Using reduce() to build a dictionary in Swift

Question

I'd like to build a dictionary using a functional programming style. My reduce() doesn't seem to work - I get a "fatal error: unexpectedly found nil while unwrapping an Optional value"

func loadMoveToCalendarsRules(calendarIndex: Int) -> [String]? {
//    return something like ["phone call", "buzz", "ring"]
    return NSUserDefaults.standardUserDefaults().objectForKey(generateMoveToCalendarsRules_NSUserDefaultsKey(calendarIndex)) as? [String]
}

// Add indeces to an array of any type
func addIndices<T>(toArray: [T]) -> [(index: Int, value: T)] {
    return Array(zip(toArray.indices, toArray))
}

typealias CalendarRules = [EKCalendar : [String]]?
func buildCalendarRules(cals: [EKCalendar]) -> CalendarRules {
    let sortedCals = cals.sort { $0.title.lowercaseString < $1.title.lowercaseString }

    // build move to cal rules.
    let indexedCalList = addIndices(sortedCals)
    // go through the sorted calendars and build a dictionary that associates each calendar with a string array. (These are keywords that apply to the given calendar.)
    let calendarRules = indexedCalList.reduce(nil as CalendarRules) {
        accumulator, nextValue in
        var retVal: [EKCalendar : [String]]? = accumulator
        // if there are values found in NSUserDefaults for this calendar index then retrieve them.
        if let rulesForCurrentCal = loadMoveToCalendarsRules(nextValue.index) {
            retVal![nextValue.value] = rulesForCurrentCal       // fatal error: unexpectedly found nil while unwrapping an Optional value
        }
        return retVal
    }

    print("------------ built calendar rules -------------")
    print(Array(arrayLiteral: calendarRules?.keys))
    print(Array(arrayLiteral: calendarRules?.values))

    return calendarRules
}

jtbandes · Accepted Answer · 2016-04-11 07:54:43Z

1

Your retVal is optional, and starts as nil (the initial value you pass in), yet you are using retVal! to force-unwrap it. You could just use [:] (an empty dictionary) as the initial value, and then retVal wouldn't need to be optional at all.

answered Apr 11, 2016 at 7:54

jtbandes

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Comments

Rob · Accepted Answer · 2016-04-11 09:33:46Z

You are starting with nil, and never instantiate a CalendarRules dictionary, so the attempt to performed a forced unwrapping of it with ! is going to fail. Instead, test to see if it's nil and if so, instantiate one.

Before I get to that, I'd first suggest defining calendar rules as a non-optional type. It makes things less confusing this way:

typealias CalendarRules = [EKCalendar : [String]]

Then, you could use nil-coalescing operator, ??, to instantiate the CalendarRules object when needed:

let calendarRules = indexedCalList.reduce(nil as CalendarRules?) { accumulator, nextValue in
    if let rulesForCurrentCal = loadMoveToCalendarsRules(nextValue.index) {
        var retVal = accumulator ?? CalendarRules()
        retVal[nextValue.value] = rulesForCurrentCal
        return retVal
    }
    return accumulator
}

It strikes me that there might be more efficient approaches, but this should address your "unexpectedly found nil" error.

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