Vector Erase Error

Question

I have the following code in C++:

#include <iostream>
#include <cstdlib>
#include <ctime>
#include <vector>
int main ()
{
    srand(time(0));
    int noOfElements = 9;
    for (int a = 0; a < 9; a++)
    {
        std::vector<int> poss;
        for (int a = 1; a <= 9; a++)
            poss.push_back(a);
        for (int b = 0; b < 9; b++)
        {
            int random = rand() % 9;
            std::cout << poss[random];
            poss.erase(random);
            noOfElements--;
        }
        std::cout << "\n";
    }
}

Yet when I run it, it returns this:

error: no matching function for call to 'std::vector<int>::erase(int&)'

for line 13.

Why is this and how can I correct it?

Answer 1

You cannot erase values from a vector directly (vectors are sequence containers, not associative containers): you need to provide an iterator to the element that you want to be erased.

In order to get an iterator, you may either:

• find the element based on its value (e.g. by using std::find()) and then provide the returned iterator in input to the erase() member function, or
• get it by applying an offset to the iterator which points to the beginning of your vector (i.e. the object returned by the begin() member function).

In the first case:

#include <vector>
#include <algorithm>

int main()
{
    std::vector<int> v { 1, 2, 3};
    auto i = std::find(begin(v), end(v), 2);
    v.erase(i);
}

The above code uses some C++11 features. In C++03, it would look as follows:

#include <vector>
#include <algorithm>

int main()
{
    std::vector<int> v;

    v.push_back(1);
    v.push_back(2);
    v.push_back(3);

    std::vector<int>::iterator i = std::find(v.begin(), v.end(), 2);
    v.erase(i);
}

In the second case, if you know the index of your element inside the vector (say, pos), then you can easily get an iterator this way:

v.begin() + pos

Alternatively (C++11 only) you could do:

next(begin(v), pos);

Answer 2

you have to pass an iterator to erase. So try

poss.erase(poss.begin() + random);