static void Main(string[] args)
{
var expression = "1 + 2 - 3 * 4 / 5";
var lexer = new Lexer(new SourceScanner(expression));
Console.WriteLine($"Lexing expression: {expression}");
Token token = null;
do
{
token = lexer.ReadNext( );
Console.WriteLine($"Found Token Type {token.Type} at Position {token.Position} with a value of {token.Value}");
} while(token.Type != Token.TokenType.EOE);
var expressions = new string[] {
"1 + 2", //3
"2 * 3", //5
"1 + 2 * 3", //7
"1",
"1 / 2"
};
var expressionEngine = new ExpressionEngine();
foreach (var e in expressions)
{
Console.WriteLine($"{e} = {expressionEngine.Evaluate( e )}");
}
}
static void Main(string[] args)
{
var expression = "1 + 2 - 3 * 4 / 5";
var lexer = new Lexer(new SourceScanner(expression));
Console.WriteLine($"Lexing expression: {expression}");
Token token = null;
do
{
token = lexer.ReadNext( );
Console.WriteLine($"Found Token Type {token.Type} at Position {token.Position} with a value of {token.Value}");
} while(token.Type != Token.TokenType.EOE);
}
/// <summary>
/// Parses the TERM Production Rule
/// TERM: FACTOR [('*'|'/') FACTOR]*
/// </summary>
private static ASTNode ParseTerm(Lexer lexer)
{
var left = ParseFactor(lexer);
var peekToken = lexer.Peek(); //look ahead 1 token
while (peekToken.Type == Token.TokenType.Multiplication || peekToken.Type == Token.TokenType.Division)
{
var op = lexer.ReadNext( ); //read the operator
var right = ParseFactor(lexer);
if (right == null)
{
throw new Exception($"Invalid Expression. FACTOR Expected at position {lexer.Position}");
}
left = CreateBinaryOperator(op, left, right);
peekToken = lexer.Peek( ); //lookahead of 1 token LL(k) where K = 1
}
return(left);
}
private Token Accept( ) => _lexer.ReadNext( );
