# 3.15 FAQ-240 Why is my fitting function not compiling?

Last Update: 2/4/2015

When defining a function using Origin C, meaning you need to follow C-language syntax when defining your function. Note that C is case-sensitive. Therefore, what you enter in the Parameter Names edit box, must match what is entered in the Function list box.

For instance, if your parameter name is P1, you cannot use p1 in your function code. Besides, most mathematical functions in Origin C are lower-case, so compiling Sin() function will fail (use sin() instead). However, pi, Pi, PI are all $\pi;$ since they are all predefined.

For general information on creating and compling a user-defined fitting function using Origin C, please refer to the following page of the Origin Help file:

Keywords:user-defined, expression, equation, Origin C