Re: index algorithm for list

From:
Thomas Maeder <maeder@glue.ch>
Newsgroups:
comp.lang.c++.moderated
Date:
21 Dec 2006 14:21:26 -0500
Message-ID:
<m2d56dw12v.fsf@glue.ch>
"Olivier Langlois" <olanglois@sympatico.ca> writes:

Here is what I would like to express by using the STL algorithms but it
is not clear for me how to proceed effectively:

size_t index(list<int> &lst, int val)


[It looks like lst should be a reference to const. I also think that
lst is a bad variable name, because it looks a lot like 1st and
doesn't buy you anything as an abbreviation.]

{
   size_t idx = 0;
   list<int>::iterator it = lst.begin();
   while( it != lst.end() )
   {
     if(*it == val) return idx;
     idx++;
     ++it;
   }
   return idx; // if idx == lst.size() means that the val is not found.
}

If predicates could contain state, I guess that I could use find_if()
and store the current index in the predicate object but this is
something not allowed.


But the predicate can have a reference to an object holding the state.

The real problem is that the Standard doesn't give a guarantee that
find() calls the predicate exactly index(list,val) times; the only
guarantee you have is that the predicate isn't called more often than
lst.size() times.

The other option could be:

distance(lst.begin(),find(lst.begin(),lst.end(),val)

but since list iterators are not random access iterators, it seems
that the distance option would be less efficient than the hand
written loop. So what would be the best way?


Start with the distance() option. Once you have determined that its
lack of efficiency is a problem, resort to a more complex, but more
efficient solution.

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