Stack Infix to Prefix not working correctly

Question

I have an assignment to make a program that converts Infix to Prefix in C++. I have written a code to do so but the problem is I am only getting the operands and not the operators in the final result. I have checked my code many time, dry run it, but I cant find the problem. Here is the code:

#include <iostream>
#include <stack>
#include <algorithm>

using namespace std;

int Prec(char c)
{
    if(c == '^')
    {
        return 3;
    }
    
    else if(c == '*' || c == '/')
    {
        return 2;
    }
    
    else if(c == '+' || c == '-')
    {
        return 1;
    }
    
    else 
    {
        return -1;
    }
} // prec

string InfixToPrefix(string s)
{
    reverse(s.begin(), s.end());
    
    stack<char> st;
    string res;
    
    for(int i=0; i<s.length(); i++)
    {
        if((s[i] >= 'a' && s[i] <= 'z') || (s[i] >= 'A' && s[i] <= 'Z'))
        {
            res += s[i];
        }
        
        else if(s[i] == ')')
        {
            st.push(s[i]);
        }
        
        else if(s[i] == '(')
        {
            while(!st.empty() && st.top() != ')')
            {
                res += st.top();
                st.pop();
            }
            
            if(!st.empty())
            {
                st.pop();
            }
            
            else
            {
                while(!st.empty() && Prec(st.top()) >= Prec(s[i]))
                {
                    res += st.top();
                    st.pop();
                }
                
                st.push(s[i]);
            }
        }
    } // for loop
    
    while(!st.empty())
    {
        res+=st.top();
        st.pop();
    }
    
    reverse(res.begin(), res.end());
    
    return res;
} // InfixToPrefix()

int main()
{
    cout<<InfixToPrefix("(a-b/c)*(a/k-l)")<<endl;
}

Can someone please help?
The correct output should be "*-a/bc-/akl" but I am only getting "abcakl". Please help. Thank you.

Answer 1

You need to put the operator logic in the mainloop.

#include <iostream>
#include <stack>
#include <algorithm>

using namespace std;

int Prec(char c)
{
    if(c == '^')
    {
        return 3;
    }

    else if(c == '*' || c == '/')
    {
        return 2;
    }

    else if(c == '+' || c == '-')
    {
        return 1;
    }

    else
    {
        return -1;
    }
} // prec

string InfixToPrefix(string s)
{
    reverse(s.begin(), s.end());

    stack<char> st;
    string res;

    for(int i=0; i<s.length(); i++)
    {
        if((s[i] >= 'a' && s[i] <= 'z') || (s[i] >= 'A' && s[i] <= 'Z'))
        {
            res += s[i];
        }

        else if(s[i] == ')')
        {
            st.push(s[i]);
        }

        else if(s[i] == '(')
        {
            while(!st.empty() && st.top() != ')')
            {
                res += st.top();
                st.pop();
            }

            if(!st.empty())
            {
                st.pop();
            }

            else
            {
                while(!st.empty() && Prec(st.top()) >= Prec(s[i]))
                {
                    res += st.top();
                    st.pop();
                }

                st.push(s[i]);
            }
        }
        else {   // added operator logic to the main portion of the parse loop
            if (!st.empty() && Prec(st.top()) >= Prec(s[i])){
                    res += st.top();
                    st.pop();
            }

                st.push(s[i]);
        }
    } // for loop

    while(!st.empty())
    {
        res+=st.top();
        st.pop();
    }

    reverse(res.begin(), res.end());

    return res;
} // InfixToPrefix()

int main()
{
    cout<<InfixToPrefix("(a-b/c)*(a/k-l)")<<endl;
}