iseven.m

linear time－frequency toolbox

function t=iseven(f,centering,tol);
%ISEVEN  True if function is even
%   Usage:  t=iseven(f);
%           t=iseven(f,centering);
%           t=iseven(f,centering,tol);
%
%   ISEVEN(f) returns 1 if f is whole point even. Otherwise it returns 0.
%
%   ISEVEN(f,centering) is true if f is even according to
%   the value of centering: 0 indicates a whole-point even function
%   and 0.5 indicates a half-point even function.  
%
%   ISEVEN(f,centering,tol) does the same, using the tolerance tol
%   to measure how large the error between the two parts of the vector
%   can be. 
%

if size(f,2)>1
  if size(f,1)>1
    error('f must be a vector');
  else
    % f was a row vector.
    f=f(:);
  end;
end;

if nargin<3
  tol=1e-10;
end;

if nargin<2
  centering=0;
end;

L=size(f,1);

switch centering
  case 0
    % Determine middle point of sequence.
    if rem(L,2)==0
      middle=L/2;
    else
      middle=(L+1)/2;
    end;
    
    % Relative norm of difference between the parts of the signal.
    d=norm(f(2:middle)-conj(flipud(f(L-middle+2:L))))/norm(f);
  case .5
    
    middle=floor(L/2);

    d=norm(f(1:middle)-conj(flipud(f(L-middle+1:L))))/norm(f);
    otherwise
      error('Unsupported centering.');
  end;


% Return true if d less than tolerance.
t=d<=tol;