Re: exercise in double number arithmetic

Yet another fast variant without looping/recursion:
\ LOGBINOM
100 FLOATS BUFFER: LCS{ \ log sum array
: }  ( a i -- f: n ) floats + f@ ;
: }! ( a i f: n -- ) floats + f! ;
: PREP-LCS ( -- ) \ prepare array
  0e lcs{ 0 }! 100 1 DO
    lcs{ i 1- } i s>f flog f+ lcs{ i }!
  LOOP ;
: LBINOM ( n k -- binom ) \ calc binom coeff by array lookup
  2dup = over 0= or
  IF 0e
  ELSE 2dup lcs{ swap } lcs{ swap } - lcs{ swap } f- fswap f-
  THEN falog fround f>d ;
PREP-LCS
42 21 LBINOM D.