Problem with erasing elements from the set - c++

Question

I am having troubles with erasing elements from sets. I get BUILD FAILED from:
n2Ar.erase(it);
n3Ar.erase(it);
where it is a pointer received from find() function: e.g. it = n2Ar.find(*i);
The whole listing of the program:
#include <stdio.h>
#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
using namespace std;
#define TESTING_FILE_IN
//#define TESTING_FILE_OUT
//#define DEBUG
//#define SHOW_TIMING
int outputSet(int i) {
cout << i << endl;
}
/*
*
*/
int main() {
int n1, n2, n3;
set<int> list, n1Ar, n2Ar, n3Ar;
set<int>::iterator it;
scanf("%d", &n1);
scanf("%d", &n2);
scanf("%d", &n3);
int val = 0;
// Getting lists of voters
for (unsigned i = 0; i < n1; i++) {
cin >> val;
n1Ar.insert(val);
}
for (unsigned i = 0; i < n2; i++) {
cin >> val;
n2Ar.insert(val);
}
for (unsigned i = 0; i < n3; i++) {
cin >> val;
n3Ar.insert(val);
}
// Processing lists
for (set<int>::iterator i = n1Ar.begin(); i != n1Ar.end(); ++i) {
it = n2Ar.find(*i);
if (it != n2Ar.end()) {
list.insert(*i);
n1Ar.erase(i);
n2Ar.erase(it);
} else {
it = n3Ar.find(*i);
if (it != n3Ar.end()) {
list.insert(*i);
n1Ar.erase(i);
n3Ar.erase(it);
}
}
}
// Outputting the final list
cout << list.size() << endl;
for_each(list.begin(), list.end(), outputSet);
return 0;
}
I hope you'll be able to help me understand what I am doing wrong in here. I am only starting with C++.

Answer 1

There are two problems in your code.
First, you need return a value in the following function, or simply make it return void.
// you should return a value here or make it return void
int outputSet(int i)
{
cout << i << endl;
}
Second, the iterators in the following iterations of your for-loop are invalidated once you remove the current one. Once an element is removed, its iterator i is also invalidated, so as to the following iterators based on ++i;
And you'll get run-time error because iterator i now points to You need somehow "reset" it.
MSVC Implementation
for (set<int>::iterator i = n1Ar.begin(); i != n1Ar.end(); ++i) {
it = n2Ar.find(*i);
if (it != n2Ar.end()) {
list.insert(*i);
// the following iterators become invalidated after the
// current one is removed. You need reset it like
// i = n1Ar.erase(i);
n1Ar.erase(i);
n2Ar.erase(it);
} else {
it = n3Ar.find(*i);
if (it != n3Ar.end()) {
list.insert(*i);
// the following iterators become invalidated after the
// current one is removed. You need reset it like
// i = n1Ar.erase(i);
n1Ar.erase(i);
n3Ar.erase(it);
}
}
}
Edit: Note that returning a new iterator from set::erase() is not a Standard way. That's mainly for the purpose of performance.
A More Portable Solution
The basic idea is to correctly set the next iterator before removing the current one.
set<int>::iterator i = n1Ar.begin();
while (i != n1Ar.end())
{
it = n2Ar.find(*i);
if (it != n2Ar.end())
{
// the trick is to use "i++" where i is incremented by one while "old" i
// is removed.
list.insert(*i);
n1Ar.erase(i++);
n2Ar.erase(it);
}
else
{
it = n3Ar.find(*i);
if (it != n3Ar.end())
{
list.insert(*i);
n1Ar.erase(i++);
n3Ar.erase(it);
}
else
{
++i;
}
}
}

Answer 2

n1Ar.erase(i);
The std::set::erase function invalidates the iterator i and causes the problem. Consider change to the following:
for (set<int>::iterator i = n1Ar.begin(); i != n1Ar.end(); ++i) {
it = n2Ar.find(*i);
if (it != n2Ar.end()) {
list.insert(*i);
i = n1Ar.erase(i);
if(i == n1Ar.cend())
break;
n2Ar.erase(it);
} else {
The if(i == n1Ar.cend()) break; check helps to ensure the invalidated iterator will not ruin the loop.

Answer 3

Erase method invalidates the iterator i.
Erase method does not return iterator.
EDIT: repeating Eric idea you can use the following code:
for (set<int>::iterator i = n1Ar.begin(); i != n1Ar.end(); )
if ( n2Ar.erase(*i) || n3Ar.erase(*i) ) {
list.insert(*i);
n1Ar.erase(i++);
} else i++;
Also this problem could be solved using standard algorithms. But this solution seems to be less efficient:
set<int> tmp;
std::set_union( n2Ar.begin(), n2Ar.end(),
n3Ar.begin(), n3Ar.end(), std::inserter(tmp,tmp.begin()) );
std::set_intersection( n1Ar.begin(), n1Ar.end(),
tmp.begin(), tmp.end(), std::inserter(list,list.begin()) );
Finally i suggest to use stl for your output (you have to include iterator library):
std::copy( list.begin(), list.end(), std::ostream_iterator<int>(std::cout,"\n"));