Parsing if-else if statement algorithm

Question

I am trying to create a very simple parser for an if-else type structure that will build and execute a SQL statement.

Rather than testing conditions for executing statem

Answer 1

I wrote a simple parser, which I tested against the example you provided. If you want to know more about parsing I suggest you to read Compiler Construction from Niklaus Wirth.

The first step is always to write down the syntax of your language in an appropriate way. I have chosen EBNF, which is very simple to understand.

| separates alternatives.

[ and ] enclose options.

{ and } denote repetitions (zero, one or more).

( and ) group expressions (not used here).

This description is not complete but the link I provided describes it in more detail.

The EBNF syntax

LineSequence = { TextLine | IfStatement }.
TextLine     = .
IfStatement  = IfLine LineSequence { ElseIfLine LineSequence } [ ElseLine LineSequence ] EndLine.
IfLine       = "#if" "(" Condition ")".
ElseLine     = "#else".
ElseIfLine   = "#else" "if" "(" Condition ")".
EndLine      = "#end".
Condition    = Identifier "=" Identifier.
Identifier   =  {  |  }.

The parser follows closely the syntax, i.e. a repetition is translated into a loop, an alternative into an if-else statement, and so on.

using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using System.Windows.Forms;

namespace Example.SqlPreprocessor
{
    class Parser
    {
        enum Symbol
        {
            None,
            LPar,
            RPar,
            Equals,
            Text,
            NumberIf,
            If,
            NumberElse,
            NumberEnd,
            Identifier
        }

        List _input; // Raw SQL with preprocessor directives.
        int _currentLineIndex = 0;

        // Simulates variables used in conditions
        Dictionary _variableValues = new Dictionary { 
            { "VariableA", "Case1" },
            { "VariableB", "CaseX" }
        };

        Symbol _sy; // Current symbol.
        string _string; // Identifier or text line;
        Queue _textQueue = new Queue(); // Buffered text parts of a single line.
        int _lineNo; // Current line number for error messages.
        string _line; // Current line for error messages.

        /// 

        /// Get the next line from the input.
        /// 
        /// Input line or null if no more lines are available.
        string GetLine()
        {
            if (_currentLineIndex >= _input.Count) {
                return null;
            }
            _line = _input[_currentLineIndex++];
            _lineNo = _currentLineIndex;
            return _line;
        }

        /// 

        /// Get the next symbol from the input stream and stores it in _sy.
        /// 
        void GetSy()
        {
            string s;
            if (_textQueue.Count > 0) { // Buffered text parts available, use one from these.
                s = _textQueue.Dequeue();
                switch (s.ToLower()) {
                    case "(":
                        _sy = Symbol.LPar;
                        break;
                    case ")":
                        _sy = Symbol.RPar;
                        break;
                    case "=":
                        _sy = Symbol.Equals;
                        break;
                    case "if":
                        _sy = Symbol.If;
                        break;
                    default:
                        _sy = Symbol.Identifier;
                        _string = s;
                        break;
                }
                return;
            }

            // Get next line from input.
            s = GetLine();
            if (s == null) {
                _sy = Symbol.None;
                return;
            }

            s = s.Trim(' ', '\t');
            if (s[0] == '#') { // We have a preprocessor directive.
                // Split the line in order to be able get its symbols.
                string[] parts = Regex.Split(s, @"\b|[^#_a-zA-Z0-9()=]");
                // parts[0] = #
                // parts[1] = if, else, end
                switch (parts[1].ToLower()) {
                    case "if":
                        _sy = Symbol.NumberIf;
                        break;
                    case "else":
                        _sy = Symbol.NumberElse;
                        break;
                    case "end":
                        _sy = Symbol.NumberEnd;
                        break;
                    default:
                        Error("Invalid symbol #{0}", parts[1]);
                        break;
                }

                // Store the remaining parts for later.
                for (int i = 2; i < parts.Length; i++) {
                    string part = parts[i].Trim(' ', '\t');
                    if (part != "") {
                        _textQueue.Enqueue(part);
                    }
                }
            } else { // We have an ordinary SQL text line.
                _sy = Symbol.Text;
                _string = s;
            }
        }

        void Error(string message, params  object[] args)
        {
            // Make sure parsing stops here
            _sy = Symbol.None;
            _textQueue.Clear();
            _input.Clear();

            message = String.Format(message, args) +
                      String.Format(" in line {0}\r\n\r\n{1}", _lineNo, _line);
            Output("------");
            Output(message);
            MessageBox.Show(message, "Error");
        }

        /// 

        /// Writes the processed line to a (simulated) output stream.
        /// 
        /// Line to be written to output
        void Output(string line)
        {
            Console.WriteLine(line);
        }

