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#include <iostream> #include <string> #include <cctype> #include <stdexcept> #include <map> using namespace std; // Token types enum class TokenType { NUMBER, PLUS, MINUS, MULTIPLY, DIVIDE, END, INVALID }; // Token structure struct Token { TokenType type; int value; // For NUMBER token }; // Lexer class class Lexer { public: Lexer(const string& text) : text(text), pos(0), currentChar(text[pos]) {} Token getNextToken() { while (currentChar != '\0') { if (isspace(currentChar)) { skipWhitespace(); continue; } if (isdigit(currentChar)) { return Token{TokenType::NUMBER, getNumber()}; } if (currentChar == '+') { advance(); return Token{TokenType::PLUS, 0}; } if (currentChar == '-') { advance(); return Token{TokenType::MINUS, 0}; } if (currentChar == '*') { advance(); return Token{TokenType::MULTIPLY, 0}; } if (currentChar == '/') { advance(); return Token{TokenType::DIVIDE, 0}; } throw runtime_error("Invalid character"); } return Token{TokenType::END, 0}; } private: string text; size_t pos; char currentChar; void advance() { ++pos; if (pos >= text.length()) { currentChar = '\0'; } else { currentChar = text[pos]; } } void skipWhitespace() { while (currentChar != '\0' && isspace(currentChar)) { advance(); } } int getNumber() { string number; while (currentChar != '\0' && isdigit(currentChar)) { number += currentChar; advance(); } return stoi(number); } }; // Parser class class Parser { public: Parser(Lexer lexer) : lexer(lexer), currentToken(lexer.getNextToken()) {} int parse() { int result = expression(); if (currentToken.type != TokenType::END) { throw runtime_error("Unexpected token after end of expression"); } return result; } private: Lexer lexer; Token currentToken; void eat(TokenType type) { if (currentToken.type == type) { currentToken = lexer.getNextToken(); } else { throw runtime_error("Unexpected token type"); } } int factor() { if (currentToken.type == TokenType::NUMBER) { int value = currentToken.value; eat(TokenType::NUMBER); return value; } throw runtime_error("Expected number"); } int term() { int result = factor(); while (currentToken.type == TokenType::MULTIPLY || currentToken.type == TokenType::DIVIDE) { TokenType op = currentToken.type; eat(op); if (op == TokenType::MULTIPLY) { result *= factor(); } else { result /= factor(); } } return result; } int expression() { int result = term(); while (currentToken.type == TokenType::PLUS || currentToken.type == TokenType::MINUS) { TokenType op = currentToken.type; eat(op); if (op == TokenType::PLUS) { result += term(); } else { result -= term(); } } return result; } }; int main() { string text; cout << "Enter an arithmetic expression: "; getline(cin, text); try { Lexer lexer(text); Parser parser(lexer); int result = parser.parse(); cout << "Result: " << result << endl; } catch (const runtime_error& e) { cerr << "Error: " << e.what() << endl; } return 0; } |
Explanation
- Token Types:
TokenTypeis an enum class that defines different types of tokens:NUMBER,PLUS,MINUS,MULTIPLY,DIVIDE,END, andINVALID.
- Token Structure:
Tokenholds the type of token and its value (if it’s a number).
- Lexer Class:
- Purpose: Breaks the input text into tokens.
- Methods:
getNextToken(): Reads characters from the input and generates tokens. It handles numbers and arithmetic operators.advance(): Moves to the next character in the input.skipWhitespace(): Skips over whitespace characters.getNumber(): Extracts multi-digit numbers from the input.
- Parser Class:
- Purpose: Parses the tokens generated by the lexer and evaluates the arithmetic expression.
- Methods:
parse(): Starts the parsing process and returns the result.eat(TokenType type): Consumes a token of the specified type.factor(): Parses a number token.term(): Handles multiplication and division operations.expression(): Handles addition and subtraction operations.
- Main Function (
main):- Reads an arithmetic expression from the user.
- Initializes the lexer and parser.
- Parses the expression and outputs the result.
- Handles errors gracefully using exception handling.