x <- numeric(0) for (...) { x[length(x)+1] <- ... } works. You can build a matrix by building a vector one element at a time this way, and then reshaping it at the end. That only works if you don't need it to be a matrix at all times. Another approach is to build a list of rows. It's not a matrix, but a list of rows can be a *ragged* matrix with rows of varying length.
On Wed, 28 Feb 2024 at 21:57, Steven Yen <st...@ntu.edu.tw> wrote: > > Is there as way to initialize a vector (matrix) with an unknown length > (dimension)? NULL does not seem to work. The lines below work with a > vector of length 4 and a matrix of 4 x 4. What if I do not know > initially the length/dimension of the vector/matrix? > > All I want is to add up (accumulate) the vector and matrix as I go > through the loop. > > Or, are there other ways to accumulate such vectors and matrices? > > > x<-rep(0,4) # this works but I like to leave the length open > > for (i in 1:3){ > + x1<-1:4 > + x<-x+x1 > + } > > x > [1] 3 6 9 12 > > > y = 0*matrix(1:16, nrow = 4, ncol = 4); # this works but I like to > leave the dimension open > [,1] [,2] [,3] [,4] > [1,] 0 0 0 0 > [2,] 0 0 0 0 > [3,] 0 0 0 0 > [4,] 0 0 0 0 > > for (i in 1:3){ > + y1<-matrix(17:32, nrow = 4, ncol = 4) > + y<-y+y1 > + } > > y > [,1] [,2] [,3] [,4] > [1,] 51 63 75 87 > [2,] 54 66 78 90 > [3,] 57 69 81 93 > [4,] 60 72 84 96 > > > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.