objective-c unique identifier for each iOS device

 if ([[UIDevice currentDevice] respondsToSelector:@selector(identifierForVendor)]) {

        

       str1=[NSString stringWithFormat:@"%@",

                             [[[UIDevice currentDevice] identifierForVendor] UUIDString]];

             

         }

objective-c substring of length

NSString* feng = [string substringToIndex:4]
NSString* zhong = [string substringFromIndex:5]

NSString to NSData

SString* myStr = @"This is the Testing String";

NSData* cData = [myStr dataUsingEncoding:NSUTF8StringEncoding];