01: #include <iostream>
02: #include <cctype>
04: using namespace std;
06: int term_value();
07: int factor_value();
09: /**
10:    Evaluates the next expression found in cin
11:    @return the value of the expression.
12: */
13: int expression_value()
14: {  int result = term_value();
16:    bool more = true;
17:    while (more)
18:    {  char op = cin.peek();
20:       if (op == '+' || op == '-')
21:       {  cin.get();
23:          int value = term_value();
24:          if (op == '+') result = result + value;
25:          else result = result - value;
26:       }
27:       else more = false;
28:    }
29:    return result;
30: }
32: /**
33:    Evaluates the next term found in cin
34:    @return the value of the term.
35: */
36: int term_value()
37: {  int result = factor_value();
39:    bool more = true;
40:    while (more)
41:    {  char op = cin.peek();
43:       if (op == '*' || op == '/')
44:       {  cin.get();
46:          int value = factor_value();
47:          if (op == '*') result = result * value;
48:          else result = result / value;
49:       }
50:       else more = false;
51:    }
52:    return result;
53: }
55: /**
56:    Evaluates the next factor found in cin
57:    @return the value of the factor.
58: */
59: int factor_value()
60: {  int result = 0;
62:    char c = cin.peek();
63:    if (c == '(')
64:    {  cin.get();
66:       result = expression_value();
67:       cin.get(); // read ")"
68:    }
69:    else // assemble number value from digits
70:    {  while (isdigit(c))
72:       {  result = 10 * result + c - '0';
74:          cin.get();
75:          c = cin.peek();
76:       } 
77:    }
78:    return result;
79: }
80: 
81: int main()
82: {  cout << "Enter an expression: ";
84:    cout << expression_value() << endl;
85:    return 0;
86: }