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You have an NSString. You want an NSNumber. Previously, you would have to rely on abusing NSPredicate to turn your string into an NSExpression that you could then evaluate. However, this has a major flaw: extending it to support functions that aren't built-in to NSExpression provided for some awkward syntax. So if you really need sin(), you have to jump through some intricate hoops to get it.

You could also have used GCMathParser. This, however, isn't extensible at all. So if you really need stddev() or nthroot() functions, you're out of luck.

Thus, DDMathParser. It is written to be identical to NSExpression in all the ways that matter, but with the major addition that you can define new functions as you need.