std::map insert or std::map find?

typedef map<int, int> MapType;    // Your map type may vary, just change the typedef

MapType mymap;
// Add elements to map here
int k = 4;   // assume we're searching for keys equal to 4
int v = 0;   // assume we want the value 0 associated with the key of 4

MapType::iterator lb = mymap.lower_bound(k);

if(lb != mymap.end() && !(mymap.key_comp()(k, lb->first)))
{
    // key already exists
    // update lb->second if you care to
}
else
{
    // the key does not exist in the map
    // add it to the map
    mymap.insert(lb, MapType::value_type(k, v));    // Use lb as a hint to insert,
                                                    // so it can avoid another lookup
}

The answer to this question also depends on how expensive it is to create the value type you're storing in the map:

typedef std::map <int, int> MapOfInts;
typedef std::pair <MapOfInts::iterator, bool> IResult;

void foo (MapOfInts & m, int k, int v) {
  IResult ir = m.insert (std::make_pair (k, v));
  if (ir.second) {
    // insertion took place (ie. new entry)
  }
  else if ( replaceEntry ( ir.first->first ) ) {
    ir.first->second = v;
  }
}

For a value type such as an int, the above will more efficient than a find followed by an insert (in the absence of compiler optimizations). As stated above, this is because the search through the map only takes place once.

However, the call to insert requires that you already have the new "value" constructed:

class LargeDataType { /* ... */ };
typedef std::map <int, LargeDataType> MapOfLargeDataType;
typedef std::pair <MapOfLargeDataType::iterator, bool> IResult;

void foo (MapOfLargeDataType & m, int k) {

  // This call is more expensive than a find through the map:
  LargeDataType const & v = VeryExpensiveCall ( /* ... */ );

  IResult ir = m.insert (std::make_pair (k, v));
  if (ir.second) {
    // insertion took place (ie. new entry)
  }
  else if ( replaceEntry ( ir.first->first ) ) {
    ir.first->second = v;
  }
}

In order to call 'insert' we are paying for the expensive call to construct our value type - and from what you said in the question you won't use this new value 20% of the time. In the above case, if changing the map value type is not an option then it is more efficient to first perform the 'find' to check if we need to construct the element.

Alternatively, the value type of the map can be changed to store handles to the data using your favourite smart pointer type. The call to insert uses a null pointer (very cheap to construct) and only if necessary is the new data type constructed.

There will be barely any difference in speed between the 2, find will return an iterator, insert does the same and will search the map anyway to determine if the entry already exists.

So.. its down to personal preference. I always try insert and then update if necessary, but some people don't like handling the pair that is returned.