        /// 

        /// Starts the parsing process.
        /// 
        public void Parse()
        {
            // Simulate an input stream.
            _input = new List {
                "select column1",
                "from",
                "#if(VariableA = Case1)",
                "    #if(VariableB = Case3)",
                "        table3",
                "    #else",
                "        table4",
                "    #end",
                "#else if(VariableA = Case2)",
                "    table2",
                "#else",
                "    defaultTable",
                "#end"
            };

            // Clear previous parsing
            _textQueue.Clear();
            _currentLineIndex = 0;

            // Get first symbol and start parsing
            GetSy();
            if (LineSequence(true)) { // Finished parsing successfully.
                //TODO: Do something with the generated SQL
            } else { // Error encountered.
                Output("*** ABORTED ***");
            }
        }

        // The following methods parse according the the EBNF syntax.

        bool LineSequence(bool writeOutput)
        {   
            // EBNF:  LineSequence = { TextLine | IfStatement }.
            while (_sy == Symbol.Text || _sy == Symbol.NumberIf) {
                if (_sy == Symbol.Text) {
                    if (!TextLine(writeOutput)) {
                        return false;
                    }
                } else { // _sy == Symbol.NumberIf
                    if (!IfStatement(writeOutput)) {
                        return false;
                    }
                }
            }
            return true;
        }

        bool TextLine(bool writeOutput)
        {
            // EBNF:  TextLine = .
            if (writeOutput) {
                Output(_string);
            }
            GetSy();
            return true;
        }

        bool IfStatement(bool writeOutput)
        {
            // EBNF:  IfStatement = IfLine LineSequence { ElseIfLine LineSequence } [ ElseLine LineSequence ] EndLine.
            bool result;
            if (IfLine(out result) && LineSequence(writeOutput && result)) {
                writeOutput &= !result; // Only one section can produce an output.
                while (_sy == Symbol.NumberElse) {
                    GetSy();
                    if (_sy == Symbol.If) { // We have an #else if
                        if (!ElseIfLine(out result)) {
                            return false;
                        }
                        if (!LineSequence(writeOutput && result)) {
                            return false;
                        }
                        writeOutput &= !result; // Only one section can produce an output.
                    } else { // We have a simple #else
                        if (!LineSequence(writeOutput)) {
                            return false;
                        }
                        break; // We can have only one #else statement.
                    }
                }
                if (_sy != Symbol.NumberEnd) {
                    Error("'#end' expected");
                    return false;
                }
                GetSy();
                return true;
            }
            return false;
        }

        bool IfLine(out bool result)
        {
            // EBNF:  IfLine = "#if" "(" Condition ")".
            result = false;
            GetSy();
            if (_sy != Symbol.LPar) {
                Error("'(' expected");
                return false;
            }
            GetSy();
            if (!Condition(out result)) {
                return false;
            }
            if (_sy != Symbol.RPar) {
                Error("')' expected");
                return false;
            }
            GetSy();
            return true;
        }

        private bool Condition(out bool result)
        {
            // EBNF:  Condition = Identifier "=" Identifier.
            string variable;
            string expectedValue;
            string variableValue;

            result = false;
            // Identifier "=" Identifier
            if (_sy != Symbol.Identifier) {
                Error("Identifier expected");
                return false;
            }
            variable = _string; // The first identifier is a variable.
            GetSy();
            if (_sy != Symbol.Equals) {
                Error("'=' expected");
                return false;
            }
            GetSy();
            if (_sy != Symbol.Identifier) {
                Error("Value expected");
                return false;
            }
            expectedValue = _string;  // The second identifier is a value.

            // Search the variable
            if (_variableValues.TryGetValue(variable, out variableValue)) {
                result = variableValue == expectedValue; // Perform the comparison.
            } else {
                Error("Variable '{0}' not found", variable);
                return false;
            }

            GetSy();
            return true;
        }

        bool ElseIfLine(out bool result)
        {
            // EBNF:  ElseIfLine = "#else" "if" "(" Condition ")".
            result = false;
            GetSy(); // "#else" already processed here, we are only called if the symbol is "if"
            if (_sy != Symbol.LPar) {
                Error("'(' expected");
                return false;
            }
            GetSy();
            if (!Condition(out result)) {
                return false;
            }
            if (_sy != Symbol.RPar) {
                Error("')' expected");
                return false;
            }
            GetSy();
            return true;
        }
    }
}

Note that the nested if-statements are processed automatically in a quite natural way. First, the grammar is expressed recursively. A LineSequence can contain IfStatments and IfStatments contain LineSequences. Second, this results in syntax processing methods that call each other in a recursive manner. The nesting of syntax elements is therefore translated into recursive method calls.