Consecutive removing of vector elements - c++

Question

I need to remove element from vector a and b, if these elements are equal. As I understand, when I remove an element, other elements move. So, a[1] become a[0], a[2] -- a[1] and so on. But my code doesn`t work properly:
#include <iostream>
#include <vector>
using namespace std;
bool remove(vector<int>& a, vector<int>& b){
for (size_t i = 0; i < a.size(); ++i )
{
while ( true )
{
std::vector<int>::iterator it;
it = std::find(b.begin(), b.end(), a[i]);
if ( it != b.end() )
{
i = 0; //because of moving elements
b.erase(b.begin()+i);
a.erase(a.begin()+i);
break;
}
}
}
return true;
}
int main(){
vector<int> a;
vector<int> b;
a.push_back(1);
a.push_back(2);
a.push_back(3);
b.push_back(1);
b.push_back(2);
b.push_back(3);
remove(a,b);
return 0;
}
The problem is - the last equal elements stay, so I can't remove them. How can I fix this, taking into consideration that unequal elements can be in the middle of the vector: a = 1, 2, 3; b = 1, 3, 3 -- 2 from vector a and 3 from vector b shouldn't be deleted?

Answer 1

As I understand, when I remove an element, other elements move.
That's partially true. If you remove an element, the elements after it get pushed down to fill the hole. If you remove the 5th element, the element that was the 6th element now becomes the 5th. But the element that was the 4th still stays the 4th. And this is certainly true only after you erase. So your code is:
i = // the element from a
it = // the iterator pointing to an element from b
i = 0;
b.erase(b.begin()+i); // erase the 1st element from b
a.erase(a.begin()+i); // erase the 1st element from a
You do all this work to find a pair of equal elements, and then immediately discard it - erasing the front element of both vectors. That's why it doesn't work. You want to erase the elements that you found:
b.erase(it);
a.erase(a.begin() + i); // no i = 0
Note that if you want to erase every element that has a match in the other array, this fix won't cut it - if you have two 1s in a but ony one in b you'd never remove the second one. For this, we need to take advantage of the Erase-remove idiom:
for (std::vector<int>::iterator itA = a.begin(); itA != a.end(); /* nothing */)
{
std::vector<int>::iterator itB = std::find(b.begin(), b.end(), *itA);
if (itB == b.end()) {
// done with this element, move on
++itA;
}
else {
// erase EVERYTHING
b.erase(std::remove(itB, b.end(), *itA), b.end());
itA = a.erase(std::remove(itA, a.end(), *itA), a.end());
}
}