conversion from infix to postfix

[Nhd]

Hi... I need a program to convert an infix arithmetic expression to postfix with the use of stacks... Can anyone tell me how do i go about implementing this? thanks

[D_C]

There is a standard algorithm to do this. The last paragraph of this website has the steps.

Print the numbers out in normal order, push parenthesis and operators onto the stack. When parenthesis line up, ignore them, don't print anything. If they don't line up, that's an error. Then, depending upon priority of the operator (PEMDAS) push or pop the operator off the stack. If you pop it, display it in the output.

The website explains it better, why don't you go have a look.

[Nhd]

Hi can anyone help me to check this... Im lost in my own program... I noe some part of it is wrong... can anyone tell me... thanks

#include <iostream>
#include <stack>
#include <vector>
#include <sstream>
using namespace std;

int main()
{
string line, str;
int number;

cout<<"Sample Input 1"<<" "<<endl;
cin>>line;

getline(cin, line);
istringstream iss(line); //create string stream

while(!iss.eof( ))
{
iss >> str; //reads from iss into str until next space is met
}

//Postfix evaluation
int eval_postfix(co nst vector<char>&v, stack<int>&st)
{
for (vector<char>:: const_iterator str = v.begin(begin() ; str !=v.end(); ++str)
{
if ('0'<= str && str<='9')
st.push(str-'0')
else
{
int arg2 = st.top();
st.pop();
int arg1 = st.top();
st.pop();
int result;

if (str == "+" || str == "-") //when you compare for char, it's '+' and '-'
{
case '+': result = arg1 + arg2;
return 1;
break;
case '-': result = arg1 - arg2;
resturn 2;
break;
}
else if (str == "*" || str == "/")
{
case '*': result = arg1 * arg2;
break;
return 3;
case '/': result = arg1 / arg2;
break;
return 4;
} //end if else

st.push(result) ;
} //end else
}//end for

int i = st.top();
st.pop();
return i;
}//end eval_postfix


system("pause") ;
return 0;
}

[Nhd]

Hi i modify my codes but i'm unable to execute it.... can anyone help me to spot the mistake.. thanks

#include <iostream>
#include <stack>
#include <vector>
#include <sstream>
using namespace std;

int main()
{
      string line, str;  
      int number;
      
      cout<<"Sample Input:"<<" "<<endl;
      cin>>line;
      
      getline(cin, line);
      istringstream iss(line); //create string stream
      
      while(!iss.eof()) 
      {
           iss >> str; //reads from iss into str until next space is met
      }
      
      //Code of Postfix Evaluation
      int eval_postfix(const vector<char>&v, stack<int>&st)
      {
          for (vector<char>::const_iterator ptr = v.begin(begin(); ptr !=v.end(); ++ptr)
          {
              if ('0'<= *ptr && *ptr<='9')
              st.push(*ptr-'0')
              else
              {
                  int arg2 = st.top();
                  st.pop();
                  int arg1 = st.top();
                  st.pop();
                  int result;
                  
              switch (*ptr)
              {
                     case '+': result = arg1 + arg2;
                     break;
                     case '-': result = arg1 - arg2;
                     break;
                     case '*': result = arg1 * arg2;
                     break;
                     case '/': result = arg1 / arg2;
                     break; 
              } //end switch
              st.push(result);
              } //end else
          }//end for
          
          int i = st.top();
          st.pop();
          return i;
      }//end eval_postfix 

       cout<<i<<"Sample Output:"<<" "<<endl;
             
       system("pause");
       return 0;
}

[Banfa]

You have tried to define the function

int eval_postfix(co nst vector<char>&v, stack<int>&st)

inside the function

int main()

This is not allowed in C/C++ move the definition of the function eval_postfix outside the function main.

[Nhd]

Hi... I put the "int eval_postfix(co nst vector<char>&v, stack<int>&st)" out of the main, i still get error.. the error i get is something to do with this "int main()".. but i cant remove the "int main()"..... I'm sory to bother u but im really weak in my programming...

#include <iostream>
#include <stack>
#include <vector>
#include <sstream>
using namespace std;
int eval_postfix(co nst vector<char>&v, stack<int>&st)
int main()
{
string line, str;
int number;
char *cstr;

cout<<"Sample Input:"<<" "<<endl;
cin>>line;

getline(cin, line);
istringstream iss(line); //create string stream

while(!iss.eof( ))
{
iss >> str; //reads from iss into str until next space is met
}

//Code of Postfix Evaluation
for (vector<char>:: const_iterator ptr = v.begin(begin() ; ptr !=v.end(); ++ptr)
{
if ('0'<= *ptr && *ptr<='9')
st.push(*ptr-'0')
else
{
int arg2 = st.top();
st.pop();
int arg1 = st.top();
st.pop();
int result;

switch (*ptr)
{
case '+': result = arg1 + arg2;
break;
case '-': result = arg1 - arg2;
break;
case '*': result = arg1 * arg2;
break;
case '/': result = arg1 / arg2;
break;
} //end switch
st.push(result) ;
} //end else
}//end for

int i = st.top();
st.pop();
return i;
}//end eval_postfix

//convert string to integer
number = atoi(str.c_str( )); //now number is 1234

//convert integer into string
sprintf(cstr, "%d", number); //print number into cstring
string str(cstr); //makes cstring a string

//checking if all characters of the string are digits
cstr = str.c_str();
if ( isdigit(*cstr) ) //checks the first character of cstr is digit or not
return true;

cout<<"Sample Output:"<<cin<< " "<<endl;

system("pause") ;
return 0;
}

[Banfa]

