library(Matrix)
set.seed(1)
mat <- Matrix(0,5,5)

idx.mat <- do.call(rbind,lapply(1:5,function(i) sample(1:5,3,replace=F)))
val.mat <- matrix(runif(15,1,10),5,3)

mat <- do.call(rbind,lapply(1:nrow(mat),function(i) {
  mat[i,idx.mat[i,]] <- val.mat[i,]
  return(mat[i,])
}))