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"Floating Point Exception"

zeppelyn (4)
So, I'm trying to finish a program that converts an infix expression to postfix then evaluates that postfix expression. I'm using an input file and when I getline the first line of the file, it'll convert it to postfix, but when I evaluate it, all I get is "floating point exception".

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#include <iostream>
#include <fstream>
#include <stack>
#include <ctype.h>
#include <sstream>
using namespace std;

int prec(char elem)
  {
    switch(elem)
      {
        case '^' : return 3;
        case '*' : return 2;
        case '/' : return 2;
        case '+' : return 1;
        case '-' : return 1;
        case '(' : return 0;

        default  : return -1;
      }
  }

int eval(char opn1, char elem, char opn2)
  {
    int result;
    switch(elem)
      {
        case '+' : result = opn1 + opn2;
        case '-' : result = opn1 - opn2;
        case '*' : result = opn1 * opn2;
        case '/' : result = opn1 / opn2;
        case '^' : result = opn1 ^ opn2;
      }
    return result;
  }

void in_to_post(string st)
  {
    string post;
    char tok, opn1, opn2;
    int result;
    stack <char> ifx;
    stack <char> pfx;
    stringstream output, out2;

    for(int i = 0; i <= st.length(); i++)
      {
        if(st[i] == ' ') i++;
        if(st[i] == '^' || st[i] == '*' || st[i] == '/' ||
           st[i] == '+' || st[i] == '-')
          {
            while(!ifx.empty() && prec(st[i]) <= prec(ifx.top()))
              {
                output << ifx.top();
                ifx.pop();
              }
            ifx.push(st[i]);
          }
        else if(st[i] == '(')
          {
            ifx.push(st[i]);
          }
        else if(st[i] == ')')
          {
            while(ifx.top() != '(')
              {
                output << ifx.top();
                ifx.pop();
              }
            ifx.pop();
          }
        else
          {
            output << st[i];
          }
      }
    while(!ifx.empty())
      {
        output << ifx.top();
        ifx.pop();
      }

    cout << output.str() <<endl;
    post = output.str();

    for(int i = 0; i < post.length(); i++)
      {
        tok = post[i];
        if(isdigit(tok))
          {
            pfx.push(tok);
          }
        else
          {
            opn2 = pfx.top();
            pfx.pop();
            opn1 = pfx.top();
            result = eval(opn1, tok, opn2);
            pfx.push(result);
          }
      }
    while(!pfx.empty())
      {
        out2 << pfx.top();
        pfx.pop();
      }
    cout << out2.str() << endl;
  }

int main()
  {
    string st, post;
    ifstream infix;
    stringstream output;

    infix.open("x.data");
    while(getline(infix, st))
      {
        cout << st << endl;
        in_to_post(st);
      }
    infix.close();
    return 0;
  }


So, I know the problem is in the second for loop, but IDK how to fix it.

Input file: "x.data"

2 + 3 * 5
2 + 3 * 5 ^ 6
2 + 3 - 5 + 6 - 4 + 2 - 1
2 + 3 * ( 5 - 6 ) - 4
2 + 3 ^ 5 * 6 - 4
2 + 3 * 6 ^ 2
( ( ( ( 2 + 3 - 4 ) / 2 + 8 ) * 3 * ( 4 + 5 ) / 2 / 3 + 9 ) )
histrungalot (272)
Take a look at the comments in the code.
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#include <iostream>
#include <fstream>
#include <stack>
#include <ctype.h>
#include <sstream>
//----------------------------
// Added for pow function
#include <cmath>
using namespace std;

int prec(char elem)
  {
    switch(elem)
      {
        case '^' : return 3;
        case '*' : return 2;
        case '/' : return 2;
        case '+' : return 1;
        case '-' : return 1;
        case '(' : return 0;

