divisor.m

function d = divisor(n)
%% divisor : provides a list of integer divisors of a number.
% divisor(n) : row vector of all distinct divisors of a positive integer N, 
%               including 1 and N.
%
% Remark:
%   This function uses the default factor() routine in Matlab and hence is 
% limited to input values upto 2^32. However if factor() routine does get
% updated for larger integers, this function will still work fine.
%   Using factor() provides a significant speed improvement over manually 
% seaching for the each divisor of n.
%
% Example:
%   a = divisor(12);
%   returns -> a = [1, 2, 3, 4, 6, 12];
%
% See Also:
%   factor, primes

% Author: Yash Kochar ( yashkochar@yahoo.com )
% Last modified: 21st June 2009
%-------------------------------------------------------------------------------

%% Input error check :
%   Check whether input is positive integer and scalar.
if ~isscalar(n)
    error('divisor:NonScalarInput','Input must be a scalar.');
end
if (n < 1) || (floor(n) ~= n)
  error('divisor:PositiveIntegerOnly', 'Input must be a positive integer.'); 
end

%% Find prime factors of number :
pf = factor(n);         % Prime factors of n
upf = unique(pf);       % Unique

%% Calculate the divisors
d = upf(1).^(0:1:sum(pf == upf(1)))';
for f = upf(2:end)
    d = d*(f.^(0:1:sum(pf == f)));
    d = d(:);
end
d = sort(d)';   % To further improve the speed one may remove this sort command
                %   Just remember to take the transpose of "d" to get a result
                %   as a row vector instead of a column vector.