How do I turn a string into a maths equation? : <p>A user will give me a string as their input, which will be a maths equation.    E.g.  <em>(21 + 3) / 4.</em> </p>

<p>I want to turn that string into a double that is equal to the answer (6.0) Is there any way to do this in swift 3, XCode 8?</p>
