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#include <iostream> #include <vector> #include <string> #include <cctype> // Token types enum class TokenType { Number, Plus, Minus, Multiply, Divide, End, Invalid }; // Token class class Token { public: Token(TokenType type, std::string value) : type(type), value(value) {} TokenType getType() const { return type; } std::string getValue() const { return value; } private: TokenType type; std::string value; }; // Lexer class class Lexer { public: Lexer(const std::string& input) : input(input), index(0) {} Token nextToken() { while (index < input.size() && std::isspace(input[index])) { ++index; } if (index >= input.size()) { return Token(TokenType::End, ""); } char current = input[index]; if (std::isdigit(current)) { std::string number; while (index < input.size() && std::isdigit(input[index])) { number += input[index++]; } return Token(TokenType::Number, number); } switch (current) { case '+': ++index; return Token(TokenType::Plus, "+"); case '-': ++index; return Token(TokenType::Minus, "-"); case '*': ++index; return Token(TokenType::Multiply, "*"); case '/': ++index; return Token(TokenType::Divide, "/"); default: return Token(TokenType::Invalid, std::string(1, current)); } } private: std::string input; size_t index; }; // Parser class class Parser { public: Parser(Lexer& lexer) : lexer(lexer), currentToken(lexer.nextToken()) {} void parse() { while (currentToken.getType() != TokenType::End) { if (currentToken.getType() == TokenType::Number) { std::cout << "Number: " << currentToken.getValue() << std::endl; } else { std::cout << "Operator: " << currentToken.getValue() << std::endl; } currentToken = lexer.nextToken(); } } private: Lexer& lexer; Token currentToken; }; // Main function int main() { std::string input = "10 + 20 * 30 - 40 / 2"; Lexer lexer(input); Parser parser(lexer); std::cout << "Parsing input: " << input << std::endl; parser.parse(); return 0; } |
Explanation:
- TokenType Enum:
- Defines various token types like
Number,Plus,Minus, etc.
- Defines various token types like
- Token Class:
- Represents a token with a type and value.
Token(TokenType type, std::string value): Constructor to initialize the token.getType() const: Returns the type of the token.getValue() const: Returns the value of the token.
- Lexer Class:
Lexer(const std::string& input): Constructor to initialize the lexer with input.nextToken(): Processes the input to generate the next token. It handles numbers and operators, skipping whitespace.
- Parser Class:
Parser(Lexer& lexer): Constructor to initialize the parser with a lexer.parse(): Processes tokens from the lexer and prints out whether they are numbers or operators.
- Main Function:
- Initializes a
Lexerwith a sample input string. - Creates a
Parserusing the lexer. - Calls the
parse()method to process the input and print the results.
- Initializes a