How to remove a pair of repeated occurrences in a list in c++? (keep elements that appear only once)

Question

I've tried using unique but unique removes the duplicate only.

What my program is suppose to do is for example the list contains 1,2,2,2,3,4,4. I am suppose to only remove the the pair of the number that is repeated and the output should be 1,2,3 (a pair of 2 and 4 is removed).

Answer 1

Iterate through data and remove pairs (Live code):

list<int> data{1, 2, 2, 2, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 6};

for (auto i = data.begin(); i != data.end();)
{
    auto n = std::next(i);

    if (n == data.end())
        break;

    if (*i == *n)
    {
        i = data.erase(i);
        i = data.erase(i);
    }
    else
        i++;
}

Output

1 2 3 4 5 6 

Also 1 2 3 for 1 2 2 2 3 4 4.

 

Above code works since C++11, if you don't have it, try this:

for (list<int>::iterator i = data.begin(); i != data.end();)
{
    list<int>::iterator n = i; 
    n++;

    if (n == data.end())
        break;

    if (*i == *n)
    {
        i = data.erase(i);
        i = data.erase(i);
    }
    else
    i++;
}

Answer 2

I would recommend using sort and unique functions to do this.

std::sort (my_vector.begin(), my_vector.end() );
std::vector<int>::iterator it;
it = std::unique (my_vector.begin(), my_vector.end() );
my_vector.resize( std::distance(my_vector.begin(),it) );

Reference: http://www.cplusplus.com/reference/algorithm/unique/ -- this has an example where you can use predicate comparison to customize the behavior of unique.

EDIT -- You may also want to look at std::adjacent_find if you are interested in removing consecutive elements.

EDIT -- If all you care is to delete consecutive elements, first sort the list, and then iterate over it. If two elements are consecutive, delete both of them using std::remove_if or something like that.