how to save old and new data with NSUserDefault - ios

Question

When I save data using NSUserDefault first time it saves data successfully.When I close my application and start again then I get old data but again I use NSUserDefault with same key.The results is new data comes but my old data does not come.I want both old and new data. So what is the solution for this?
store data
NSUserDefaults *userDefaults ;
if(![[NSUserDefaults standardUserDefaults] objectForKey:#"mydata"])
{
userDefaults = [NSUserDefaults standardUserDefaults];
[userDefaults setObject:app.arr_defult forKey:#"mydata"];
[userDefaults synchronize];
}
else
{
NSLog(#"Leaderboards Dict Exists %#", [NSUserDefaults standardUserDefaults]);
userDefaults = [NSUserDefaults standardUserDefaults];
[userDefaults setObject:app.arr_defult forKey:#"mydata"];
[userDefaults synchronize];
}
read DAta :
NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
arrayOfImages = [userDefaults objectForKey:#"mydata"];

Answer 1

It's not a good practice to store huge data set in NSUserDefaults.
NSUserDefaults is only for storing small amount of data like app preferences,
setting preferences, etc.
Use Core Data to handle your scenarios.

Answer 2

Everything is fine .just you add new data with same key first you get old data from nsuserdefault and add you both data(old and new) in nsuserdefault.i think is work fine..
// First Time
NSUserDefaults *userDefaults ;
userDefaults = [NSUserDefaults standardUserDefaults];
[userDefaults setObject:app.arr_defult forKey:#"mydata"];
[userDefaults synchronize];
// second Time
NSMutableArray *temparray1=[[NSMutableArray alloc] init];
[temparray1 addObject:[[NSUserDefaults standardUserDefaults] objectForKey:#"mydata"]];
[temparray1 addObject:app.arr_defult];
[userDefaults setObject:temparray1t forKey:#"mydata"];
[userDefaults synchronize];