如何在地图上找到最小值? [英] How can I find the minimum value in a map?

问题描述

我有一个map，我想在地图上找到最小值(右侧).这是我的操作方式:

I have a map and I want to find the minimum value (right-hand side) in the map. Here is how I did it:

bool compare(std::pair<std::string ,int> i, pair<std::string, int> j) {
  return i.second < j.second;
}
////////////////////////////////////////////////////
std::map<std::string, int> mymap;

mymap["key1"] = 50;
mymap["key2"] = 20;
mymap["key3"] = 100;

std::pair<char, int> min = *min_element(mymap.begin(), mymap.end(), compare); 
std::cout << "min " << min.second<< " " << std::endl;

上面的代码工作正常，我能够获得最小值.但是，当我按如下所示将此代码放入我的类中时，它似乎不起作用:

The code above works fine and I'm able to get the minimum value. However, when I put this code inside my class as follows, it doesn't seem to work:

int MyClass::getMin(std::map<std::string, int> mymap) {
  std::pair<std::string, int> min = *min_element(mymap.begin(), mymap.end(), 
                                                 (*this).compare);
                                                 // Error probably due to "this".
  return min.second; 
}

bool MyClass::compare(
    std::pair<std::string, int> i, std::pair<std::string, int> j) { 
  return i.second < j.second; 
}

如何使代码与我的班级一起工作?另外，是否有更好的解决方案，不需要编写其他compare函数?

How can I make the code work with my class? Also, is there a better solution which doesn't require writing the additional compare function?

推荐答案

您有一些选择.实现此功能的最佳"方法是使用 functor ，这可以保证是最快的调用方法:

You have a few options. The "best" way to do this is with a functor, this is guaranteed to be the fastest to call:

typedef std::pair<std::string, int> MyPairType;
struct CompareSecond
{
    bool operator()(const MyPairType& left, const MyPairType& right) const
    {
        return left.second < right.second;
    }
};



int MyClass::getMin(std::map<std::string, int> mymap) 
{
  std::pair<std::string, int> min 
      = *min_element(mymap.begin(), mymap.end(), CompareSecond());
  return min.second; 
}

(您也可以将CompareSecond类嵌套在MyClass中.

(You can also nest the CompareSecond class inside MyClass.

使用现在拥有的代码，您可以轻松地对其进行修改以使其起作用.只需使函数static并使用正确的语法即可:

With the code you have now, you can easily modify it to work, however. Just make the function static and use the correct syntax:

static bool 
MyClass::compare(std::pair<std::string, int> i, std::pair<std::string, int> j) 
{ 
  return i.second < j.second; 
}

int MyClass::getMin(std::map<std::string, int> mymap) 
{
  std::pair<std::string, int> min = *min_element(mymap.begin(), mymap.end(), 
                                                 &MyClass::compare);
  return min.second; 
}

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