How can I remove a number if it exists in the vector?

Question

Take this code:

using ColumnIndexVector = std::vector<int>;
ColumnIndexVector m_vColumnIndexesToExclude;

bool bModified = false;

for (int iColumn = 1; iColumn < m_Grid.GetColumnCount(); iColumn++)
{
    if (m_bNotUsed) // Not used
    {
        if (m_vColumnIndexesToExclude.erase(iColumn) > 0)
        {
            bModified = true;
        }
    }
}

The line if (m_vColumnIndexesToExclude.erase(iColumn) > 0) will not compile. I complains that iColumn is not a ColumnIndexVector.

How can I remove a number if it exists in the vector?

what is `mGrid` ? Please post a [mcve] and the complete error message — 463035818_is_not_an_ai, Mar 17 '22 at 08:27
@463035818_is_not_a_number That was not relevant to the issue so I simplified the code to just a `bool` variable. — Andrew Truckle, Mar 17 '22 at 08:29
@Quimby it is not "index". The integer being passed is a item value in the vector. — Andrew Truckle, Mar 17 '22 at 08:30
if it is not relevant then you can create a [mcve] without `mGrid` — 463035818_is_not_an_ai, Mar 17 '22 at 08:30
@Evg - I am trying that code in the other answer. *But* how do we determine the number of items erased? — Andrew Truckle, Mar 17 '22 at 08:34
From your code you don't need a number of items, only the fact that at least one item was erased. You can find that by checking an iterator value returned by `std::find()` or `std::remove()` against `m_vColumnIndexesToExclude.end()`. — Evg, Mar 17 '22 at 08:37
@AndrewTruckle You didn't do a [mcve]. See here: https://godbolt.org/z/cn6Tf1aoj The example isn't complete nor verifiable. — eerorika, Mar 17 '22 at 08:37
Note that if you want to erase several elements, it is generally more efficient and less error-prone to create a new vector instead of trying to modify several times the existing one. — Damien, Mar 17 '22 at 08:38

eerorika · Answer 1 · 2022-03-17T08:42:34.850

This is the error that my compiler says (after adding definitions that you forgot):

error: no matching function for call to 'std::vector<int>::erase(int&)'
   22 |         if (m_vColumnIndexesToExclude.erase(iColumn) > 0)
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
In file included from /opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/vector:67,
                 from <source>:1:
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_vector.h:1430:7: note: candidate: 'std::vector<_Tp, _Alloc>::iterator std::vector<_Tp, _Alloc>::erase(std::vector<_Tp, _Alloc>::const_iterator) [with _Tp = int; _Alloc = std::allocator<int>; std::vector<_Tp, _Alloc>::iterator = std::vector<int>::iterator; std::vector<_Tp, _Alloc>::const_iterator = std::vector<int>::const_iterator]'
 1430 |       erase(const_iterator __position)
      |       ^~~~~
/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/bits/stl_vector.h:1430:28: note:   no known conversion for argument 1 from 'int' to 'std::vector<int>::const_iterator'
 1430 |       erase(const_iterator __position)
      |             ~~~~~~~~~~~~~~~^~~~~~~~~~

As the compiler explains to you, there is no erase function that would accept an int as an argument. There is an erase function that accepts an iterator. The iterator is what you must pass to erase an element from a vector.

The typical way to erase all elements with particular value is the erase-remove idiom. With certain pre-conditions you have more efficient options available:

If the container is sorted, then you can do binary search using std::lower_bound for single element or std::equal_range for multiple.
If the container isn't sorted and you don't need to keep the order, then you can do a linear search, swap with last element, and pop_back.

However, considering the apparent use case, an unsorted vector would be an inefficient choice. std::unordered_set would likely be more efficient if there are many indices.

How can I remove a number if it exists in the vector?

1 Answers1