# 15.3.4.4.1 A Simple Example

## Summary

This tutorial will show you how to create a fitting function with the Fitting Function Builder.

Minimum Origin Version Required: Origin 8.5 SR0

## What you will learn

• How to define a peak fitting function
• How to write function body
• How to write initialization codes
• How to set bounds for parameters

## Steps

1. In Origin's main menu, click Tools, and then click Fitting Function Builder to open the tool. Click Create a New Function on the first (Goal) page, and then click Next to go to the Name and Type page.
2. On the Name and Type page, we will create a category for the new fitting function, then name the function and select a Function Type.
• Click New to open the Category Name dialog, and input MyPeakFitting as the new category name.
• Input AsymmetricGauss in the Function Name edit box.
• Select Origin C from the Function Type list. You should find that the Hints in the left panel have updated accordingly.
• Click the Next button to go to the Variables and Parameters page.
3. On the Variables and Parameters page, we will do the following:
• Input x and Y as the independent variable and the dependent variable.
• Input y0,xc,sig1,sig2 and A in the Parameters edit box.
• Enable the Peak Function box so we can use this function for replica fitting in the NLFit tool or fitting peaks in the Peak Analyzer. ( Note: Before using it in the Peak Analyzer, make sure you have shared this function to the PFW category in the Fitting Function Organizer.)
• Click Next to go to the Function Body page.
4. On the Function Body page, we will input the function body and choose the peak attribute for each parameter.
• Click the cell in the Peak Attribute column and xc row of the Parameters tab. Select X Center as the peak attribute. Then select X Width and Amplitude/Area for the parameters sig1 and A.
• Enter initial values and parameter meanings in the Initial Value and Meaning columns.
• Copy and paste all of the following lines into the Function Body edit box.
double B,sig;
B=A/(0.5*(sig1+sig2)*sqrt(2*pi));
sig = x<xc ? sig1:sig2;
y = y0 + B * exp(-0.5 *((x-xc)/sig)^2);
• After entering values for the independent variables, click the Evaluate button to quickly check the fitting function.
5. Click Next to go to the Parameter Initialization Code page, and enter the following codes into the Initialization Code edit box.
double cent;
double sig;
xc = peak_pos(x_y_curve, &sig, &y0, NULL, &A, &cent);
sig1 = 0.42466 * (sig - 2 * (cent-xc));
sig2 = 0.42466 * (sig + 2 * (cent-xc));
6. Click Next to go the Bounds and General Linear Constraints dialog, and then set the dialog as in the image below:
7. Click Finish.
 NOTE: To define a peak function available in Quick Peaks Gadget or Multiple Peak Fit tool, the user-defined peak function must be in the category of Peak Functions.

## Where to Access the User-defined Function

1. Use the user-defined function to perform curve fitting with the Nonlinear Fitter. For more details about the Nonlinear Fitter, please refer to Nonlinear Curve Fitting.
2. Find the function in the Fitting Function Organizer. For more details about the Fitting Function Organizer, please refer to Fitting Function Organizer.
3. In the Fitting Function Organizer, right-click this function and select Share To to share this function to the PFW category. You can then use the function to perform peak fitting with the Peak Analyzer.