PostfixNotation.PostfixNotation.NumberCharProcess C# (CSharp) Method

        private void NumberCharProcess(string function, ref int i)
        {
            if (0 <= (int)function[i] && (int)function[i] <= 9)
            {
                while (i < function.Length && 0 <= (int)function[i] && (int)function[i] <= 9)
                {
                    postfixNotation = postfixNotation + function[i];
                    ++i;
                }
                postfixNotation = postfixNotation + " ";
                i--;
            }
            else
            {
                while (i < function.Length && !operations.Contains(function[i].ToString()))
                {
                    postfixNotation = postfixNotation + function[i];
                    ++i;
                }
                postfixNotation = postfixNotation + " ";
                i--;
            }
        }