Removing Value from NSMutableArray error - ios

Question

I'm trying to created an fun facts app. It displays a random string from an array every time, then that fact is deleted from the array. For me code, when the app is first launched it gets new array of facts and it saves the data when the app is closed and uses the array from the previous launch every time after the initial launch. My problem is I get an error "Thread 1: signal SIGABRT" when I try to remove a string from my array on my 4th last line. Please tell me what corrections I need to make. I am fairly new to programming. I appreciate all the help I get. Thanks for your time
import Foundation
let userDefaults = NSUserDefaults.standardUserDefaults()
func isAppAlreadyLaunchedOnce()->Bool{
let defaults = NSUserDefaults.standardUserDefaults()
if let isAppAlreadyLaunchedOnce = defaults.stringForKey("isAppAlreadyLaunchedOnce"){
println("App already launched")
return true
}
else{
defaults.setBool(true, forKey: "isAppAlreadyLaunchedOnce")
println("App launched first time")
return false
}
}
struct newFactBook {
let factsArray : NSMutableArray = [
"Ants stretch when they wake up in the morning.",
"Ostriches can run faster than horses.",
"Olympic gold medals are actually made mostly of silver.",
"You are born with 300 bones; by the time you are an adult you will have 206.",
"It takes about 8 minutes for light from the Sun to reach Earth.",
"Some bamboo plants can grow almost a meter in just one day.",
"The state of Florida is bigger than England.",
"Some penguins can leap 2-3 meters out of the water.",
"On average, it takes 66 days to form a new habit.",
"Mammoths still walked the earth when the Great Pyramid was being built."]
}
var checkLaunch = isAppAlreadyLaunchedOnce()
var oldFunFactsArray : NSMutableArray = []
if(checkLaunch == false){
oldFunFactsArray = newFactBook().factsArray
}
else if (checkLaunch == true){
oldFunFactsArray = userDefaults.objectForKey("key") as! NSMutableArray
}
func randomFacts1() -> (String, Int){
var unsignedArrayCount = UInt32(oldFunFactsArray.count)
var unsignedRandomNumber = arc4random_uniform(unsignedArrayCount)
var randomNumber = Int(unsignedRandomNumber)
return (oldFunFactsArray[randomNumber] as! String, randomNumber)
}
oldFunFactsArray.removeObjectAtIndex(randomFacts1().1) //this gives me the error
//oldFunFactsArray.removeValueAtIndex(randomFacts1().1, fromPropertyWithKey: "key") //this gives me the same error
//oldFunFactsArray.removeAtIndex(randomFacts1().1) //This gives me the error "NSMutableArray does not have a member named 'removeAtIndex'
userDefaults.setObject(oldFunFactsArray, forKey:"key")
userDefaults.synchronize()
println(oldFunFactsArray)

Answer 1

You will have to create a mutableCopy of the facts array to removeObjects from it.
if(checkLaunch == false){
oldFunFactsArray = newFactBook().factsArray.mutableCopy
}