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

mat <- matrix(runif(300),ncol=3)
mat[] <- as.numeric(t(apply(mat, 1, function(r) r == max(r))))