function plotdefield(defn,axisvec,ngrid)
% PLOTDEFIELD plots the direction field for a single first order
% differential equation.
%
% plotdefield(defn,axisvec) uses the d.e. defined 
% in normal form in function whose name is in the string defn.
% The function must have calling structure defn(t,y)
% and work with vector input arguments.
%
% axisvec is the vector [tmin,tmax,ymin,ymax] giving
% the minimum and maximum desired values of t and y.
%
% ngrid is an optional argument specifying the number of line 
% segments in t and y directions; the default value is 9.
%
% Specify "axis('square')" before calling this routine to get
% a square diagram if desired.

% Author: Colin Fox, January 1995.
% Modified for MatLab 4 Aug 1995
% Modified for MATLAB 5 June 23 1997 Michelle Wood
if nargin < 3, ngrid = 9; end 
gap = (axisvec([2,4]) - axisvec([1,3]))/ngrid;

t = linspace(axisvec(1),axisvec(2),ngrid);
y = linspace(axisvec(3),axisvec(4),ngrid);
[tt,yy] = meshgrid(t,y);

dy = zeros(size(tt)); dt = ones(size(tt));

for tcount = 1:ngrid
  for ycount = 1:ngrid
     dy(ycount,tcount)...
       = feval(defn,tt(ycount,tcount),yy(ycount,tcount));
  end
end

dirfield(dt,dy,t,y)

axis((axisvec + (gap([1,1,2,2]) .* [-1,1,-1,1])))