compress.m

为stanford大学donoho教授所编写的redgit变换源代码。是用c编写的

function [c, err] = compress(a, ratio)
% COMPRESS Compress an array
%
%	[c, err] = compress(a, ratio)
%
% The "compression" is achieved by zero-out 100*ratio percent of array
% element with smallest magnitute (hence, ratio is between 0 and 1).

if (ratio < 0) | (ratio > 1)
    error('ratio must be between 0 and 1');
end

% Sort the coefficient in the order of energy.
asort = sort(abs(a(:)));
err = cumsum(asort.^2);
err = flipud(err);

% Threshold.
mind = floor(ratio * prod(size(a)));
if (mind > 0)
    thresh = asort(mind);
    c = a .* (abs(a) > thresh);
else
    c = a;
end