# 14.6. A function on `Complex` numbers¶

A natural operation we might want to perform on complex numbers is addition. If the numbers are in Cartesian coordinates, addition is easy: you just add the real parts together and the imaginary parts together. If the numbers are in polar coordinates, it is easiest to convert them to Cartesian coordinates and then add them.

Again, it is easy to deal with these cases if we use the accessor functions:

```Complex add (Complex& a, Complex& b)
{
double real = a.getReal() + b.getReal();
double imag = a.getImag() + b.getImag();
Complex sum (real, imag);
return sum;
}
```

Notice that the arguments to `add` are not `const` because they might be modified when we invoke the accessors. To invoke this function, we would pass both operands as arguments:

```Complex c1 (2.0, 3.0);
Complex c2 (3.0, 4.0);

Complex sum = add (c1, c2);
sum.printCartesian();
```

The output of this program is

```5 + 7i
```

The active code below uses the `add` function for `Complex` objects. As an exercise, write the `subtract` function for `Complex` objects in the commented area of the active code. If you get stuck, you can reveal the extra problem at the end for help. Once you are finished, feel free to modify the code and experiment around!