C++ STL stack pop operation giving segmentation fault


I implemented a programming question from <a href="http://www.geeksforgeeks.org/next-greater-element/" rel="nofollow">this link</a> in C++ but I am getting a segmentation fault in the pop() operation with my code. I am fairly new to C++ and can not seem to find the error myself.

#include<iostream> #include<stack> using namespace std; void printNge(int *arr); int main() { int arr[] = {1,4,2,6,3,8,7,2,6}; printNge(arr); return 0; } void printNge(int *arr) { stack<int> st; st.push(arr[0]); for(int i=1; i<9;i++) { while((st.top() < arr[i]) && (!st.empty())) { cout << "Element is:" << st.top() << " NGE is:" << arr[i] << endl; cout << "Removing element: " << st.top() << endl; st.pop(); } cout << "Pushing element: " << arr[i] << endl; st.push(arr[i]); } while(!st.empty()) { cout << "Element is:" << st.top() << " NGE is:" << -1 << endl; st.pop(); } }

Thanks for the help.


This line

while((st.top() < arr[i]) && (!st.empty())) {

is what is causing the segfault. You have to check the stack for being empty before you try to access top, as caling top on empty stack invokes UB.


Calling pop_back on an empty container is undefined. std::stack uses std::deque by default, see <a href="http://en.cppreference.com/w/cpp/container/deque/pop_back" rel="nofollow">std::deque::pop_back</a>:


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