# Custom Function

Essential Calculate holds many functions from statistics, finance and mathematics, along with other general purpose functions. There are
more than 400+ entries in the library. Also, it is easy to add the own calculations with custom functions.

## LibraryFunction

A property which holds the collection of library functions in Essential Calculate is LibraryFunctions. The function name in the library should only contain letters, digits or underscore. The function name serves as the hash key and the function delegate serves as the hash value.

Adding a custom function to the Formula Library in Essential Calculate is a two step process. The first step is to write a method that actually does the calculation work for your
custom function. The second step is to register this method with the CalcEngine. So, when the `CalcEngine` object is a member of an application, the additional
function methods can be added to the application and then these methods should be registered with the `CalcEngine` object after the object is created.

### Write a method

The method must have the signature specified by the delegate, LibraryFunction. The method must accept a string argument and returns a string value.

For writing a custom formula method, any convention with respect to passing arguments can be used within the implementation code. Thus, arguments can be a single entry
like A1 or 153 or it can be more complex like A1:C15. The computed value will be returned as a string. The arguments can be enhanced with standard items like cell references,
numbers, other formulas, etc., using the parsing tools of `CalcEngine` to minimize the amount of code that is required to be written.

Below code example shows implementing the custom function of computing minimum value,

``````public string CustomMin(string args)
{
double min = double.MaxValue;
double d;
var splitArgs = args.Split(new char[] { CalcEngine.ParseArgumentSeparator });
foreach (string s in splitArgs)
{
if (double.TryParse(s, NumberStyles.Number | NumberStyles.AllowExponent, null, out d))
min = Math.Min(min, d);
}
}``````

### Register the method with CalcEngine

The second step for adding the own formula is to register the custom method with the `CalcEngine` object. This is done with the help of AddFunction method of `CalcEngine` which accepts the string that is used when you refer the function, and the second argument is a delegate for method name. The only requirement here is that the function name should
start with an alpha character and should only contain alpha-numeric characters. Additionally, the string cannot be the name of any existing library function.

``````//Class derived from ICalcData,
CalcData calcData = new CalcData();

CalcEngine engine = new CalcEngine(calcData);

//Adding custom function to the library with user defined formula name,

### Compute the Custom Function

To compute the custom formula, you need to pass the registered custom formula in `PareAndComputeFormula` method of `CalcEngine`.

``````//Set value 100 to variable "A1"
calcData.SetValueRowCol(100, 1, 1);

//Set value 200 to variable "B1"
calcData.SetValueRowCol(200, 1, 2);

//Set value 300 to variable "C1"
calcData.SetValueRowCol(300, 1, 2);

//compute the registered method using CalcEngine.
var r = engine.ParseAndComputeFormula("=CustomMin(A1,B1,C1)");``````

## Remove Custom Function

To remove a single function from the Function Library, use RemoveFunction method of `CalcEngine`, passing a formula name as the string that references this function and to remove all functions, you can clear the hash table that holds them by using the `Clear` method of `LibraryFunction`.

``````//To remove a single function from library,
engine.RemoveFunction("CheckMin");

//To remove all functions from library,
engine.LibraryFunctions.Clear();``````

## Replace Function

To replace a function with another implementation, remove the original name and add the same name again with a different method name.

``````// Remove formula name "CheckMin" from the library,
engine.RemoveFunction("CheckMin");

//Adds formula name "Minimum" to the library,
using a selected few Library functions, you may want to remove the unused ones. This can be done using the `Clear` method of `LibraryFunction`.
and after clearing all functions, you can add few functions that will be used often by using `AddFunction` method of `CalcEngine`.