factor.m

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function f = factor(n)
%FACTOR Prime factors.
%   FACTOR(N) returns a vector containing the prime factors of N.
%
%   This function uses the simple sieve approach. It may require large
%   memory allocation if the number given is too big. Technically it is
%   possible to improve this algorithm, allocating less memory for most
%   cases and resulting in a faster execution time. However, it will still
%   have problems in the worst case, so we choose to impose an upper bound 
%   on the input number and error out for n > 2^32. 
% 
%   Class support for input N:
%      float: double, single
%
%   See also PRIMES, ISPRIME.

%   Copyright 1984-2004 The MathWorks, Inc. 
%   $Revision: 1.14.4.2 $  $Date: 2004/07/05 17:01:58 $

if numel(n)~=1, error('MATLAB:factor:NonScalarInput','N must be a scalar.'); end
if (n < 0) || (floor(n) ~= n)
  error('MATLAB:factor:InputNotPosInt', 'N must be a positive integer.'); 
end
if n > 2^32
    error('MATLAB:factor:InputOutOfRange','The maximum value of n allowed is 2^32.');
end

if n < 4
   f=n; 
   return
else
   f = [];
end

p = primes(sqrt(n));
while n>1,
  d = find(rem(n,p)==0);
  if isempty(d)
    f = [f n];
    break; 
  end
  p = p(d);
  f = [f p];
  n = n/prod(p);
end

f = sort(f);