## Copyright (C) 1996 John W. Eaton
##
## This file is part of Octave.
##
## Octave is free software; you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 2, or (at your option)
## any later version.
##
## Octave is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
## General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with Octave; see the file COPYING.  If not, write to the Free
## Software Foundation, 59 Temple Place - Suite 330, Boston, MA
## 02111-1307, USA.

## usage: list_primes (n)
##
## List the first n primes.  If n is unspecified, the first 30 primes
## are listed.
##
## The algorithm used is from page 218 of the TeXbook.

## Author: jwe

function retval = list_primes (n)

  if (nargin > 0)
    if (! is_scalar (n))
      error ("list_primes: argument must be a scalar");
    endif
  endif

  if (nargin == 0)
    n = 30;
  endif

  if (n == 1)
    retval = 2;
    return;
  endif

  if (n == 2)
    retval = [2; 3];
    return;
  endif

  retval = zeros (1, n);
  retval (1) = 2;
  retval (2) = 3;

  n = n - 2;
  i = 3;
  p = 5;
  while (n > 0)

    is_prime = 1;
    is_unknown = 1;
    d = 3;
    while (is_unknown)
      a = fix (p / d);
      if (a <= d)
        is_unknown = 0;
      endif
      if (a * d == p)
        is_prime = 0;
        is_unknown = 0;
      endif
      d = d + 2;
    endwhile

    if (is_prime)
      retval (i++) = p;
      n--;
    endif
    p = p + 2;

  endwhile

endfunction
