Generate a random alphanumeric string in Objective c at iOS

    NSUInteger len =12;
    NSString *letterd =@"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789#&@!?";
    NSMutableString *randomString = [NSMutableString stringWithCapacity: len];
    
     for (NSUInteger i = 0; i < len; i++) {
        u_int32_t r = arc4random() % [letterd length];
        unichar c = [letterd characterAtIndex:r];
        [randomString appendFormat:@"%C", c];
    }
    NSLog(randomString);