How can an application execute a mathematical formula edited in an Edit

I am so tired about this problem. Please can someone help me.
In my application I need perform a procedure which can read a mathematical formula form an Edit and execute it. The problem is how can I convert this string to a real function
Thank you​

 

It is necessary to add “#include<math.h >” into C++ program to use any mathematical function. The string uses only characters so first create function and give function name, and then use any data type related to math as integer or float. If require convert string into integer and float. Thus, it is possible to create mathematical formula with function and edit it.

 

Can you give an example of the kind of input you want to give, and what output you would expect?