Erasing last item of a vector with iterator? 1

[Sen7inel]

Hi,

I guess this is pretty basic, but I can't seem to sort this out right now: I'm going thru a vector with an iterator, and if get a match, want to erase the item. Something like this:

for (vector<int>::iterator i = v.begin(); i != v.end(); ++i)
  if ((*i) == 0)
    v.erase(i);

Now, if the last item is erased, the iteration goes thru one more time, with the size of v being 0, and me getting an access violation. i is not end(). How is this supposed to be done without having to check size() or empty()?

 

[chipperMDW]

For your specific example, try:

remove( v.begin(), v.end(), 0 );

If you want to test for a more complex condition than equality, you can create a function object and use it in [tt]remove_if[/tt].

Both [tt]remove[/tt] and [tt]remove_if[/tt] are in the [tt]<algorithm>[/tt] header, and in the [tt]std[/tt] namespace.

 

[xwb]

What a lot of people do not realize is that erase returns the next iterator. If you want a loop, your loop should look like

for (vector<int>::iterator i = v.begin(); i != v.end();)
  if ((*i) == 0)
    i = v.erase(i);
  else
     ++i;

 

[cpjust]

iterator erase(iterator it);
iterator erase(iterator first, iterator last);

The first member function removes the element of the controlled sequence pointed to by it. The second member function removes the elements of the controlled sequence in the range [first, last). Both return an iterator that designates the first element remaining beyond any elements removed, or end() if no such element exists.

xwb: I don't see why the original form won't work if the last iterator that erase() returns is end()?

 

[uolj]

The original code might "work", but really it is undefined behavior because even if the element that is erased is the last element in the vector, the for loop still increments the iterator after the call to erase and before it checks it against v.end(). The iterator was invalidated after the call to erase, and so incrementing it is undefined.

 

[xwb]

After the ++i, i = v.end () + 1. You will never reach the condition i != v.end ().

 

[cpjust]

Oh crap, you're right! Thanks.