Generate a random matrix in R with m columns and n rows where rows sum to 1

Question

I want to generate an nxm matrix. Suppose its 100x3. I want each row to sum to 1 (so two \"0\"\'s and one \"1\").

sample(c(0,0,1),3) 

will

Answer 1

No loops, no transposition. Just create a matrix of zeros and replace one entry per row with 1 by sampling the rows.

m <- matrix(0, 100, 3)
nr <- nrow(m)
m[cbind(1:nr, sample(ncol(m), nr, TRUE))] <- 1

all(rowSums(m) == 1)
# [1] TRUE