Can I use NonlinearModelFit to model some data with a piece wise linear function??

• To: mathgroup at smc.vnet.net
• Subject: [mg119989] Can I use NonlinearModelFit to model some data with a piece wise linear function??
• From: morris <morris.pearl at gmail.com>
• Date: Mon, 4 Jul 2011 06:44:35 -0400 (EDT)

```I attempt to do this, but it does not seem to work very well.  See
example session below.  I first create some data out of three linear
functions, plus a little bit of random perturbation.  Then I have a
function f[x] which is a piecewise linear function, with three pieces.

NonlinearModelFit seems however to respond with only two pieces, by
giving the same value to the two parameters which represent the cut
points (c1 and c2 in the below example).

Thank you for any thoughts.

Happy Independence Day to Americans.

---------------------------------------------------------------------------

In[49]:=

curve = Join[Table[{i, -2i},{i,-14,6}], Table[{i,5},{i,7,10}],
Table[{i, 3 i}, {i, 11, 20}]] + Table[Random[Real,0.25],{35}]

Out[49]= {{-13.782,28.218},{-12.8514,26.1486},{-11.9445,24.0555},
{-10.9566,22.0434},{-9.85094,20.1491},{-8.75561,18.2444},
{-7.92158,16.0784},{-6.99195,14.008},{-5.79096,12.209},
{-4.95951,10.0405},{-3.96988,8.03012},{-2.92945,6.07055},
{-1.98475,4.01525},{-0.877226,2.12277},{0.157389,0.157389},
{1.10679,-1.89321},{2.17641,-3.82359},{3.22352,-5.77648},
{4.04408,-7.95592},{5.17738,-9.82262},{6.16403,-11.836},
{7.23777,5.23777},{8.03634,5.03634},{9.01375,5.01375},{10.196,5.19598},
{11.0892,33.0892},{12.2308,36.2308},{13.2204,39.2204},
{14.0469,42.0469},{15.0948,45.0948},{16.1524,48.1524},
{17.2123,51.2123},{18.0879,54.0879},{19.0543,57.0543},
{20.1223,60.1223}}

In[50]:=   f[x_] := Piecewise[{  {a1 + x b1, x < c1}, {a2 + x b2, c1
<= x <= c2}, {a3 + x b3, x  > c2}}]

Out[50]= \[Piecewise]	a1+b1 x	x<c1
a2+b2 x	c1<=x<=c2
a3+b3 x	x>c2
0	True

In[51]:= m = NonlinearModelFit[curve, f[x], {c1,c2,a1,b1,a2,b2,a3,b3},
{x}]

]
In[52]:= Normal[m]

Out[52]= \[Piecewise]	0.227214 -2.0132 x	x<1.
1. +1. x	1.<=x<=1.
-22.4279+4.20451 x	x>1.
0	True

```

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