        default  : return -1;
      }
  }

int eval(char opn1, char elem, char opn2)
  {
    int result;
    switch(elem)
      {
        //--------------------------------------------------
        // Added the breaks
        case '+' : result = opn1 + opn2;break;
        case '-' : result = opn1 - opn2;break;
        case '*' : result = opn1 * opn2;break;
        case '/' : result = opn1 / opn2;break;
        //--------------------------------------------------
        // This has to be a the pow function because
        // ^ is XOR not to the power
        case '^' : result = int(pow(float(opn1),float(opn2)));break;
      }
    return result;
  }

void in_to_post(string st)
  {
    string post;
    char tok,opn1,opn2;
    int result;
    stack <char> ifx;
    stack <char> pfx;
    stringstream output, out2;

    //------------------------------------------------------
    // i should be just < not <= st.length()
    for(int i = 0; i < st.length(); i++)
      {
        if(st[i] == ' ') i++;
        if(st[i] == '^' || st[i] == '*' || st[i] == '/' ||
           st[i] == '+' || st[i] == '-')
          {
            while(!ifx.empty() && prec(st[i]) <= prec(ifx.top()))
              {
                output << ifx.top();
                ifx.pop();
              }
            ifx.push(st[i]);
          }
        else if(st[i] == '(')
          {
            ifx.push(st[i]);
          }
        else if(st[i] == ')')
          {
            while(ifx.top() != '(')
              {
                output << ifx.top();
                ifx.pop();
              }
            ifx.pop();
          }
        else
          {
            output << st[i];
          }
      }
    while(!ifx.empty())
      {
        output << ifx.top();
        ifx.pop();
      }

    cout << output.str() <<endl;
    post = output.str();

    for(int i = 0; i < post.length(); i++)
      {
        tok = post[i];
        if(isdigit(tok))
          {
            pfx.push(tok);
          }
        else
          {
            opn2 = pfx.top();
            pfx.pop();
            opn1 = pfx.top();
            //---------------------------------------------
            // Added the pop
            pfx.pop();

//-----------------------------------------------------
// Cheated here by substacting the 0x30, but this
// is where your real issues start.  This will work for 
// the first equation because the value of the result and
// all intermeditate results are less than 8 bits.  As soon
// as you try the second equation, the 5^6 (15,625) blows 
// that apart and the code is broken.  Once you start to 
// populate pfx, the values stored must be integers and 
// not chars.  In that case the 0x30s would come out of 
// the function call below and other spots below.  But then 
// where do you do that?  If all of the numbers in the 
// x.data file are never larger that one character than 
// down here maybe, if not than you would have to change 
// the loop above and the parsing becomes more complex.
// 
// Take a look and see if just making the changes have pfx
// a stack<int> is OK.

            result = eval(opn1-0x30, tok, opn2-0x30);
            pfx.push(result+0x30);
          }
      }
    while(!pfx.empty())
      {
        out2 << pfx.top()-0x30;
        pfx.pop();
      }
    cout << out2.str() << endl;
  }

int main()
  {
    string st, post;
    ifstream infix;
    stringstream output;

    infix.open("x.data");
    while(getline(infix, st))
      {
        cout << st << endl;
        in_to_post(st);
      }
    infix.close();
    return 0;
  }
 
$ ./a.out
2 + 3 * 5
235*+
17                      <------- Right
2 + 3 * 5 ^ 6
2356^*+
29                      <------- Wrong because of 5^6
2 + 3 - 5 + 6 - 4 + 2 - 1
23+5-6+4-2+1-
3                        <------- Right
2 + 3 * ( 5 - 6 ) - 4
2356-*+4-
-5                       <------- Right
2 + 3 ^ 5 * 6 - 4
235^6*+4-
-80                     <------- Wrong
2 + 3 * 6 ^ 2
2362^*+
-146                   <------- Wrong
( ( ( ( 2 + 3 - 4 ) / 2 + 8 ) * 3 * ( 4 + 5 ) / 2 / 3 + 9 ) )
23+4-2/8+3*45+*2/3/9+
3                       <------- Wrong because of fractions
$
Last edited on
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