#! /usr/bin/env python3
#
def double_factorial ( n ):

#*****************************************************************************80
#
## double_factorial() evaluates the double factorial function.
#
#  Discussion:
#
#    The double factorial function of n is represented by n!!
#
#    If n is even, n!! = 2 * 4 * ... * n-2 * n
#
#    If n is odd, n!! = 1 * 3 * ... * n-2 * n
#
#  Modified:
#
#    17 January 2023
#
#  Author:
#
#    John Burkardt
#
#  Input:
#
#    integer n: the argument.
#
#  Output:
#
#    integer value: the value of n!!
#
  value = 1
  
  while ( 0 < n ):
    value = value * n
    n = n - 2

  return value

def double_factorial_test ( ):

#*****************************************************************************80
#
## double_factorial() evaluates the double factorial function.
#
  exact = [ 1, 1, 2, 3, 8, 15, 48, 105, 384, 945, 3840, 10395, 46080, 125135, 645120 ]
  exact_num = len ( exact )

  print ( '' )
  print ( 'double_factorial_test():' )
  print ( '  Test double_factorial(n)' )
  print ( '' )

  for n in range ( 0, exact_num ):
    value = double_factorial ( n )
    print ( n, value, exact[n] )

  return

if ( __name__ == "__main__" ):
  double_factorial_test ( )