I have a vector of items (data).  I have to do a pairwise
comparison of each item to each other item and apply logic to see if
one should be deleted.  In the past I have done this using for
statements, like:

for (int i = 0; i < input_vector.size(); i++)
     for (int j = i; i < input_vector.size(); j++)
          {
               . . . process data . . .
          }

The problem is that I have do delete items in the middle, which makes
index-watching tricky.  So, in the past, I have copied the vector
first and manipulated the copy vice the original when I process the
data.  However, then I have to keep track of which items in the
original vector have already been merged away.  For example, I compare
items 1 and 2, and I decide to delete 2.  So, then after I finish all
the comparisons with item 1, I need to do that with item 3 (not 2,
which I`ve deleted in the copied vector.

     In addition to being complicated in the past, this has been slow.

     Is there a better approach?   I come across the same scenario
frequently.

Or write a proper functor and use 'remove_if'.  You can rely on the
requirement that elements of a vector exist in an array...

And if you post a bit more of the code that illustrates your problem
we might point out inefficiencies further (if you want).