Why is 1 smaller than 0?
- To: mathgroup at smc.vnet.net
- Subject: [mg73749] Why is 1 smaller than 0?
- From: p at dirac.org (Peter Jay Salzman)
- Date: Tue, 27 Feb 2007 05:49:58 -0500 (EST)
This is an implementation of "steepest descent" to minimize a function. It
runs for 36 iterations. On the 36th iteration, it claims:
0.0274 > .00001 True
which is true. After the While loop exits, it claims:
0.03 > .00001 False
which is true, not false, as Mathematica claims.
Why is the While loop exiting prematurely? How do I write this so that it
runs for as long as f(x,y) is greater than .00001?
Thanks!
Pete
(* Implements Minimization via method of steepest descent. *)
Clear[f, x, s, delf, a , xNew, iteration, delta, tolerance, theA, min];
(* Function to minimize *)
f[x_, y_] := 100`50*(y - x^2)^2 + (1 - x)^2;
delf[x_, y_] := {-2*(1 - x) - 400.0`50*x(-x^2 + y), 200.0`50*(-x^2 + y)};
iteration = 0.0`50;
(* Initial guess. *)
x = { {5.0`50,1.0`50} };
(* Points "downhill" from the current position. *)
s = {};
While[ f[Last[x][[1]],Last[x][[2]]] > .00001,
Print[f[Last[x][[1]],Last[x][[2]]], " ", f[Last[x][[1]],Last[x][[2]]] > .00001];
(* Get direction to travel in (downhill) from grad f. *)
s = Append[s, -delf[Last[x][[1]], Last[x][[2]]]];
(* a tells us how far to travel. Need to minimize f to find it. *)
xNew = Last[x] + a*delf[Last[x][[1]], Last[x][[2]]];
(* Minimizes f with respect to a. *)
{min, theA} = Minimize[f[xNew[[1]], xNew[[2]]], {a}];
(* Update x using the direction *)
xNew = xNew /. theA;
delta = Norm[Last[x] - xNew];
x = Append[x, xNew /. theA];
++iteration;
];
Print["Convergence in ", iteration, " iterations.\n",
"Delta: ", delta, ", tolerance: ", N[tolerance], "\n",
"Minimum point at ", Last[x], "\n",
"Value of f at min point: ", f[Last[x][[1]], Last[x][[2]]], "\n"];
Print[f[Last[x][[1]],Last[x][[2]]], " ", f[Last[x][[1]],Last[x][[2]]] > .00001];