function dirfield(t,x,y,f)
% usage dirfield(t,x,y,f)
% description: this function plots a direction field with x and y
% as x-y coordinates for the grid, t the time and f the name of the
% function f(t,x,y) for the direction vectors.It also sets hold on.

m = length(x);
n = length(y);
[xx,yy]=meshgrid(x,y);
uu = xx;
vv = yy;
for ii=1:n
    for jj=1:m
        vtmp = feval(f,t,[xx(ii,jj);yy(ii,jj)]);
        xxrhs(ii,jj)=vtmp(1);
        yyrhs(ii,jj)=vtmp(2);
    end
end
quiver(xx,yy,xxrhs,yyrhs);
hold on;