Spoj question ONP Transform the expression giving signal abort - algorithm

Question

I am trying to solve ONP - Transform the Expression in spoj. The question is to transform infix expression into postfix expression. I have used std::stack as my data structure and shunting-yard algorithm for solving it. The code runs fine on my computer using g++. But on spoj, it gives SIGABRT error. Even on ideone, it gives run time error free() invalid pointer.
I have tried several test cases. At first, I thought that my program was taking too much memory, but upon testing with time -v (ubuntu), I found that the maximum space taken was in KB.
// ------------------- infix to postfix conversion ---------------
#include <iostream>
#include <string>
#include <stack>
#include <algorithm>
#include <utility>
using std::stack;
using std::pair;
using std::cout;
using std::cin;
using std::endl;
using std::string;
stack< pair<char, short> > op_st; // operator stack
short op_precedence(char op) {
// return operator precedence
// input: operator; output: its precedence value
switch (op) {
case '+': return 0;
case '-': return 1;
case '*': return 2;
case '/': return 3;
case '^': return 4;
case '(': return 6;
}
}
inline bool is_operator(char sym) {
// is sym an operator?
return (sym == '+' || sym == '-' || sym == '*' || sym == '/' || sym == '^' || sym == '(');
}
inline bool is_operand(char sym) {
// is sym an operand?
return (sym >= 'a' && sym <= 'z');
}
void in_to_post(string & expr) {
// infix to postfix converter
// input: infix expression
for (int i = 0; i < expr.length(); ++i) {
if (is_operator(expr[i])) { // operator
// pop op_stack until the
// top of the stack has less precedence
// than curr operator or stack is empty
while(1) {
if (op_st.empty()) { // stack is empty; straight away push
op_st.push(std::make_pair(expr[i], op_precedence(expr[i])));
break;
}
pair <char, short> & top_op = op_st.top();
if (op_precedence(top_op.second) >= op_precedence(expr[i])) {
cout << top_op.first;
op_st.pop();
}
else {
op_st.push(std::make_pair(expr[i], op_precedence(expr[i])));
break;
}
}
}
else if (is_operand(expr[i])) { // operand; push it to output queue immediately
cout << expr[i];
}
else if (expr[i] == ')') { // right paranthesis
while (1) {
if (op_st.empty()) { // invalid expression; ')' reached before matching '('
//cout << "No matching '(' found\n";
abort();
}
pair <char, short> & top_op = op_st.top();
if (top_op.first == '(') { // matching '(' found; stop
op_st.pop();
break;
}
else {
cout << top_op.first;
op_st.pop();
}
}
}
}
// pop out the whole op_st (if any)
while (!(op_st.empty())) {
pair <char, short> & top_op = op_st.top();
cout << top_op.first;
op_st.pop();
}
}
int main() {
int t;
cin >> t;
for (int i = 0; i < t; ++i) {
string expr;
cin >> expr;
//cout << expr.length() << endl;
in_to_post(expr);
cout << "\n";
}
return 0;
}
Input to program given on my system:
((a+b)-c*(d+e))^((a+b)-c*(d+e))+((a+b)-c*(d+e))-((a+b)-c*(d+e))+((a+b)-c*(d+e))^((a+b)-c*(d+e))+((a+b)-c*(d+e))-((a+b)-c*(d+e))-((a+b)-c*(d+e))^((a+b)-c*(d+e))+((a+b)-c*(d+e))-((a+b)-c*(d+e))*((a+b)-c*(d+e))^((a+b)-c*(d+e))+((a+b)-c*(d+e))-((a+b)-c*(d+e))^((a+b)-c*(d+e))^((a+b)-c*(d+e))+((a+b)-c*(d+e))-((a+b)-c*(d+e))+((a+b)-c*(d+e))^((a+b)-c*(d+e))+((a+b)-c*(d+e))-((a+b)-c*(d+e))^((a+b)-c*(d+e))^((a+b)-c*(d+e))+((a+b)-c*(d+e))-((a+b)-c*(d+e))
Successfully gives the output:
ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*-ab+cde+*--+^^-+^+-+^^-+^*-+^--+^+-+^.
But, the same code gives free() invalid pointer error in ideone. Why is that?

