randpermpiotrdollar.m

来自「一个Matlab写的关于图理论以及其在机器学习中应用的教学用GUI软件」· M 代码 · 共 41 行

% Returns a random permutation of integers. 
%
% randperm2(n) is a random permutation of the integers from 1 to n.  For example,
% randperm2(6) might be [2 4 5 6 1 3].  randperm2(n,k) is only returns the first k
% elements of the permuation, so for example randperm2(6) might be [2 4].
% 
% This is a faster version of randperm.m if only need first k<<n elements of the random
% permutation.  Also uses less random bits (only k).  Note that this is an implementation
% O(k), versus the matlab implementation which is O(nlogn), however, in practice it is
% often slower for k=n because it uses a loop.
%
% INPUTS
%   n 	- permute 1:n
%   k   - keep only first k outputs
%
% OUTPUTS
%   p 	- k length vector of permutations
%
% EXAMPLE
%   randperm2(10,5)
%
% DATESTAMP
%   29-Sep-2005  2:00pm
%
% See also RANDPERM

% Piotr's Image&Video Toolbox      Version 1.03   
% Written and maintained by Piotr Dollar    pdollar-at-cs.ucsd.edu 
% Please email me if you find bugs, or have suggestions or questions! 
 
function p = randperm2(n,k);

    if (nargin<2) k=n; else k = min(k,n); end

    p = 1:n;
    for i=1:k
        r = i + floor( (n-i+1)*rand );     
        t = p(r);  p(r) = p(i);  p(i) = t;
    end
    p = p(1:k);