largestn.m

linear time－frequency toolbox

function [xo]=largestn(xi,N)
%LARGESTN   Keep N largest coefficients.
%   Usage:  xo=largestn(x,N);
%
%   LARGESTN(x,N) returns an array of the same size as x keeping
%   the N largest coefficients.
%
%   If the coefficients represents a signal expanded in an orthonormal
%   basis, then this will be the best N-term approximation.
%
%   SEE ALSO:  LARGESTR
%
%   REFERENCES:
%     S. Mallat. A wavelet tour of signal processing. Academic Press, San Diego,
%     CA, 1998.

if nargin~=2
  error('Wrong number of input arguments.');
end;

if (prod(size(N))~=1 || ~isnumeric(N))
  error('N must be a scalar.');
end;

% Determine the size of the array.
ss=prod(size(xi));

% Handle the trivial cases.
if N>=ss
  xo=xi;
  return;
end;

if N<=0
  xo=zeros(size(xi));
  return;
end;

% Sort the absolute values of the coefficients.
sxi=sort(abs(xi(:)));

% Find the coeffiecient sitting at position N through the array,
% and use this as a threshing value. 
threshval=sxi(ss-N+1);

% Do the threshing.
xo=xi.*(abs(xi)>=threshval);