# 4.2.2.21 Fitting with Piecewise Functions

## Summary

We will show you how to define piecewise fitting function in this tutorial.

Minimum Origin Version Required: Origin 8.0 SR6

## What you will learn

This tutorial will show you how to:

• Define piecewise (conditional) fitting functions.

## Example and Steps

We can start this tutorial by importing the sample \Samples\Curve Fitting\Exponential Decay.dat data file. Highlight column D and plot a Scatter Graph. You can fit this curve using built-in functions under Growth/Sigmoidal category, however, in this tutorial, we will separate the curve into two parts by a piecewise function. So the equation will be: $y = \begin{cases} a+bx+e^{-\frac{x-x_c}{t1}}, & \mbox{if } x

### Define the Function

Press F9 to open the Fitting Function Organizer and define a function like:

 Function Name: piecewise Function Type: User-Defined Independent Variables: x Dependent Variables: y Parameter Names: xc, a, b, t1 Function Form: Origin C Function:

Click the button on the right of the Function edit box and define the fitting function in Code Builder using:

void _nlsfpiecewise(
// Fit Parameter(s):
double xc, double a, double b, double t1,
// Independent Variable(s):
double x,
// Dependent Variable(s):
double& y)
{
// Beginning of editable part
// Divide the curve by if condition.
if(x<xc) {
y = a+b*x+exp(-(x-xc)/t1);
} else {
y = a+b*x;
}
// End of editable part
}

### Fit the Curve

Press Ctrl + Y to bring up NLFit dialog with the graph window active. Select the piecewise function we defined and initialize the parameter values:

 xc: 1 a: 1 b: -1 t1: 0.1

Click Fit button to generate the results:

 xc: 0.24 a: 36.7659 b: -24.6288 t1: 0.04961

Note that this function is sensitive to xc and t1, different initial values could generate different results.