C++ STL stack pop operation giving Segment fault though I checked for empty stack

Question

I performed the pop operation on S[position] stack and it's giving segment fault but the top operation giving the result without any error and the empty check also performed before the pop operation.

Can't figure out what is causing the problem.

#include<iostream>
#include<stack>
#include<stdlib.h>
using namespace std;

int main(){
    int bar,position;
    cout<<"Enter the number of bars: ";cin>>bar;
    cout<<"Enter the position of the bar where you want to place: ";cin>>position;
    position = position - 1;
    stack<int> S[bar];
    for(int i = bar; i>0; i--){
        S[0].push(i);
    }

    S[0].pop();

    for(int i = 1; i<bar; i++){
        if(i == position){
            S[i].push(1);
            continue;
        }
        S[i].push(S[0].top());
        S[0].pop();
    }

    for(int i = 1; i< bar; i++){
        if(S[i].top() == 2){
            if(!S[position].empty()){
                cout<<S[position].top();
                S[position].pop(); //This line generating segment fault
            }
        }
    }

}

Output:

Enter the number of bars: 3
Enter the position of the bar where you want to place: 2
Segmentation fault (core dumped)

Answer 1

1.You can use one library - bits/stdc++.h. This lib contains almost everyone lib.

    #include <bits/stdc++.h>

---

2.If bars=b, stack have k elements, 0 - k-1, and you go in 1. You must go on 0, or make stack with k+1 elements.

stack<int> s[k+1] and for(long long i=1 ; i<=bar ; i++);

or

for(i=0 ; i<bars ; i++)

---

3.The real error - the one you are looking for is not on the order in which you think. The error is on line when you make check for its value. line:

if(S[i].top() == 2)

The error is here, because you are not sure are this stack empty. You check for stack[position]. If stack is empty and you get .pop() or .top() the compiler give you Segmentation fault (core dumped). In my opinion you are confused about where to write i and position, but check -!S[i].empty() must be before if(S[i] == 2). Or you just need to check the stack with index i if it's empty. After the changes, the code look like this:

    #include<bits/stdc++.h>

using namespace std;

int main(){ int bar,position;

cout<<"Enter the number of bars: ";cin>>bar;
cout<<"Enter the position of the bar where you want to place: ";cin>>position;
position = position - 1;
stack<int> S[bar];
for(int i = bar; i>0; i--){
    S[0].push(i);
}

S[0].pop();

for(int i = 0; i<bar; i++){
    if(i == position){
        S[i].push(1);
        continue;
    }
    S[i].push(S[0].top());
    S[0].pop();
}
    /*
    for(int i = 0; i < bar; i++){
            if( !S[i].empty() ){
                    if(S[i].top() == 2){
                            cout<<S[i].top();
                             S[i].pop();
                   }
            }
    }
     OR
     */
for(int i = 0; i < bar; i++){
    if( !S[i].empty() ){
            if(S[i].top() == 2){///this line gave you a segmentation fault (core dumped)
                    if(!S[position].empty()){
                            cout<<S[position].top();
                             S[position].pop(); 
                    }
            }
    }
}
    return 0;

}