Catching NSInvalidArgumentException from NSExpression

Question

In my code I am evaluating strings as mathematical expressions for example:

NSString *formula=@\"9*7\";
NSExpression *expr =[NSExpression expressionWithFormat:fo         


        

Answer 1

The reason this exception is not caught with your current code is that the exception is being thrown from this line:

NSExpression *expr =[NSExpression expressionWithFormat:formula];

You need to move this line into the @try block.

Answer 2

What you need is a maths parser. NSExpression was designed to take well-formed input, and doesn't handle errors. A quick Google will give this.