xt<-function(n, nmin=4){
  #error handling
  if(class(n)!="numeric"){stop("Must be numeric.")}
  if(floor(n)!=n){stop("Must be whole number.")}
  if(n<nmin){stop("n must be at least nmin.")}
  #set up vector for rows
  v<-vector("character", (2*n)-1)
  #determine length of each row of stars.
  for(i in 1:(n-1)){
    mid<-(length(v)+1)/2
    lbound<-mid-(i-1)
    rbound<-mid+(i-1)
    #fill in each row of stars.
    for(j in lbound:rbound){    
      v[j]<-"*"
    }
    print(noquote(v))
  }
  #put in the last row for the stem.
  v<-vector("character", (2*n)-1)
  v[mid]<-"*";print(noquote(v))
}
xt(15)