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// Copyright INRIA
//
// sharks and sardins: Lotka-Volterra ODE
//
mode(-1)
xdel(winsid())
toolbar(0,'off');
titlepage(["Lotka-Volterra:";...
"dy1/dt= 3*y1 - 2*y1*y2";..
"dy2/dt=-2*y2 + y1*y2"]);
halt(); clf();
mode(1)
oldln=lines();
lines(0)
deff("yprim=f(t,y)",..
["yprim1=aa*y(1)-cc*y(1)*y(2)";..
"yprim2=-bb*y(2)+ff*y(1)*y(2)";..
"yprim=[yprim1;yprim2]"])
aa=3; bb=2; cc=2; ff=1;
xmin=0; xmax=4; ymin=0; ymax=4;
fx=xmin:0.5:xmax; fy=ymin:0.5:ymax;
toolbar(0,'off');
fchamp(f,1,fx,fy)
a=gca();
a.x_label.text="y1",a.x_label.font_size=3;
a.y_label.text="y2",a.y_label.font_size=3;
xstring(1,4.15,['Click left to start a new trajectory, move the mouse'
' and click again to fix the desired trajectory'
'Click right to exit'])
t0=0; tmax=10;
t=t0:0.05:tmax;
oldx0=10*xmax; oldy0=10*ymax;
dx=0.1; dy=0.1;
rtol=0.0001; atol=rtol;
fig=gcf();fig.pixmap='on';
show_pixmap();
while (%t)
[b,x0,y0]=xclick();
if or(b==[2 5]) then break end;
if or(b==[0 3]) & xmin<x0 & x0<xmax & ymin<y0 & y0<ymax then
sol=ode([x0;y0],t0,t,rtol,atol,f);
xpoly(sol(1,:)',sol(2,:)');
p=gce();p.thickness=2;p.foreground=5;
show_pixmap()
rep=[x0,y0,-1];
while rep(3)==-1 then
rep=xgetmouse(0);
x0=rep(1); y0=rep(2);
if (xmin<x0 & x0<xmax & ymin<y0 & y0<ymax) &..
(abs(x0-oldx0)>=dx | abs(y0-oldy0)>=dy) then
sol=ode([x0;y0],t0,t,rtol,atol,f);
p.data=[sol(1,:)' sol(2,:)'];
show_pixmap()
oldx0=x0; oldy0=y0;
end
end
end
end
fig.pixmap='off';
lines(oldln(1))
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