EXC Bad Instruction - swift

Question

In was wondering why I keep getting this error message, EXC Bad Instruction could someone help me out and tell me why.
Here is the code.
func updateStocks() {
let stockManager:StockManagerSingleton = StockManagerSingleton.sharedInstance
stockManager.updateListOfSymbols(stocks)
//Repeat this method after 15 secs. (For simplicity of the tutorial we are not cancelling it never)
dispatch_after(
dispatch_time(
DISPATCH_TIME_NOW,
Int64(15 * Double(NSEC_PER_SEC))
),
dispatch_get_main_queue(),
{
self.updateStocks()
}
)
}
//4
func stocksUpdated(notification: NSNotification) {
let values = (notification.userInfo as! Dictionary<String,NSArray>)
let stocksReceived:NSArray = values[kNotificationStocksUpdated]!
stocks.removeAll(keepCapacity: false)
for quote in stocksReceived {
let quoteDict:NSDictionary = quote as! NSDictionary
let changeInPercentString = quoteDict["ChangeInPercent"] as! String
let changeInPercentStringClean: NSString = (changeInPercentString as NSString).substringToIndex((changeInPercentString as NSString).length-1)
stocks.append(quoteDict["symbol"] as! String,changeInPercentStringClean.doubleValue)
}
tableView.reloadData()
NSLog("Symbols Values updated :)")
}
}
The line with the error in it is,
let changeInPercentString = quoteDict["ChangeInPercent"] as! String

Answer 1

The error states that Swift attempted to unwrap a nil value, as you stated on this line
let changeInPercentString = quoteDict["ChangeInPercent"] as! String
Swift attempts to force setting the value of quoteDict["ChangeInPercent"] to a String, because you use as!, instead, you should use as?, which will set the value to nil if the value cannot be found
let changeInPercentString = quoteDict["ChangeInPercent"] as? String
You could set this to a default value by using the ?? operator. For example, if you wanted the default value to be 0.0%, you could use
let changeInPercentString = (quoteDict["ChangeInPercent"] as? String) ?? "0.0%"
The inherent problem is most likely either that quoteDict["ChangeInPercent"] does not exist, or quoteDict["ChangeInPercent"] is not a String - it may be an NSString or simply a Double value.
If you find out that it is supposed to be an NSString, for example, you will need to change how you cast the value
let changeInPercentString: NSString = (quoteDict["ChangeInPercent"] as? NSString) ?? "0.0%"