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  1. #include <iostream>
  2. #include <stack>
  3. #include <sstream>
  4.  
  5. namespace Parser {
  6. class Token {
  7. public:
  8. 	enum Kind {
  9. 		NUMBER,
  10. 		PLUS, MINUS, MUL, DIV,
  11. 		PRINT, EOS, UNKNOWN
  12. 	};
  13.  
  14. 	Token() :k{EOS} {}
  15. 	Token(Kind x) :k{x} {}
  16. 	Token(double x) :k{NUMBER}, n{x} {}
  17.  
  18. 	double number() const { return n; }
  19. 	Kind kind() const { return k; }
  20.  
  21. private:
  22. 	Kind k;
  23. 	double n;
  24. };
  25.  
  26. class TokenStream {
  27. public:
  28. 	TokenStream(std::istream& s) :is{s} {}
  29.  
  30. 	const Token& get() {
  31. 		char c;
  32. 		is >> c;
  33.  
  34. 		if (is.eof())
  35. 			return curr_tok = {Token::EOS};
  36.  
  37. 		if (c >= '0' && c <= '9') {
  38. 			is.putback(c);
  39. 			double d;
  40. 			is >> d;
  41. 			return curr_tok = {d};
  42. 		}
  43. 		switch (c) {
  44. 			case '+': return curr_tok = {Token::PLUS};
  45. 			case '-': return curr_tok = {Token::MINUS};
  46. 			case '*': return curr_tok = {Token::MUL};
  47. 			case '/': return curr_tok = {Token::DIV};
  48. 			case ';':
  49. 			case '\n': return curr_tok = {Token::PRINT};
  50. 			default: return curr_tok = {Token::UNKNOWN};
  51. 		}
  52. 	}
  53.  
  54. 	const Token& current() const {
  55. 		return curr_tok;
  56. 	}
  57.  
  58. private:
  59. 	std::istream& is;
  60. 	Token curr_tok;
  61. };
  62. } //namespace
  63.  
  64. struct SyntaxError : public std::runtime_error {
  65. 	SyntaxError(const char* s) :runtime_error{s} {}
  66. };
  67.  
  68. double builtin_op(double x, double y, Parser::Token t) {
  69. 	using Parser::Token;
  70. 	switch (t.kind()) {
  71. 		case Token::PLUS: return x + y;
  72. 		case Token::MINUS: return x - y;
  73. 		case Token::MUL: return x * y;
  74. 		case Token::DIV: return x / y;
  75. 	}
  76. }
  77.  
  78. void calculate(std::istream& is) {
  79. 	using Parser::Token;
  80. 	Parser::TokenStream ts {is};
  81. 	std::stack<double> st;
  82.  
  83. 	while (ts.get().kind() != Token::EOS) {
  84. 		auto tok = ts.current();
  85. 		switch (tok.kind()) {
  86. 			case Token::PRINT:
  87. 			{
  88. 				if (st.empty())
  89. 					throw SyntaxError {"Nothing to print"};
  90. 				double res = st.top();
  91. 				st.pop();
  92. 				if (!st.empty())
  93. 					throw SyntaxError {"Too many values"};
  94. 				std::cout << "= " << res << '\n';
  95. 				break;
  96. 			}
  97. 			case Token::NUMBER:
  98. 				st.push(tok.number());
  99. 				break;
  100. 			case Token::PLUS:
  101. 			case Token::MINUS:
  102. 			case Token::MUL:
  103. 			case Token::DIV:
  104. 			{
  105. 				if (st.empty())
  106. 					throw SyntaxError{"Expected a number before the operator"};
  107. 				double n1 = st.top();
  108. 				st.pop();
  109. 				if (st.empty())
  110. 					throw SyntaxError{"Expected a number before the operator"};
  111. 				double n2 = st.top();
  112. 				st.pop();
  113. 				st.push(builtin_op(n2, n1, tok));
  114. 				break;
  115. 			}
  116. 			default: throw SyntaxError {"Unexpected token"};
  117. 		}
  118. 	}
  119. }
  120.  
  121. int main() {
  122. 	//calculate(std::cin);
  123. 	std::istringstream is {"5 1 2 + 4 * + 3 -; 2 2 +; 2 2 -;"};
  124. 	calculate(is);
  125. 	return EXIT_SUCCESS;
  126. }
Success #stdin #stdout 0s 3472KB
comments (?)
stdin 
Standard input is empty
stdout 
= 14
= 4
= 0
https://ideone.com/n3xPkv
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
secret

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