You have to move the entire function, not just the header line

This

int main()
{
  // main code

  int fn()
  {
    // function code
  }

  // more main code
}

becomes

int fn();

int main()
{
  // main code

  // more main code
}

int fn()
{
  // function code
}

[Nhd]

for this function, for the 2nd sentence , may i noe how do i declare "begin"? i hav error regarding this.... should i declare it in main function or in its own function itself...

int eval_postfix(co nst vector<char>&v, stack<int>&st)
{
for (vector<char>:: const_iterator ptr = v.begin(begin() ; ptr !=v.end(); ++ptr)
{
if ('0'<= *ptr && *ptr<='9')
st.push(*ptr-'0');
else
{
int arg2 = st.top();
st.pop();
int arg1 = st.top();
st.pop();
int result;

switch (*ptr)
{
case '+': result = arg1 + arg2;
break;
case '-': result = arg1 - arg2;
break;
case '*': result = arg1 * arg2;
break;
case '/': result = arg1 / arg2;
break;
} //end switch
st.push(result) ;
} //end else
}//end for

int i = st.top();
st.pop();
return i;
}//end eval_postfix

[Banfa]

Not really sure what you are saying but

for (vector<char>:: const_iterator ptr = v.begin(begin() ; ptr !=v.end(); ++ptr)

should be

for (vector<char>:: const_iterator ptr = v.begin(); ptr !=v.end(); ++ptr)

[Nhd]

Thanks Banfa for ur help.... but do u mind helping me to check my program please.... I've been struggling from juz now and i still cant do it....

#include <iostream>
#include <stack>
#include <vector>
#include <sstream>
using namespace std;

int eval_postfix(const vector<char>&v, stack<int>&st);
int main()
{
      string line, str;
      int number;
      char *cstr;
      
      cout<<"Sample Input:"<<" "<<endl;
      cin>>line;
      
      getline(cin, line);
      istringstream iss(line); //create string stream
      
      while(!iss.eof()) 
      {
       iss >> str; //reads from iss into str until next space is met
      }
      
      //convert string to integer
      number = atoi(str.c_str()); //now number is 1234
      
      cout<<"Sample Output:"<<str<<" "<<endl;
             
      system("pause");
      return 0;
}
	         
     //Code of Postfix Evaluation
      int eval_postfix(const vector<char>&v, stack<int>&st)
      {
          for (vector<char>::const_iterator ptr = v.begin(); ptr !=v.end(); ++ptr)
          {
              if ('0'<= *ptr && *ptr<='9')
              st.push(*ptr-'0');
              else
              {
                  int arg2 = st.top();
                  st.pop();
                  int arg1 = st.top();
                  st.pop();
                  int result;
                  
              switch (*ptr)
              {
                     case '+': result = arg1 + arg2;
                     break;
                     case '-': result = arg1 - arg2;
                     break;
                     case '*': result = arg1 * arg2;
                     break;
                     case '/': result = arg1 / arg2;
                     break; 
              } //end switch
              st.push(result);
              } //end else
          }//end for
          
          int i = st.top();
          st.pop();
          return i;
      }//end eval_postfix 
 
      //Conversion from Infix to Postfix expression
      vector<char> postFix;
      for (vector<char>::const_iterator ptr = v.begin(); ptr !=v.end(); ++ptr)
      {
        switch(ch)
        {
                  case operand: 
                       postfixExp = postfixExp + ch;
                       break;
                  case '(': 
                       Stack.push (ch);
                       Break;
                  case ')':
                       while (stack.top()!='(')
                       {
                             postfixExp = postfixExp + stack.top();
                             Stack.pop();
                       } //end while
                       
                       Stack.pop();
                       break;
                       
                  //infix to postfix - use stack for storing operators
                  case operator:
                       
                       while (!Stack.isEmpty() && Stack.top != '(' && precedence(ch) >= precedence(stack.top()))
                       {
                             postfixExp = postfixExp + Stack.top();
                             Stack.pop();
                        } end while
             }//end switch
    
    } //end for
    
    while (!Stack.isEmpty())
    {
          postfixExp = postfixExp + Stack.top();
          Stack.pop()
    }// end while

[Banfa]

None of this code

      //Conversion from Infix to Postfix expression
      vector<char> postFix;
      for (vector<char>::const_iterator ptr = v.begin(); ptr !=v.end(); ++ptr)
      {
        switch(ch)
        {
                  case operand: 
                       postfixExp = postfixExp + ch;
                       break;
                  case '(': 
                       Stack.push (ch);
                       Break;
                  case ')':
                       while (stack.top()!='(')
                       {
                             postfixExp = postfixExp + stack.top();
                             Stack.pop();
                       } //end while
                       
                       Stack.pop();
                       break;
                       
                  //infix to postfix - use stack for storing operators
                  case operator:
                       
                       while (!Stack.isEmpty() && Stack.top != '(' && precedence(ch) >= precedence(stack.top()))
                       {
                             postfixExp = postfixExp + Stack.top();
                             Stack.pop();
                        } end while
             }//end switch
    
    } //end for
    
    while (!Stack.isEmpty())
    {
          postfixExp = postfixExp + Stack.top();
          Stack.pop()
    }// end while

Is inside a function and is causing a syntax error

It doesn't appear to have any purpose so I guess you can just delete it as the program compiles fine without it.

[Nhd]

hi Banfa.. my purpose in typing out those codes is to convert infix to postfix..

for example:
Sample input 1
1 + 2 * 3 - 4 / 2 + 5 * 6
Sample output 1
35

Sample input 2
( 10 + 20 ) * 30 - 4000 / 20 + 50 * 60
Sample output 2
3700

I'm trying to write a program for this... and my notes shows only these codes, the one that i type out...

what should i do now... :(