problem in Conditional orders

Its because your check column contain 0 or something else not 1.
that's why its excuting i++ command.
or
Use Trim() function

if (ds.Tables[0].Rows[i]["check"].ToString().Trim() == "1") 

or

if (Convert.ToInt32(ds.Tables[0].Rows[i]["check"]))== 1)

Hi there, I've restructured your code as much as possible even though the logic you've used, does not make ANY sense :S Anyways, you ought learn so many basic level coding 'stuff' like condition checking, try catch blocks, managing db connections, commenting, variable naming conventions, string manipulation, etc.

Take it slow and one by one; but learn them all.

I hope the restructuring helps the performance. If you explain what you really want to do, it would make it a lot easier to help you out.

Hope this helps, Regards

int i = 0;

// This condition prevents the logic even starting if there are no valid data
if ((ds.Tables.Count > 0) && (ds.Tables[0].Rows.Count > 0))
{
	for (int counter = 0; counter < ds.Tables[0].Rows.Count; counter++)
	{
		DataRow currentRow = ds.Tables[0].Rows[counter];

		try
		{
			// I don't know why you've declared these 
			// 'div' & 'digit' variables and logic
			// if you don't use it :S
			int digit = (int) currentRow["digit"];
			int div = digit/i;
		} 
		catch
		{
			// I've not caught the exception to a variable, because you don't use it

			SqlConnection con = new SqlConnection();
			SqlCommand cmd = con.CreateCommand();
			cmd.CommandText = "UPDATE [mytb] SET [check]=0 WHERE [id] = @id";
			cmd.CommandType = CommandType.Text;
			cmd.Parameters.AddWithValue("@id", currentRow["digit"]);

			try
			{
				con.Open();
				cmd.ExecuteNonQuery();
			} 
			catch
			{
				if (con.State != ConnectionState.Closed)
					con.Close();
			}
		}

		Label1.Text += string.Format("{0}, ", currentRow["id"].ToString());
	}
}