is_prime1 <- function ( n )

#*****************************************************************************80
#
## is_prime1() reports whether an integer is prime.
#
#  Licensing:
#
#    This code is distributed under the MIT license.
#
#  Modified:
#
#    29 December 2022
#
#  Author:
#
#    John Burkardt.
#
#  Input:
#
#    integer n: the value to be tested.
#
#  Output:
#
#    logical value: true if n is a prime.
#
{
   if ( n != round ( n ) )
   {
     stop ( 'is_prime1(): input n is not an integer!\n' )
   }

  if ( n < 0 )
  {
    return ( FALSE )
  }

  if ( n == 0 | n == 1 )
  {
    return ( FALSE )
  }
#
#  I had to insert this check to avoid the catastrophic
#  idiocy of the for loop limits.
#
  if ( n == 2 )
  {
    return ( TRUE )
  }
#
#  Astoundingly, R parses ( i in 2 : n - 1 ) as (i in (2:n)-1)!
#
#  R morons execute ( i in 2 : 1 ) as 2 downto 1.
#  To avoid this, we have to make 2 a special case.
#
  for ( i in 2 : ( n - 1 ) )
  {
    if ( ( n %% i ) == 0 )
    {
      return ( FALSE )
    }
  }

  return ( TRUE )
}