Thread: How do I allow a user to input a function?

Depends on how complicated you want to make the function.
A straight-forward polynomial like that with a single variable should be pretty easy.

Right. If the input is always in a particular format this shouldn't be too hard. But, it gets tricky if the user is allowed to use parentheses to change the order/precedence of the operations, etc.

The key is parsing the string. This means stepping-through the characters in the string, and determining if the character is a number (or part of a number*), an operator like " ^ " or " + " , or a variable " x ", etc.

Once you've "extracted" a coefficient or an exponent, you (your program) can perform the appropriate mathematical operation.

*You'll need to allow for a multi-digit coefficient (i.e. 12 in your sample). You might want to get the program working with single digit numbers first, then add multi-digit capability when everything else works.

Right now I have it set up piece wise, as in enter the first coefficient etc. and I can get the function to display but I don't know how to get it to work as a function.

The short answer is you can't. Your input is a string which is either of type const char[] or type string, and neither of those types is consistent with the type of a function. A function has a type that reflects its return type and parameter types, e.g.:

int (*)(double, string)

compare that to type const char[] or type string. The bottom line is you can't execute a string. This doesn't work:

Code:

#include <string>
#include <iostream>

using namespace std;

void greeting()
{
     cout<<"hello world";
}

int main()
{
     string str = "greeting";
     str();  //error--"term does not evaluate to a function"
     
     return 0;
}

> "5x^4-3x^2+12x-6"
Step 1 is write a function which tokenises the string to

5x^4
-3x^2
+12x
-6

x by itself is x^1
the constant is -6x^0

Then split each one of those up into two values, and store them in say

Code:

struct foo {
  int scale;
  int power;
} terms[10];

From which you get, as you parse each term

Code:

term[0].scale = 5;
term[0].power = 4;
term[1].scale = -3;
term[1].power = 2;
term[2].scale = 12;
term[2].power = 1;
term[3].scale = -6;
term[3].power = 0;

Then it's simply a matter of inputting a value for x, and looping over those terms to do

Code:

sum += term[i].scale * pow( x, term[i].power );

7stud's not wrong when it comes to standard data types, but if you wanted, you could create a class and overload the () operator to allow for something similar to f(x) notation in code.