Why is there segment fault line 46? - c++

Question

evaluation of infix expression. there is segmentation fault in line 40.
#include <iostream>
#include <stack>
#include <string>
using namespace std;
int pre(char);
int operation(int, int, char);
int fun(string);
int main() {
string s;
cin >> s;
int res = fun(s);
cout << res;
return 0;
}
// function to get result
int fun(string s) {
stack<int> s1;
stack<char> s2;
for (int i = 0; i < s.length(); i++) {
if (isdigit(s[i])) {
int val = 0;
while (i < s.length() && isdigit(s[i])) {
val = val * 10 + (s[i] - '0');
i++;
}
s1.push(val);
} else if (s[i] == '(')
s2.push(s[i]);
else if (s[i] == ')') {
while (!s2.empty() && s2.top() != '(') {
char oprt = s2.top();
s2.pop();
int v2 = s1.top();
s1.pop();
int v1 = s1.top();
s1.pop();
int r = operation(v1, v2, oprt);
s1.push(r);
}
if (!s2.empty())
s2.pop();
} else if (s[i] == '+' || s[i] == '-' || s[i] == '*' || s[i] == '/') {
while (s2.size() > 0 && pre(s[i]) <= pre(s2.top())) {
char oprt = s2.top();
int v2 = s1.top();
s1.pop();
int v1 = s1.top();
s1.pop();
int r = operation(v1, v2, oprt);
s1.push(r);
}
s2.push(s[i]);
}
}
while (!s2.empty()) {
int v2 = s1.top();
s1.pop();
int v1 = s1.top();
s1.pop();
char oprt = s2.top();
s2.pop();
int r = operation(v1, v2, oprt);
s1.push(r);
}
return s1.top();
}
int operation(int v1, int v2, char oprt) {
int r;
switch (oprt) {
case '+':
r = v1 + v2;
break;
case '-':
r = v1 - v2;
break;
case '/':
r = v1 / v2;
break;
case '*':
r = v1 * v2;
break;
}
return r;
}
int pre(char c) {
if (c == '+' || c == '-')
return 1;
else
return 2;
}

Answer 1

Look at this code:
for (int i = 0; i < s.length(); i++) {
if (isdigit(s[i])) {
int val = 0;
while (i < s.length() && isdigit(s[i])) {
val = val * 10 + (s[i] - '0');
i++;
}
s1.push(val);
}
You use the same variable in both loops. As the result, you would miss some characters. For example: "123()". In this example you would encounter the first digit at the position 0 and start the inner loop. The inner loop would stop at the position 3 where the symbol '(' is located. Then you finish the iteration and the new iteration starts... after the i++ increment of the outer loop. As the result you would miss the symbol.
Sooner or later you would get into this loop, where you check the stack for emptiness one time but pop three times:
while (!s2.empty() && s2.top() != '(') {
char oprt = s2.top();
s2.pop();
int v2 = s1.top();
s1.pop();
int v1 = s1.top();
s1.pop();
int r = operation(v1, v2, oprt);
s1.push(r);
}
Should be no surprise that on some inputs you would get the segmentation fault.