How to turn a vector into a matrix in R?

Question

I have a vector with 49 numeric values. I want to have a 7x7 numeric matrix instead.

Is there some sort of convenient automatic conversion statement I can use, or do I have to do 7 separate column assignments of the correct vector subsets to a new matrix? I hope that there is something like the oposite of c(myMatrix), with the option of giving the number of rows and/or columns I want to have, of course.

Answer 1

Just use matrix:

matrix(vec,nrow = 7,ncol = 7) 

One advantage of using matrix rather than simply altering the dimension attribute as Gavin points out, is that you can specify whether the matrix is filled by row or column using the byrow argument in matrix.

Answer 2

A matrix is really just a vector with a dim attribute (for the dimensions). So you can add dimensions to vec using the dim() function and vec will then be a matrix:

vec <- 1:49 dim(vec) <- c(7, 7)  ## (rows, cols) vec  > vec <- 1:49 > dim(vec) <- c(7, 7)  ## (rows, cols) > vec      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [1,]    1    8   15   22   29   36   43 [2,]    2    9   16   23   30   37   44 [3,]    3   10   17   24   31   38   45 [4,]    4   11   18   25   32   39   46 [5,]    5   12   19   26   33   40   47 [6,]    6   13   20   27   34   41   48 [7,]    7   14   21   28   35   42   49