simps.m

function z = simps(x,y,dim)
%SIMPS  Simpson's numerical integration.
%   Z = SIMPS(Y) computes an approximation of the integral of Y via the
%   Simpson's method (with unit spacing). To compute the integral for
%   spacing different from one, multiply Z by the spacing increment.
%
%   For vectors, SIMPS(Y) is the integral of Y. For matrices, SIMPS(Y) is a
%   row vector with the integral over each column. For N-D arrays, SIMPS(Y)
%   works across the first non-singleton dimension.
%
%   Z = SIMPS(X,Y) computes the integral of Y with respect to X using the
%   Simpson's rule. X and Y must be vectors of the same length, or X must
%   be a column vector and Y an array whose first non-singleton dimension
%   is length(X). SIMPS operates along this dimension.
%
%   Z = SIMPS(X,Y,DIM) or SIMPS(Y,DIM) integrates across dimension DIM of
%   Y. The length of X must be the same as size(Y,DIM).
%
%   Examples:
%   --------
%   % The integration of sin(x) on [0,pi] is 2
%   % Let us compare TRAPZ and SIMPS
%   x = linspace(0,pi,6);
%   y = sin(x);
%   trapz(x,y) % returns 1.9338
%   simps(x,y) % returns 2.0071
%
%   If Y = [0 1 2
%           3 4 5
%           6 7 8]
%   then simps(Y,1) is [6 8 10] and simps(Y,2) is [2; 8; 14]
%
%
%   Class support for inputs X, Y:
%      float: double, single
%
%   -- Damien Garcia -- 08/2007, revised 11/2009
%      directly adapted from TRAPZ
%
%   See also CUMSIMPS, TRAPZ, QUAD.


%%   Make sure x and y are column vectors, or y is a matrix.
perm = []; nshifts = 0;
if nargin == 3 % simps(x,y,dim)
  perm = [dim:max(ndims(y),dim) 1:dim-1];
  yp = permute(y,perm);
  [m,n] = size(yp);
elseif nargin==2 && isscalar(y) % simps(y,dim)
  dim = y; y = x;
  perm = [dim:max(ndims(y),dim) 1:dim-1];
  yp = permute(y,perm);
  [m,n] = size(yp);
  x = 1:m;
else % simps(y) or simps(x,y)
  if nargin < 2, y = x; end
  [yp,nshifts] = shiftdim(y);
  [m,n] = size(yp);
  if nargin < 2, x = 1:m; end
end
x = x(:);
if length(x) ~= m
  if isempty(perm) % dim argument not given
    error('MATLAB:simps:LengthXmismatchY',...
          'LENGTH(X) must equal the length of the first non-singleton dimension of Y.');
  else
    error('MATLAB:simps:LengthXmismatchY',...
          'LENGTH(X) must equal the length of the DIM''th dimension of Y.');
  end
end

%% The output size for [] is a special case when DIM is not given.
if isempty(perm) && isequal(y,[])
  z = zeros(1,class(y));
  return;
end

%% Use TRAPZ if m<3
if m<3
    if exist('dim','var')
        z = trapz(x,y,dim);
    else
        z = trapz(x,y);
    end
    return
end

%% Simpson's rule
y = yp;
clear yp

dx = repmat(diff(x,1,1),1,n);
dx1 = dx(1:end-1,:);
dx2 = dx(2:end,:);

alpha = (dx1+dx2)./dx1/6;
a0 = alpha.*(2*dx1-dx2);
a1 = alpha.*(dx1+dx2).^2./dx2;
a2 = alpha.*dx1./dx2.*(2*dx2-dx1);

z = sum(a0(1:2:end,:).*y(1:2:m-2,:) +...
    a1(1:2:end,:).*y(2:2:m-1,:) +...
    a2(1:2:end,:).*y(3:2:m,:),1);

if rem(m,2) == 0 % Adjusting if length(x) is even   
    state0 = warning('query','MATLAB:nearlySingularMatrix');
    state0 = state0.state;
    warning('off','MATLAB:nearlySingularMatrix')
    C = vander(x(end-2:end))\y(end-2:end,:);
    z = z + C(1,:).*(x(end,:).^3-x(end-1,:).^3)/3 +...
        C(2,:).*(x(end,:).^2-x(end-1,:).^2)/2 +...
        C(3,:).*dx(end,:);
    warning(state0,'MATLAB:nearlySingularMatrix')
end

%% Resizing
siz = size(y); siz(1) = 1;
z = reshape(z,[ones(1,nshifts),siz]);
if ~isempty(perm), z = ipermute(z,perm); end