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  1. #include <iostream>
  2. #include <cctype>
  3.  
  4. using namespace std;
  5.  
  6.     bool eval_expr(const char **pe, int &lhs, bool inside = false);
  7.     char skip_ws(const char **pe) {
  8.         while (**pe == ' ') ++(*pe);
  9.         return **pe;
  10.     }
  11.     bool eval_prim(const char **pe, int &res) {
  12.         char c = skip_ws(pe);
  13.         if (c == '(') {
  14.             ++(*pe);
  15.             if (!eval_expr(pe, res, true)) return false;
  16.             ++(*pe);
  17.             return true;
  18.         }
  19.         if (isdigit(c)) {
  20.             res = 0;
  21.             while (isdigit(c)) {
  22.                 res = 10*res + c - '0';
  23.                 c = *(++(*pe));
  24.             }
  25.             return true;
  26.         }
  27.         return false;
  28.     }
  29.     bool eval_expr(const char **pe, int &lhs, bool inside) {
  30.         if (!eval_prim(pe, lhs)) return false;
  31.         char op;
  32.         while ((op = skip_ws(pe)) && (op == '+' || op == '-' || op == '*' || op == '/')) {
  33.             ++(*pe);
  34.             int rhs;
  35.             if (!eval_prim(pe, rhs)) return false;
  36.             switch (op) {
  37.                 case '+': lhs += rhs; break;
  38.                 case '-': lhs -= rhs; break;
  39.                 case '*': lhs *= rhs; break;
  40.                 case '/': lhs /= rhs; break;
  41.             }
  42.         }
  43.         return inside ? op == ')' : !op;
  44.     }
  45.  
  46.     bool evaluate(const char *e, int &result) {
  47.         return eval_expr(&e, result);
  48.     }
  49.  
  50. int main() {
  51.     int res;
  52.     if (evaluate("2", res)) {
  53.         cout << res << endl;
  54.     } else {
  55.         cout << "invalid" << endl;
  56.     }
  57.     if (evaluate("2+3", res)) {
  58.         cout << res << endl;
  59.     } else {
  60.         cout << "invalid" << endl;
  61.     }
  62.     if (evaluate("2*3-4", res)) {
  63.         cout << res << endl;
  64.     } else {
  65.         cout << "invalid" << endl;
  66.     }
  67.     if (evaluate("(1+2)+5", res)) {
  68.         cout << res << endl;
  69.     } else {
  70.         cout << "invalid" << endl;
  71.     }
  72.     if (evaluate("", res)) {
  73.         cout << res << endl;
  74.     } else {
  75.         cout << "invalid" << endl;
  76.     }
  77.     if (evaluate("2+", res)) {
  78.         cout << res << endl;
  79.     } else {
  80.         cout << "invalid" << endl;
  81.     }
  82.     return 0;
  83. }
Success #stdin #stdout 0.01s 2724KB
comments (?)
stdin
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Standard input is empty
stdout
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2
5
2
8
invalid
invalid
https://ideone.com/fvV8C
language:
C++ (gcc 8.3)
created:
13 years ago
visibility:
public

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