From octave-sources-request at bevo dot che dot wisc dot edu  Tue Jun 27 07:57:49 2000
Subject: Re: primes.m
From: Francesco Potorti` <F dot Potorti at cnuce dot cnr dot it>
To: pkienzle at kienzle dot powernet dot co dot uk (Paul Kienzle)
CC: octave-sources at bevo dot che dot wisc dot edu
Date: Tue, 27 Jun 2000 14:56:27 +0200

Your algorithm, optimized.

About three times faster, and needs half the memory :-)

function x=primes(p)
  len = (p-1)/2;		# length of the sieve
  sieve = ones (1, len);	# assume every odd number is prime
  for i=1:(sqrt(p)/2)           # check up to sqrt(p)
    if (sieve(i))		# if i is prime, eliminate multiples of i
      sieve(3*i+1:2*i+1:len) = 0; # do it
    endif
  endfor
  x = [2, 1+2*find(sieve)];	# primes remaining after sieve
endfunction

-- 
Francesco Potort́ (researcher)         Voice: +39 050 315 3058 (op.2215)
Area della ricerca CNR - CNUCE         Fax:   +39 050 3138091
via Vittorio Alfieri 1                 Email: F dot Potorti at cnuce dot cnr dot it
I-56010 Ghezzano, Pisa                 Web:   http://fly.cnuce.cnr.it/



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