in2post cpp

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


void push(char);
char pop();
string in2postfix(string);
int priority ( char );
char stk[20];
int top=-1;


int main()
{
    int cont;
    string infix, postfix;
    cout<<"\n enter the infix expression : ";
    //cin>>infix;
    postfix = in2postfix("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 postfix
// all the strings are assumed to be valid expressions
string in2postfix(string infix)
{
    int i=0;
    string postfix = "";

    // 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')
        {
            postfix.insert(postfix.end(),infix[i]);
            i++;
        }

        // if an opening bracket is found then put it in stack
        else if(infix[i]=='(' || infix[i]=='{'  || infix[i]=='[')
        {
            push(infix[i]);
            i++;
        }

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

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

            if(infix[i]=='}')
            {
                while(stk[top]!='{')
                {
                    postfix.insert(postfix.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 less than or equal to the stack top then
            // pop the stack top and add it to the postfix string
            else if( priority(infix[i]) <= priority(stk[top])) {
                postfix.insert(postfix.end(),pop());

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

            // if the priority of current operator if high then push it onto the stack
            else if(priority(infix[i]) > priority(stk[top])) {
                push(infix[i]);
                i++;
            }
        }
    }

    // at the end remove all the operators from the stack
    while(top!=-1)
    {
        postfix.insert(postfix.end(),pop());
    }
    cout<<"The converted postfix string is : "<<postfix;
    return postfix;
}

// 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;
}