infix to postfix program not producing the correct output - c++

Question

#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <iomanip>
using namespace std;
class Stack {
public:
string stack[100][1];
int size;
Stack() { size = 0; }
~Stack() {};
void push(string data) {
stack[size][0] = data;
size += 1;
}
bool empty() {
if (size == 0)
{
return true;
}
else
{
return false;
}
}
string pop() {
size -= 1;
return stack[size][0];
}
string top() {
if (empty())
cout << "Stack is empty";
return stack[size - 1][0];
}
};
int pre(char op)
{
switch (op)
{
case '(': case ')': return 0;
case '+': case '-': return 1;
case '*': case '/': return 2;
}
return -1;
}
vector<string> infix_to_postfix(const vector<string>& expr) {
vector<string> postfix;
string op;
Stack st;
for (int i = 0; i < expr.size(); i++) {
if (expr[i][0] == '+' || expr[i][0] == '-' || expr[i][0] == '*' || expr[i][0] == '/') {
while (!st.empty()) {
op = (st.top()[0]);
if (pre(expr[i][0]) <= pre(op[0]))
postfix.push_back(st.pop());
else break;
}
postfix.push_back(expr[i]);
}
else if (expr[i][0] == '(') {
st.push(expr[i]);
}
else if (expr[i][0] == ')') {
while (!st.empty()) {
op = st.pop();
if (op[0] == '(') break;
else {
postfix.push_back(op);
}
}
}
else {
postfix.push_back(expr[i]);
}
}
return postfix;
}
int main() {
auto expr = infix_to_postfix({ "(", "2", "+", "3", ")", "*", "7" });
for (auto& elem : expr)
{
std::cout << elem << ", ";
}
std::cout << "\n";
return 0;
}
This program is supposed to convert from infix to postfix but it is not producing the correct output. I'm expecting the output to be:
2, 3, +, 7, *,
but the output is
2, +, 3, *, 7,

Answer 1

You have two issues.
When handling an operator, after popping the lower precedence operators you need to push the new operator onto the stack rather than adding it to the result:
if (expr[i][0] == '+' || expr[i][0] == '-' || expr[i][0] == '*' || expr[i][0] == '/') {
while (!st.empty()) {
op = (st.top()[0]);
if (pre(expr[i][0]) <= pre(op[0]))
postfix.push_back(st.pop());
else break;
}
st.push(expr[i]);
}
At the end of the function you need to pop any remaining operators off the stack:
while (!st.empty())
{
postfix.push_back(st.pop());
}
return postfix;