Answer 1

op_precedence(top_op.second) calls op_precedence with the number returned by earlier op_precedence call - not with the operator character.
When op_precedence is passed an argument that doesn't match one of the recognized operators, the program exhibits undefined behavior, by way of reaching the end of a non-void function without encountering a return statement.

Answer 2

So, after Igor Tandetnik kindly pointed the mistake to me, in line 58, I changed op_precedence(top_op.second) to top_op.second. I also added default case to op_precedence function to correct the warning. After compiling and running, this code did actually abort() on line 75 for simple input ((a+b)). It turns out, that my implementation of the algorithm was wrong. My code didn't take into consideration associativity of operator '('. According to the question, we don't need to take into consideration associativity for other operators but for '(' we need its precedence to be higher than all other operators when outside of the stack, and lower than all other operators when inside of the stack. This is to ensure that when any operator other than '(' is the input and the top of the stack is '(', we could push the input operator into the stack. The corrected code is below:
// ------------------- infix to postfix conversion ---------------
#include <iostream>
#include <string>
#include <stack>
#include <algorithm>
#include <utility>
using std::stack;
using std::pair;
using std::cout;
using std::cin;
using std::endl;
using std::string;
short op_out_precedence(char op) {
// return operator precedence (when outside of stack)
// in and out precedence is to take care of associativity
// Here we don't require the associativity of any other operator except '('
// input: operator; output: its precedence value
switch (op) {
case '+': return 1;
case '-': return 2;
case '*': return 3;
case '/': return 4;
case '^': return 5;
case '(': return 6;
default : abort(); // not supposed to happen
// operator can't be other than the mentioned cases
}
}
short op_in_precedence(char op) {
// return operator precedence (when inside of stack)
// in and out precedence is to take care of associativity
// Here we don't require the associativity of any other operator except '('
// input: operator; output: its precedence value
switch (op) {
case '+': return 1;
case '-': return 2;
case '*': return 3;
case '/': return 4;
case '^': return 5;
case '(': return 0;
default : abort(); // not supposed to happen
// operator can't be other than the mentioned cases
}
}
inline bool is_operator(char sym) {
// is sym an operator?
return (sym == '+' || sym == '-' || sym == '*' || sym == '/' || sym == '^' || sym == '(');
}
inline bool is_operand(char sym) {
// is sym an operand?
return (sym >= 'a' && sym <= 'z');
}
void in_to_post(string & expr) {
// infix to postfix converter
// input: infix expression
stack< pair<char, short> > op_st; // operator stack
int len = expr.length();
for (int i = 0; i < len; ++i) {
if (is_operator(expr[i])) { // operator
// pop op_stack until the
// top of the stack has less or equal precedence
// than curr operator or stack is empty
while(1) {
if (op_st.empty()) { // stack is empty; straight away push
op_st.push(std::make_pair(expr[i], op_in_precedence(expr[i])));
break;
}
pair <char, short> & top_op = op_st.top();
if (top_op.second > op_out_precedence(expr[i])) {
cout << top_op.first;
op_st.pop();
}
else {
op_st.push(std::make_pair(expr[i], op_in_precedence(expr[i])));
break;
}
}
}
else if (is_operand(expr[i])) { // operand; push it to output queue immediately
cout << expr[i];
}
else if (expr[i] == ')') { // right paranthesis
while (1) {
if (op_st.empty()) { // invalid expression; ')' reached before matching '('
cout << "No matching '(' found\n";
abort();
}
pair <char, short> & top_op = op_st.top();
if (top_op.first == '(') { // matching '(' found; stop
op_st.pop();
break;
}
else {
cout << top_op.first;
op_st.pop();
}
}
}
}
// pop out the whole op_st (if any)
while (!(op_st.empty())) {
pair <char, short> & top_op = op_st.top();
cout << top_op.first;
op_st.pop();
}
}
int main() {
int t;
cin >> t;
for (int i = 0; i < t; ++i) {
string expr;
cin >> expr;
//cout << expr.length() << endl;
in_to_post(expr);
cout << "\n";
}
return 0;
}