ransort.m

来自「Polynomial Root Finder is a reliable and」· M 代码 · 共 26 行

function [ind] = ransort(n)

% function [ind] = ransort(n)
%
% function computes an index vector ind of length n which contains
% all integers 1:n but in a quasi random order.
%
% m-file bitrev must be available
% author: m. lang 17.12.91

% This is very advantageous for example for the function poly:
% compute z = roots([1 zeros(1:m) 1]); p = poly(z).
% There is a large error in p, which stems from the second operation
% poly. If the roots z are sorted with increasing angle in zs, then
% ph = poly(zs(ransort(n))) yields much less error than p.
%
% ransort uses bitreversal which is known as van der corput sequence
% in the theory of uniform distributed sequences.

m = ceil(log2(n));
ind = 1:2^m;
ind =  bitrev(ind);
ind = ind(find(ind<=n));