Swift - Resolving a math operation in a string

Question

just a short question. In Swift it is possible to solve the following code:

var a: String;
a = "(3*3)";

The arithmetic operation in the string will be solved. But i can′t figure out, why this following variation doesn′t work.

var a: String; 
var b: String;

b = "3*3";
a = "(b)";

In this case the arithmetic operation in var a will not be resolved. Any ideas why and how i can this get to work. Some things would be much more easier if this would work. Thanks for your answers.

Answer 1

In the second case, you are interpolating a string, not an arithmetic expression. In your example, it's a string you chose at compile time, but in general it might be a string from the user, or loaded from a file or over the web. In other words, at runtime b could contain some arbitrary string. The compiler isn't available at runtime to parse an arbitrary string as arithmetic.

If you want to evaluate an arbitrary string as an arithmetic formula at runtime, you can use NSExpression. Here's a very simple example:

let expn = NSExpression(format:"3+3")
println(expn.expressionValueWithObject(nil, context: nil))
// output: 6

You can also use a third-party library like DDMathParser.