Expression.Number C# (CSharp) Method

    private static double Number(string expression, ref int index)
    {
        int endIndex = index;

        while(endIndex < expression.Length && (char.IsDigit(expression[endIndex]) || expression[endIndex] == '.'))
        {
            ++endIndex;
        }

        double result = double.Parse(expression.Substring(index, endIndex - index));
        index = endIndex;
        return result;
    }

    private Expression F()
    {
        if (lexer.Current == LexicalToken.Id)
        {
            var result = Expression.Identifier(lexer.Value);
            AddVariable(lexer.Value);
            lexer.Next();

            return(result);
        }
        else if (lexer.Current == LexicalToken.Num)
        {
            var result = Expression.Number(lexer.Value);
            lexer.Next();

            return(result);
        }
        else if (lexer.Current == LexicalToken.OpenPar)
        {
            lexer.Next();
            var e = E();
            lexer.Expect(LexicalToken.ClosePar);
            return(Expression.Parenthesis(e));
        }
        else
        {
            throw new ApplicationException(String.Format("Parse error: Incorrent token '{0}' encountered. Expected ')'.", lexer.Value));
        }
    }