C++

Forum

 
Evaluating postfix

trevormoon (45)
Hello all I am stuck trying to write a program that reads in an expression, turns that expression into postfix, and then evaluates it. The program reads in the expression and converts it to postfix, but I cant get it to evaluate correctly. For example I input 2 + 3 postfix equals 23+ and it returns 1 as the answer. Any help would be greatly appreciated!



1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
 
#include <fstream>
#include <iostream>
#include <sstream>
#include <stack>
#include <string>
using namespace std;

int evaluate (int opn1, char token, int opn2)
  { int result;
    switch (token)
      {
        case '+' : result = opn1 + opn2;
        case '-' : result = opn1 - opn2;
        case '*' : result = opn1 * opn2;
        case '/' : result = opn1 / opn2;
        case '^' : result = opn1 ^ opn2;
      }
    return result;
  }
int priority(char a)
 {
    int temp;
    if (a == '^')
        temp = 3;
    else  if (a == '*' || a == '/')
        temp = 2;
    else  if (a == '+' || a == '-')
        temp = 1;
    else if ( a == '(')
        temp = 0;
    return temp;
}

int main()
{
    string aline;
    string st;;
    int result;
    int  opn2, opn1, temp;

    stack<char> ostack;
    stack<int> istack;
    stringstream output2;
    stringstream output;
    getline(cin,aline);
    st = output.str();
    for (int i = 0; i < aline.length(); i++)
    {
          if(aline[i] == ' ')
              i++;
                if (aline[i] == '+' || aline[i] == '-' || aline[i] == '*' || aline[i] == '/' || aline[i] == '^')
                {
                    while (!ostack.empty() && priority(ostack.top()) >= priority(aline[i]))
                     {
                       output << ostack.top();
                       ostack.pop();
                     }
                  ostack.push(aline[i]);
                }
              else if (aline[i] == '(')
              {
                ostack.push(aline[i]);
              }
              else if (aline[i] == ')')
        {
                 while (ostack.top() != '(')
                {
                    output << ostack.top();
                    ostack.pop();
                }
                 ostack.pop();
        }
           else
             {
            output << aline[i];
             }
    }
    while (!ostack.empty())
     {
        output << ostack.top();
        ostack.pop();
     }

      cout<<aline<<endl;
    cout << output.str() << endl;
st = output.str();
for (int i=0; i < st.length(); i++)
  {
      if (st[i] != '+' && st[i] != '-' & st[i] != '*' & st[i] != '/' & st[i] != '^')
        {
        temp =  st[i] - 48;
           cout<< "temp " << temp <<endl;
          istack.push(temp);
        }
         else
          {
          char token = st[i];
            cout<<"token "<< token <<endl;
        opn2 =  istack.top();
              cout<<"opn2 " << opn2 <<endl;
            istack.pop();
        opn1 =  istack.top();
            cout<<"opn1 " << opn1 <<endl;
              istack.pop();

          result = evaluate(opn1, token, opn2);
          istack.push(result);
          cout <<" result " << result<<endl;
          istack.push(st[i]);
     }
 }
  }

Here is what it prints out:
2 + 3
2 + 3
23+
temp 2
temp 3
token +
opn2 3
opn1 2
result 1


zeppelyn (4)
I haven't managed to fix my postfix issues either, but I don't think you need to push both the result and st[i] at the end of your evaluation "for" loop.
trevormoon (45)
Hi zeppelyn sorry I didn't see your comment. If you add breaks after each statement in evaluate this code should work for you.
Topic archived. No new replies allowed.