-(void)checkLoginStatus
{
    NSUserDefaults *pref=[NSUserDefaults standardUserDefaults];
    NSString *userId=[pref objectForKey:@"user_id"];
    if(userId!=nil)
    {
        UIAlertView *alert=[[UIAlertView alloc]initWithTitle:@"Already Logged in!" message:@"You are already logged in." delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil];
        [alert show];
    }
    else{
        UIAlertView *alert=[[UIAlertView alloc]initWithTitle:@"please log in first!" message:@"You are not logged in." delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil];
        [alert show];
    }
}