in2pre cpp

/* Program to perform all types of stack conversions */
#include<iostream>
#include<string>
#include<algorithm>
#define MAX 20
using namespace std;


void push(char);
char pop();
string in2prefix(string);
int priority ( char );
char stk[20];
int top=-1;


int main()
{
    int cont;
    string infix, prefix;
    cout<<"\n enter the infix expression : ";
    //cin>>infix;
    prefix = in2prefix("a+b*c/d-e+f");
    return 0;
}

// method that pushes the elements onto the character stack
void push(char oper)
{
    if(top==MAX-1)
    {
        cout<<"stackfull!!!!";
    }

    else
    {
        top++;
        stk[top]=oper;
    }
}

// method that removes character from stack and returns them
char pop()
{
    char ch;
    if(top==-1)
    {
        cout<<"stackempty!!!!";
    }
    else
    {
        ch=stk[top];
        stk[top]='\0';
        top--;
        return(ch);
    }
    return 0;
}

// method that converts String from infix to prefix
// all the strings are assumed to be valid expressions
string in2prefix(string infix)
{
    int i=0;
    string prefix = "";

    reverse(infix.begin(), infix.end());
    cout<<"\n"<<infix;

    // iterating while end of string is not found
    while(infix[i]!='\0')
    {
        // if an alphabet is found then copy it to the output string
        if(infix[i]>='a' && infix[i]<='z')
        {
            prefix.insert(prefix.end(),infix[i]);
            i++;
        }

        // as we have reversed the string closing bracket will be found first
        // if an closing bracket is found then put it in stack
        else if(infix[i]=='}' || infix[i]==')'  || infix[i]==']')
        {
            push(infix[i]);
            i++;
        }

        // as we have reversed the string opening bracket will be found after the closing bracket
        // if an opening bracket is found then
        // keep removing the operators from the stack and add them to prefix string until you find the corresponding opening bracket
        else if(infix[i]=='(' || infix[i]=='{'  || infix[i]=='[')
        {
            if(infix[i]=='(')
            {
                while( stk[top] != ')' )
                {
                    prefix.insert(prefix.end(),pop());
                }
                pop();
                i++;
            }

            if(infix[i]=='[')
            {
                while(stk[top]!=']')
                {
                    prefix.insert(prefix.end(),pop());
                }
                pop();
                i++;
            }

            if(infix[i]=='{')
            {
                while(stk[top]!='}')
                {
                    prefix.insert(prefix.end(),pop());
                }
                pop();
                i++;
            }
        }

        // if none of the above cases are satisfied then we surely have an operator
        else
        {

            // if the stack if empty then we simply put the operator in stack
            if(top==-1)
            {
                push(infix[i]);
                i++;
            }

            // if the priority of current operator is greater than or equal to the stack top then push it onto the stack
            else if(priority(infix[i]) >= priority(stk[top])) {
                push(infix[i]);
                i++;
            }

            // if the priority of current operator is less than the stack top then
            // pop the stack top and add it to the prefix string
            else if( priority(infix[i]) < priority(stk[top])) {
                prefix.insert(prefix.end(),pop());

                // now if you have an operator that has greater priority than of current operator then pop
                while(priority(stk[top]) > priority(infix[i])){
                    prefix.insert(prefix.end(),pop());
                    if(top < 0) {
                        break;
                    }
                }
                push(infix[i]);
                i++;
            }
        }
    }

    // at the end remove all the operators from the stack
    while(top!=-1)
    {
        prefix.insert(prefix.end(),pop());
    }

    // reverse the final string before output
    reverse(prefix.begin(), prefix.end());

    cout<<"\nThe converted prefix string is : "<<prefix;
    return prefix;
}

// method that returns priority for operators according to their precedence
int priority ( char alpha )
{
    if(alpha == '+' || alpha =='-')
    {
        return(1);
    }

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

    if(alpha == '$')
    {
        return(3);
    }

    return 0;
}