Re: Help with some logic, best way to update a container.

"Doug Harrison [MVP]" <>
Sun, 08 Feb 2009 14:29:31 -0600
On Sun, 08 Feb 2009 02:41:57 -0500, Joseph M. Newcomer
<> wrote:

for(std::map<int, int>::const_iterator iter = newValues.begin(); iter != newValues.end();
   { /* add values in */
    OldValues.insert(std::pair(iter->first, iter->second));

Could be simply OldValues.insert(*iter). Also, it won't compile because
pair is a template. For those times when you actually have to construct a
pair, you could use make_pair to avoid having to specify the template
parameters, but it's better practice to declare a typedef MapT and use
MapT::value_type(x, y), as it can avoid conversions due to the type of the
key being const K and not just plain K. It also allows you to pass string
literals when (say) std::string is the key or value type.

   } /* add values in */

(or something close to this; I'm still not always able to type this stuff in straight, and
I've not got my reference material available)

Note that insert to a map will either add a new value if the value does not exist, or
replace the existing value if it already exists.

Actually, map::insert will *not* update an existing item; it either inserts
or does nothing. You can tell what happened by examining its return value,
and if you need to update, you will also have an iterator to the existing
item. However, if you need an "insert_or_update" function, the easiest way
is to use operator[].

So note that A=1 really stands for, what
you might implement in a map, as 65=1 (treating the letters as integers because you gave
your sample map as <int, int> and I don't want to change my example around too much
because it is hard to talk about 65=1)

So that takes care of everything except deleting the elements which are not in newValues.

So, I *could* do
for(std::map<int,int>::iterator iter = OldValues.begin(); iter != OldValues.end(); ++iter)
   if(newValues.find(iter->first) == newValues.end())
       { /* delete it */
      } /* delete it */

Incrementing iter is undefined after you erase it. The correct pattern is
something like:

   for (i = m.begin(); i != m.end(); )
      if (need to erase)
         m.erase(i++); // NB: Post-increment is necessary here.

For containers whose erase returns an iterator, you must use:

   for (i = m.begin(); i != m.end(); )
      if (need to erase)
         i = m.erase(i);

ISTR VC's map::erase returns an iterator, but that's non-standard.

Doug Harrison
Visual C++ MVP

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