/*  average run of alternating coin-flips  */
/*  using a HASH_ARRAY type  */
make_random_state(true)$
for j : 0 thru 999 do (
  old_coin : 0,  /* not 'heads' or 'tails'  */
  new_coin : random(2)+1,
  XX[j] : 0,  /* set j-th element to zero  */
  while new_coin # old_coin do (
    old_coin : new_coin,
    new_coin : random(2)+1,
    XX[j] : XX[j] + 1
  )
)$
load('descriptive)$
float(mean(makelist(XX[j],j,0,999)));