Re: Can I dynamically add new elements to vector while looping it?

From:

Alan Johnson <alanwj@no.spam.stanford.edu>

Newsgroups:

comp.lang.c++

Date:

Mon, 12 Jun 2006 23:25:34 -0700

Message-ID:

<e6llou$6q4$1@news.Stanford.EDU>

linq936@hotmail.com wrote:

Hi,
  The following is a psudo code to describe my question:

  vector<int> ints;
  vector<int>::iterator itr;
  while (itr != ints.end()){
     int j = some_function(*i);
     if (j>0){
        ints.push_back(j);
     }
     ints.erase(itr);
  }

  Can it work? I dynamically add new element into a vector while I am
looping the vector.

In general the answer is no. If insert causes reallocation to happen
(because it causes size() to exceed capacity()), then your iterator itr
will become invalid.

In your specific case, it looks as if you always decrease the size by 1
during a loop iteration, and sometimes increase the size by 1 (for a net
change of 0). I suggest you move the erase call ahead of the push_back,
in which case you can be certain that the size never increases during an
iteration, which will make the reallocation behavior predictable. That is:

   vector<int> ints;
   vector<int>::iterator itr;
   while (itr != ints.end()){
      int j = some_function(*i);
      ints.erase(itr);
      if (j>0){
         ints.push_back(j);
      }
   }

Note that in your version, during the first iteration of the loop the
size of the vector may increase, which may cause reallocation, which may
invalidate your iterator.

--
Alan Johnson