c# – 解析if-else if语句算法

我正在尝试为if-else类型结构创建一个非常简单的解析器,它将构建并执行SQL语句.

我会测试构建字符串的条件,而不是测试执行语句的条件.

一个例子是:

select column1
from
#if(VariableA = Case1)
table1
#else if(VariableA = Case2)
table2
#else
defaultTable
#end

如果VariableA等于Case1,则结果字符串应为:从table1中选择column1

一个更复杂的例子是嵌套的if语句:

select column1
from
#if(VariableA = Case1)
#if(VariableB = Case3)
    table3
#else
    table4
#else if(VariableA = Case2)
table2
#else
defaultTable
#end

这是我真正遇到麻烦的地方,我想不出一个正确识别每个if-else-end组的好方法.

另外,我不确定跟踪“else”子句中的字符串是否应该评估为true的好方法.

我一直在网上寻找不同类型的解析算法,所有这些算法看起来都非常抽象和复杂.

对于这个非计算机科学专业,有什么建议吗?

解决方法

我写了一个简单的解析器,我根据你提供的例子进行了测试.如果你想了解更多关于解析的信息,我建议你阅读Niklaus Wirth的 Compiler Construction.

第一步始终是以适当的方式记下语言的语法.我选择了EBNF,这很容易理解.

|分开替代品.

[和]附上选项.

{和}表示重复(零,一个或多个).

(和)组表达式(此处未使用).

此描述并不完整,但我提供的链接更详细地描述了它.

EBNF语法

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

解析器严格遵循语法,即将重复转换为循环,替换为if-else语句,依此类推.

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<string> _input; // Raw SQL with preprocessor directives.
        int _currentLineIndex = 0;

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

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

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

        /// <summary>
        /// Get the next symbol from the input stream and stores it in _sy.
        /// </summary>
        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");
        }

        /// <summary>
        /// Writes the processed line to a (simulated) output stream.
        /// </summary>
        /// <param name="line">Line to be written to output</param>
        void Output(string line)
        {
            Console.WriteLine(line);
        }

        /// <summary>
        /// Starts the parsing process.
        /// </summary>
        public void Parse()
        {
            // Simulate an input stream.
            _input = new List<string> {
                "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 = <string>.
            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;
        }
    }
}

请注意,嵌套的if语句以非常自然的方式自动处理.首先,语法以递归方式表达. LineSequence可以包含IfStatments,IfStatments包含LineSequences.其次,这导致语法处理方法以递归方式相互调用.因此,语法元素的嵌套被转换为递归方法调用.

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