function [K] = kernel(ker,x,y)% Calculate kernel function.   %% x: 輸入樣本,d×n1的矩陣,n1為樣本個數,d為樣本維數% y: 輸入樣本,d×n2的矩陣,n2為樣本個數,d為樣本維數%% ker  核參數(結構體變量)% the following fields:%   type   - linear :  k(x,y) = x'*y%            poly   :  k(x,y) = (x'*y+c)^d%            gauss  :  k(x,y) = exp(-0.5*(norm(x-y)/s)^2)%            tanh   :  k(x,y) = tanh(g*x'*y+c)%   degree - Degree d of polynomial kernel (positive scalar).%   offset - Offset c of polynomial and tanh kernel (scalar, negative for tanh).%   width  - Width s of Gauss kernel (positive scalar).%   gamma  - Slope g of the tanh kernel (positive scalar).%% ker = struct('type','linear');% ker = struct('type','ploy','degree',d,'offset',c);% ker = struct('type','gauss','width',s);% ker = struct('type','tanh','gamma',g,'offset',c);%% K: 輸出核參數,n1×n2的矩陣%-------------------------------------------------------------%switch ker.type    case 'linear'        K = x'*y;    case 'ploy'        d = ker.degree;        c = ker.offset;        K = (x'*y+c).^d;    case 'gauss'                s = ker.width;        rows = size(x,2);        cols = size(y,2);           tmp = zeros(rows,cols);        for i = 1:rows            for j = 1:cols                tmp(i,j) = norm(x(:,i)-y(:,j));            end        end                K = exp(-0.5*(tmp/s).^2);    case 'tanh'        g = ker.gamma;        c = ker.offset;        K = tanh(g*x'*y+c);    otherwise        K = 0;end