C ++函数插入模板地图 [英] C++ function to insert to template map

问题描述

继续我上一个问题 C ++模板类映射我已经实现了这个功能来插入一些值。此功能为一系列键插入相同的值。如果地图中存在密钥，则应覆盖旧值。功能最终是否正确有效？你能建议一个更好的方法来实现吗？

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));
    }
  }
}

我尝试：

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; 
  }
}

推荐答案

插入键是否存在：

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;

这个结构充分利用了 insert 已执行 find（），如果插入失败，您可以立即使用生成的迭代器覆盖映射值。

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.

这里有一定的隐藏成本：插入总是使元素的副本，无论其实际是否成功。为了避免这种情况，我们可以使用 lower_bound（）稍微更详细的方法来搜索所声明的密钥，同时为新元素提供正确的插入位置： p>

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;
}

双参数版本的 insert（）如果插入提示（第一个参数中的迭代器）是插入的正确位置，这是正确的，这就是 lower_bound（）提供。

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.

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