix(nrow=2, ncol=3, A)
AbyCol
AbyRow <- matrix(nrow=2, ncol=3, A, byrow=T)
AbyRow