C++ remove_if overwriting my vector

Question

My remove_if seems to be overwriting the elements that are not filtered out with values of filtered out elements. The purpose of these code is to allow user to filter and display only teacher from a certain category. (Not deleting any element) Here are some of the code

static string compare;
static string debug;

bool filter_Cat (Teacher &t) 
{ 
    return (t.getCat() != compare); 
}

void filterCat (vector<Teacher> &t)
{
   vector<Teacher>::iterator i;
   vector<Teacher>::iterator newedited = remove_if(t.begin(), t.end(), filter_Cat);
   for (i = t.begin(); i != newedited; ++i)
   {
     Teacher& te = *i;
     te.getName();
     cout << "\t";
     te.getCategory();
     cout << "\t";
     te.getLocation();
   }
 }

 void filterTutorCat(vector<Teacher> &t)
 {
    int choice;
    cout << "No\tCategory" << endl
         << "1\tEnglish" << endl
         << "2\tMath" << endl
         << "3\tScience" << endl
         << "Choose the category you wish to filter :";
    cin >> choice;
    getline(cin, debug);

    if(choice <= 3 && choice > 0)
    {
        if (choice == 1)
        {
          compare = "English";
          filterCat(t);
        }
        if (choice == 2)
        {
          compare = "Math";
          filterCat(t);
        }
        if (choice == 3)
        {
          compare = "Science";
          filterCat(t);
        }

    }
    else
    {
        cout << "Invalid Option" << endl;
    }
 }

Answer 1

remove_if shifts elements, for which the compare function returns false, from right to left; which in other words means, it overwrites the elements, for which compare returns true, with elements, for which compare returns false. The size of the vector doesn't change, however.

This reads,

Removes all elements satisfying specific criteria from the range [first, last). The first version removes all elements that are equal to value, the second version removes all elements for which predicate p returns true.

Removing is done by shifting the elements in the range in such a way that elements to be erased are overwritten. The elements between the old and the new ends of the range have unspecified values. Iterator to the new end of the range is returned. Relative order of the elements that remain is preserved.

So what you want to do should be expressed as:

void filterCat (vector<Teacher> &v)
{
   for (vector<Teacher>::iterator it = v.begin(); it != v.end() ; ++it)
   {
      if (!filter_Cat(*i))
      {
           std::cout << i->getName() <<"\t" << i->getCategory() << std::endl;
      }
   }
 }

It seems in your code, getName() prints the name which ideally it should not do, instead it should return name. So I would suggest you to change it to make it return name. And do the same for getCategory as well. Choose your name correctly. If it is getName(), you should get you name by returning it; if it is printName(), then it should print name.

---

Also, the code which you've written isn't good:

• You should avoid global variables.
• You should avoid if-else as much as possible. Learn better ways.
• You should learn about function objects (or functor)
• You should learn about const member function.
• You should understand the difference between iterator and const_iterator, and their usage.
• You should understand the difference between const reference, and non-const reference. And try using them appropriately.

So I would write your code as:

//this is functor, not a function
struct filter_cat
{
   std::string m_cat; //use member data, avoid global variable
   filter_cat(std::string const & cat) : m_cat(cat) {}
   bool operator()(Teacher const & t) const  //const member function
   { 
     return (t.getCat() != m_cat); //getCat should be const member function
   }
};

//pass vector by const reference
void filterCat (vector<Teacher> const & v, filter_cat filter)
{
   //use const_iterator here, instead of iterator 
   for (vector<Teacher>::const_iterator it = v.begin(); it != v.end() ; ++it)
   {
      if (!filter(*i))
      {
           //getName and getCategory should be const member function
           std::cout << i->getName() <<"\t" << i->getCategory() << std::endl;
      }
   }
}

void filterTutorCat(vector<Teacher> const &t)
{
    int choice;
    cout << "No\tCategory" << endl
         << "1\tEnglish" << endl
         << "2\tMath" << endl
         << "3\tScience" << endl
         << "Choose the category you wish to filter :";
    cin >> choice;
    getline(cin, debug);

    //avoid if-else as much as possible, learn better ways!
    std::string cats[] = {"English", "Math", "Science"};

    if(choice <= 3 && choice > 0)
    {
          filterCat(v, filter_cat(cats[choice-1]));
    }
    else
    {
        cout << "Invalid Option" << endl;
    }
}

As noted in the comments: getCat, getName and getCategory should be const member functions. In fact, if getCategory returns category, then getCat isn't even needed.

Solved my issue.

Answer 2

remove_if collects the values for which filter_Cat returns false at the start of the container. While it doesn't reduce the number of elements in the container it neither does make any guarantees about the values of the elements beyond the returned range. So you are loosing values when using remove_if.