Infix to Postfix expression converter.

/*
 *
 * Author: Carlos Alaniz         20244196
 * Date: 10/01/2013
 * Instructor: Zhixiang Chen;
 * Course : CMPE 3333.02
 *
 * Description: Infix to Postfix expression converter.
 **/





#include <iostream>
#include <string>
#include <cstdlib>
#include <map>

using namespace std;

class postfix {
private:
    string stack;
    string post;
    string pre;
    map <char, int> ops;
public:
    postfix(string pre) {
        post.clear();
        stack.clear();
        this->pre = pre;
        //Operator herarchy table
        ops['-'] = 1;
        ops['+'] = 1;
        ops['/'] = 2;
        ops['*'] = 2;
        ops['^'] = 4;
        ops['('] = -1;
        ops[')'] = -1;
    }

    void checksym(char sym) {
        if (stack.empty() || sym == '(') {
            stack += sym; //push into stack
            return;
        } else {
            char topOp = stack[stack.length() - 1];
            while (!stack.empty() && ops[sym] <= ops[topOp]) {
                //Pop and discard
                stack.erase(stack.end() - 1);
                if (topOp == '(' && sym == ')') {
                    //stack.erase(stack.end() - 1);
                    break;
                }
                post += topOp;
                if (!stack.empty()) {
                    topOp = stack[stack.length() - 1];
                }
            }
            if (sym != ')')
                stack += sym;
        }


    }

    void scan() {
        for (unsigned i = 0; i < pre.length(); i++) {
            if (isdigit(pre[i])) {
                //separate numbers with a "'" ex. 380'5'85'5+++ 580.58.+5.+9.+12541.+
                while (isdigit(pre[i])) {
                    post += pre[i];
                    i++;
                }
                post += ',';
                i--;
            } else if (isalpha(pre[i])) {
                if (post.find(pre[i]) >= 0)
                    post += pre[i];
                else
                    exit(-5);
            } else if (pre[i] == ' ') {
                //ignore
            } else {
                checksym(pre[i]);
            }
        }
        for (int j = stack.length() - 1; j >= 0; j--) {
            post += stack[j]; //print the rest of the stack
            stack.erase(stack.end() - 1);
        }

    }

    string get_postfix(){
        return post;
    }
};

int main() {
    //string pre = "(5+8*(8056-7))*8^5-5+(9^8*8/8-5)";
    //string pre = "2*3-4/5";
    string pre = "a+b*c-d";
    postfix post(pre);
    post.scan();
    cout << post.get_postfix() << endl;
    return 0;
}


/**
 * Sample Output:
 *
 * input : (5+8*(8056-7))*8^5-5+(9^8*8/8-5)
 * output : 5.8.8056.7.-*+8.5.^*5.-9.8.^8.*8./5.-+
 *
 * input : 2*3-4/5
 * output : 2.3.*4.5./-
 *
 * input : a+b*c-d
 * output : abc*+d-
 */