Q:

Why do I get an NSInvalidArgument Exception, reason: 'Unable to parse the format string'

Although there are a ton of threads about this subject, I haven't been able to find an answer to my problem. When I refactored this function below to pass the search argument to NSPredicate instead of hard coding the search value, I now get the error:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse the format string "formulaName == %0"'
func fetchFormula() {
    let fetchRequest = NSFetchRequest(entityName: "Formula")
    let calculation = calculator.selectedFormula
    let name = calculation.formulaName // name = Baechle
    let predicate = NSPredicate(format: "formulaName == %0", name)
    fetchRequest.predicate = predicate

    do {
      let results = try managedObjectContext.executeFetchRequest(fetchRequest) as? [Formula]
      if (results != nil) {
        formula.text = results?[0].formulaName
      }
    } catch {
      fatalError("Error fetching data!")
    }
    return
  }

Even though I think my syntax is correct, I've tried just about every other suggestion I've found here on SO and in Apple's docs:
    let predicate = NSPredicate(format: "formulaName = 'Baechle'") // started with this and learned that this is a bad idea, but it worked
    let predicate = NSPredicate(format: "formulaName = %0", name) // only 1 '=' which results in the error I've described
    let predicate = NSPredicate(format: "formulaName == %0", name) // now 2 '==', but still doesn't work, produces "formulaName == %0"'and same error
    let predicate = NSPredicate(format: "formulaName == '%0'", name) // produces formulaName == "%0" resulting in no results

I'm out of ideas. Any help greatly appreciated.

A:

Because you have been using the wrong format specifier. It should be %@ instead of %0:
let predicate = NSPredicate(format: "formulaName == %@", name)

Predicate Format String Syntax

