writeToFile fails on iphone but works on simulator

NSString *myfile = [[NSBundle] mainBundle] pathFo开发者_如何学编程rResource:@"fileName" ofType:@"plist"];    
NSMutableArray *mydata= [[NSMutableArray alloc] initWithContentsOfFile:myfile];

/* code to modify mydata */

[mydata writeToFile:myfile atomically:YES]

In case of simulator 'fileName.plist' is modified but in case of iphone device file remains unchanged. There is no exception seen either.

Is the above code expected to work fine on both iphone and simulator ?

Also in the debugger when I hover over 'mydata' I see different values in case of simulator and device. In case of simulator I see for example, '5 objects' but in case of actual device it shows '{(int)[$VAR count]}'. Could this be related to file not being written ?

You can't write to files in a bundle's resource directory. On top of that you wouldn't want to, because any changes would be overwritten when you update your app. The documents directory persists across versions and (I believe) it is backed up via itunes.

Here is a snippet that checks the documents directory for a plist. if the file doesn't exist it copies a plist out of the resources into the documents directory.

BOOL success;
NSFileManager* fileManager = [NSFileManager defaultManager]; 
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *writableDBPath = [documentsDirectory stringByAppendingPathComponent:@"score.plist"];
success = [fileManager fileExistsAtPath:writableDBPath];
if (success) return success;
// The writable database does not exist, so copy the default to the appropriate location.
NSString *defaultDBPath = [[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:@"score.plist"]];
success = [fileManager copyItemAtPath:defaultDBPath toPath:writableDBPath error:&error]; 
return success;