Why am I getting a segmentation fault in this iterator? - c++

Question

I'm writing a program that should look through a graph and calculate the minimum number of edges that need to be deleted to leave a forest where each connected group has an even number of vertices. I know how to solve the problem, but when I try and iterate through a list I get a segmentation fault and can't figure out why.
#include <cmath>
#include <cstdio>
#include <list>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int main() {
/* Enter your code here. Read input from STDIN. Print output to STDOUT */
int N;
cin >> N;
int M;
cin >> M;
// matrix of adjacency list to hold node values
vector<list<int> > adjList(N, list<int>());
// find the number of children nodes each node has
int ui, vi;
for (int i = 0; i < M; i++) {
cin >> ui;
cin >> vi;
ui--;
vi--;
//cout << ui << " " << vi << endl;
adjList[ui].push_back(vi);
adjList[vi].push_back(ui);
//cout << "list length: " << adjList[ui].size() << endl;
}
//cout << "after for loop" << endl;
// count the number of nodes with even numbers of children
for (int i = 0; i <= M; i++) {
cout << i << "-> ";
for (list<int>::iterator it = adjList[i].begin(); it != adjList[i].end(); it++) {
cout << *it << " ";
}
cout << endl;
}
int edgesRemoved = 0;
for (int i = 0; i <= M; i++) {
for (list<int>::iterator it = adjList[i].begin(); it != adjList[i].end(); ++it) {
int j = *it;
if (adjList[j].size() % 2 == 1) {
// delete vertex from current list
cout << "test" << endl;
adjList[i].erase(it);
// delete vertex on the other list
cout << "test" << endl;
cout << j << endl;
cout << *adjList[j].begin() << endl;
for (list<int>::iterator it2 = adjList[j].begin(); it2 != adjList[j].end(); ++it2) {
cout << *it2 << " ";
if (i == *it2) {
adjList[j].erase(it2);
}
}
edgesRemoved++;
}
}
}
cout << edgesRemoved << endl;
return 0;
}
After using cout statements to debug the program I figured out that the problem is here:
for (int i = 0; i <= M; i++) {
for (list<int>::iterator it = adjList[i].begin(); it != adjList[i].end(); ++it) {
int j = *it;
if (adjList[j].size() % 2 == 1) {
// delete vertex from current list
cout << "test" << endl;
adjList[i].erase(it);
// RIGHT UNDER HERE
// vvvvvvvvvv
for (list<int>::iterator it2 = adjList[j].begin(); it2 != adjList[j].end(); ++it2) {
cout << *it2 << " ";
if (i == *it2) {
adjList[j].erase(it2);
}
}
edgesRemoved++;
}
}
}
I get a segmentation fault after the program creates an iterator that is meant to go through another list in the vector. I don't understand why though, the syntax is the same as the first for loop with another iterator going through the first list.
Here is an example of what happens after I type in the input of the digits that represent a tree, the program then prints the adjacency matrix and goes on to the actual calculation (this part works fine, it's the end result during calculation):
10 9
2 1
3 1
4 3
5 2
6 1
7 2
8 6
9 8
10 8
0-> 1 2 5
1-> 0 4 6
2-> 0 3
3-> 2
4-> 1
5-> 0 7
6-> 1
7-> 5 8 9
8-> 7
9-> 7
test
test
1
0
Segmentation fault

Answer 1

Erasing the item under the iterator invalidates the iterator; using it further results in undefined behavior, so anything could happen. The usual idiom for this sort of thing is:
std::list<int>::iterator it = adjList[i].begin();
while ( it != adjList[i].end() ) {
if ( *it == i ) {
it = adjList[j].erase( it );
} else {
++ it;
}
}
The erase function returns an iterator to the element immediately following the one which was removed.
This is valid for all sequence types, not just std::list.

Answer 2

You must not delete the current item in a list (the thing the iterator is pointing to) while you are iterating over it. You could do something like this:
adjList[j].erase(it2++);
However, afaik, it is considered best practice to neither shrink nor expand a list while iterating.