CODEWITHSUNDEEP
c++templatesdictionary
arjacsoh
arjacsoh

Reputation: 9262

C++ function to insert to template map

Continuing from my last question C++ template class map I have implemented the function to insert some values. This function inserts the same value for a range of keys. If the key exists in the map it should overwrite the old values. Is the function eventually correct and efficient? Could you suggest a better way to implement it? 

void insert_ToMap( K const& keyBegin, K const& keyEnd, const V& value) 
{
  if(!(keyBegin < keyEnd))
    return;

  const_iterator it;

  for(int j=keyBegin; j<keyEnd; j++)
  {
    it = my_map.find(j);

    if(it==my_map.end())
    {
      my_map.insert(pair<K,V>(j,value));
    }
    else
    { 
      my_map.erase(it);
      my_map.insert(pair<K,V>(j, value));
    }
  }
}

I try:

int main()
{
  template_map<int,int> Map1 (10);

  Map1.insert_ToMap(3,6,20);
  Map1.insert_ToMap(4,14,30);
  Map1.insert_ToMap(34,37,12);

  for (auto i = Map1.begin(); i != Map1.end(); i++)
  {
    cout<< i->first<<"   "<<i->second<<std::endl; 
  }
}

Upvotes: 2

Views: 3000

Answers (1)

Kerrek SB
Kerrek SB

Reputation: 477650

To insert whether or not the key exists:

typedef std:::map<K, V> map_type;

std::pair<typename map_type::iterator, bool> p
         = my_map.insert(std::pair<K const &, V const &>(key, new_value));

if (!p.second) p.first->second = new_value;

This construction takes advantage of the fact that insert already performs a find(), and if the insertion fails, you can immediately use the resulting iterator to overwrite the mapped value.


There is a certain hidden cost here: The insertion always makes a copy of the element, whether or not it actually succeeds. To avoid even that, we can use a slightly more verbose approach using lower_bound() to search for the purported key and simultaneously provide the correct insertion position for the new element:

typename map_type::iterator it = my_map.lower_bound(key);

if (it == my_map.end() || it->first != key)
{
  my_map.insert(it, std::pair<K const &, V const &>(key, new_value));  // O(1) !
}
else
{
  it->second = new_value;
}

The two-argument version of insert() operates in constant time if the insertion hint (the iterator in the first argument) is the correct position for the insertion, which is precisely what lower_bound() provides.

Upvotes: 5

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