C++ 帮助在地图中找到最大值
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/9370945/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
C++ Help finding the max value in a map
提问by Sh0gun
I've been doing a basic program to find the max, min, median, variance, mode etc. of a vector. Everything went fine until I got to the mode.
我一直在做一个基本程序来查找向量的最大值、最小值、中值、方差、众数等。一切都很顺利，直到我进入模式。
The way I see it, I should be able to loop through the vector, and for each number that occurs I increment a key on the map. Finding the key with the highest value would then be the one that occured the most. Comparing to other keys would tell me if it's a single multiple or no mode answer.
在我看来，我应该能够遍历向量，并且对于出现的每个数字，我都会在地图上增加一个键。找到具有最高值的键将是出现最多的键。与其他键相比，它会告诉我它是单倍数还是无模式答案。
Here's the chunk of code that's been causing me so much trouble.
这是给我带来很多麻烦的代码块。
map<int,unsigned> frequencyCount;
// This is my attempt to increment the values
// of the map everytime one of the same numebers
for(size_t i = 0; i < v.size(); ++i)
frequencyCount[v[i]]++;
unsigned currentMax = 0;
unsigned checked = 0;
unsigned maax = 0;
for(auto it = frequencyCount.cbegin(); it != frequencyCount.cend(); ++it )
//checked = it->second;
if (it ->second > currentMax)
{
maax = it->first;
}
//if(it ->second > currentMax){
//v = it->first
cout << " The highest value within the map is: " << maax << endl;
The entire program can be seen here. http://pastebin.com/MzPENmHp
整个程序可以在这里看到。http://pastebin.com/MzPENmHp
采纳答案by YXD
You never changed currentMaxin your code.
您从未更改currentMax过您的代码。
map<int,unsigned> frequencyCount;
for(size_t i = 0; i < v.size(); ++i)
frequencyCount[v[i]]++;
unsigned currentMax = 0;
unsigned arg_max = 0;
for(auto it = frequencyCount.cbegin(); it != frequencyCount.cend(); ++it ) }
if (it ->second > currentMax) {
arg_max = it->first;
currentMax = it->second;
}
}
cout << "Value " << arg_max << " occurs " << currentMax << " times " << endl;
Another way to find the mode is to sort the vector and loop through it once, keeping track of the indices where the values change.
找到模式的另一种方法是对向量进行排序并循环遍历一次，跟踪值发生变化的索引。
回答by Rob?
You can use std::max_elementto find the highest map value (the following code requires C++11):
可以使用std::max_element查找最高映射值（以下代码需要C++11）：
std::map<int, size_t> frequencyCount;
using pair_type = decltype(frequencyCount)::value_type;
for (auto i : v)
frequencyCount[i]++;
auto pr = std::max_element
(
std::begin(frequencyCount), std::end(frequencyCount),
[] (const pair_type & p1, const pair_type & p2) {
return p1.second < p2.second;
}
);
std::cout << "A mode of the vector: " << pr->first << '\n';
回答by daknowles
Here's a templated function based on Rob's excellent answer above.
这是基于 Rob 上面的出色答案的模板化函数。
template<typename KeyType, typename ValueType>
std::pair<KeyType,ValueType> get_max( const std::map<KeyType,ValueType>& x ) {
using pairtype=std::pair<KeyType,ValueType>;
return *std::max_element(x.begin(), x.end(), [] (const pairtype & p1, const pairtype & p2) {
return p1.second < p2.second;
});
}
Example:
例子：
std::map<char,int> x = { { 'a',1 },{ 'b',2 },{'c',0}};
auto max=get_max(x);
std::cout << max.first << "=>" << max.second << std::endl;
Outputs: b=>2
输出：b=>2
回答by cosurgi
You guys write too much. This can be done in few lines, here's a full working snippet:
你们写得太多了。这可以在几行中完成，这是一个完整的工作片段：
#include <iostream>
#include <algorithm>
#include <map>
int main() {
std::map<char,int> x = { { 'a',1 },{ 'b',2 },{'c',0} };
std::map<char,int>::iterator best
= std::max_element(x.begin(),x.end(),[] (const std::pair<char,int>& a, const std::pair<char,int>& b)->bool{ return a.second < b.second; } );
std::cout << best->first << " , " << best->second << "\n";
}
回答by mav_2k
We may reuse key or, value comparator objects as per requirements in place of comparator api, while fetching min/max/ranges over any STL iterator.
我们可以根据要求重用键或值比较器对象来代替比较器 api，同时通过任何 STL 迭代器获取最小/最大/范围。
http://www.cplusplus.com/reference/map/multimap/key_comp/http://www.cplusplus.com/reference/map/multimap/value_comp/
http://www.cplusplus.com/reference/map/multimap/key_comp/ http://www.cplusplus.com/reference/map/multimap/value_comp/
==
==
Example:
例子：
// multimap::key_comp
#include <iostream>
#include <map>
int main ()
{
std::multimap<char,int> mymultimap;
std::multimap<char,int>::key_compare mycomp = mymultimap.key_comp();
mymultimap.insert (std::make_pair('a',100));
mymultimap.insert (std::make_pair('b',200));
mymultimap.insert (std::make_pair('b',211));
mymultimap.insert (std::make_pair('c',300));
std::cout << "mymultimap contains:\n";
char highest = mymultimap.rbegin()->first; // key value of last element
std::multimap<char,int>::iterator it = mymultimap.begin();
do {
std::cout << (*it).first << " => " << (*it).second << '\n';
} while ( mycomp((*it++).first, highest) );
std::cout << '\n';
return 0;
}
Output:
mymultimap contains:
a => 100
b => 200
b => 211
c => 300
==
==
回答by CapelliC
you are almost there: simply add currentMax = it->second;after maax = it->first;
你就快到了：只需在currentMax = it->second;后面添加maax = it->first;
but using a map to locate the max is overkill: simply scan the vector and store the index where you find higher numbers: very similar to what you already wrote, just simpler.
但是使用地图来定位最大值是多余的：只需扫描向量并将索引存储在您找到更高数字的位置：与您已经编写的非常相似，只是更简单。
回答by dr_g
As someone accustomed to using boost libraries, an alternative to using the anonymous function proposed by Rob is the following implementation of std::max_element:
作为习惯使用 boost 库的人，使用 Rob 提出的匿名函数的替代方法是 std::max_element 的以下实现：
std::map< int, unsigned >::const_iterator found =
std::max_element( map.begin(), map.end(),
( boost::bind(&std::map< int, unsigned >::value_type::second, _1) <
boost::bind(&std::map< int, unsigned >::value_type::second, _2 ) ) );
回答by rashedcs
We can easily do this by using max_element() function.
我们可以通过使用 max_element() 函数轻松地做到这一点。
Code Snippet :
代码片段：
#include <bits/stdc++.h>
using namespace std;
bool compare(const pair<int, int>&a, const pair<int, int>&b)
{
return a.second<b.second;
}
int main(int argc, char const *argv[])
{
int n, key, maxn;
map<int,int> mp;
cin>>n;
for (int i=0; i<n; i++)
{
cin>>key;
mp[key]++;
}
maxn = max_element(mp.begin(), mp.end(), compare)->second;
cout<<maxn<<endl;
return 0;
}
回答by shed
Beter use inner comparator map::value_comp().
最好使用内部比较器 map::value_comp()。
For example:
例如：
#include <algorithm>
...
auto max = std::max_element(freq.begin(), freq.end(), freq.value_comp());
std::cout << max->first << "=>" << max->second << std::endl
will output:
将输出：
Key => Value
