尝试按值对地图进行排序，但出现错误

Trying to sort map by values but get errors

本文关键字：排序 错误 地图      更新时间：2023-10-16

我有这个带有地图的程序，我正在尝试按值对它们进行排序，但我遇到了错误。谁能告诉我我做错了什么。错误在 28、29、30 行。谢谢

#include <iostream>
#include <cstdlib>
#include <map>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
typedef pair<string,int> pair;
void CovidMapa()
{
typedef map <string,int> covid1304;
covid1304 cd;
cd["Kumanovo"] = 44;
cd["Skopje"] = 28;
cd["Prilep"] = 11;
cd["Bitola"] = 6;
cd["Tetovo"] = 5;
cd["Veles"] = 4;
cd["Debar"] = 2;
cd["Gostivar"] = 2;
cd["Stip"] = 1;
cd["Kavadarci"] = 1;
covid1304::iterator ir;
for(ir = cd.begin(); ir != cd.end(); ++ir)
{
cout<<"Grad: "<<ir->first<<" Zaboleni: "<<ir->second<<endl;
}
vector <pair> vec;
copy (cd.begin(); cd.end(); back_inserter<vector<pair> >(vec));
sort (vec.begin(), vec.end(), [](const pair& l, pair& r)
{
if(l.second != r.second)
return l.second<r.second;
return l.first<r.first;
});
for (auto const &pair: vec)
{
cout<<"Grad: "<<pair.first<<" Zaboleni: "<<pair.second;
}
}
int main()
{
CovidMapa();
}
||=== Build: Debug in NaprednoCovid (compiler: GNU GCC Compiler) ===|
C:UsersKentDesktopNaprednoCovidmain.cpp||In function 'void CovidMapa()':|
C:UsersKentDesktopNaprednoCovidmain.cpp|28|error: template argument 1 is invalid|
C:UsersKentDesktopNaprednoCovidmain.cpp|28|error: template argument 2 is invalid|
C:UsersKentDesktopNaprednoCovidmain.cpp|28|error: invalid type in declaration before ';' token|
C:UsersKentDesktopNaprednoCovidmain.cpp|29|error: expected ')' before ';' token|
C:UsersKentDesktopNaprednoCovidmain.cpp|29|error: template argument 1 is invalid|
C:UsersKentDesktopNaprednoCovidmain.cpp|29|error: template argument 2 is invalid|
C:UsersKentDesktopNaprednoCovidmain.cpp|29|error: no matching function for call to 'back_inserter(int&)'|
C:UsersKentDesktopNaprednoCovidmain.cpp|29|note: candidate is:|
c:program files (x86)codeblocks1mingwlibgccmingw324.8.1includec++bitsstl_iterator.h|479|note: template<class _Container> std::back_insert_iterator<_Container> std::back_inserter(_Container&)|
c:program files (x86)codeblocks1mingwlibgccmingw324.8.1includec++bitsstl_iterator.h|479|note:   template argument deduction/substitution failed:|
C:UsersKentDesktopNaprednoCovidmain.cpp|29|error: template argument 1 is invalid|
C:UsersKentDesktopNaprednoCovidmain.cpp|29|error: expected ';' before ')' token|
C:UsersKentDesktopNaprednoCovidmain.cpp|30|error: request for member 'begin' in 'vec', which is of non-class type 'int'|
C:UsersKentDesktopNaprednoCovidmain.cpp|30|error: request for member 'end' in 'vec', which is of non-class type 'int'|
C:UsersKentDesktopNaprednoCovidmain.cpp|30|error: reference to 'pair' is ambiguous|
C:UsersKentDesktopNaprednoCovidmain.cpp|8|note: candidates are: typedef struct std::pair<std::basic_string<char>, int> pair|
c:program files (x86)codeblocks1mingwlibgccmingw324.8.1includec++bitsstl_pair.h|96|note:                 template<class _T1, class _T2> struct std::pair|
C:UsersKentDesktopNaprednoCovidmain.cpp|30|error: 'pair' does not name a type|
C:UsersKentDesktopNaprednoCovidmain.cpp|35|error: ISO C++ forbids declaration of 'parameter' with no type [-fpermissive]|
C:UsersKentDesktopNaprednoCovidmain.cpp|35|error: expected '{' before ';' token|
C:UsersKentDesktopNaprednoCovidmain.cpp||In function 'void CovidMapa()':|
C:UsersKentDesktopNaprednoCovidmain.cpp|35|warning: lambda expressions only available with -std=c++11 or -std=gnu++11 [enabled by default]|
C:UsersKentDesktopNaprednoCovidmain.cpp|35|error: expected ')' before ';' token|
C:UsersKentDesktopNaprednoCovidmain.cpp|36|warning: 'auto' changes meaning in C++11; please remove it [-Wc++0x-compat]|
C:UsersKentDesktopNaprednoCovidmain.cpp|36|error: ISO C++ forbids declaration of 'pair' with no type [-fpermissive]|
C:UsersKentDesktopNaprednoCovidmain.cpp|36|error: range-based 'for' loops are not allowed in C++98 mode|
C:UsersKentDesktopNaprednoCovidmain.cpp|38|error: request for member 'first' in 'pair', which is of non-class type 'const int'|
C:UsersKentDesktopNaprednoCovidmain.cpp|38|error: request for member 'second' in 'pair', which is of non-class type 'const int'|
||=== Build failed: 20 error(s), 2 warning(s) (0 minute(s), 0 second(s)) ===|

对于初学者来说，有错别字

copy (cd.begin(); cd.end(); back_inserter<vector<pair> >(vec));
^^^       ^^^

其次，对于由于 using 指令而导致的非限定名称查找，您有两个同名的实体pair。因此，编译器会发出错误。

例如写

typedef pair<string,int> Pair;
//...
vector <Pair> vec;
copy (cd.begin(), cd.end(), back_inserter(vec));
sort (vec.begin(), vec.end(), [](const Pair& l, const Pair& r)
{
if(l.second != r.second)
return l.second<r.second;
return l.first<r.first;
});

另一种方法是保持 typedef 声明不变，但随后使用限定名::pair而不是非限定名pair，例如

vector <::pair> vec;

这是显示更新的程序。

#include <iostream>
#include <cstdlib>
#include <map>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
typedef pair<string,int> pair;
void CovidMapa()
{
typedef map <string,int> covid1304;
covid1304 cd;
cd["Kumanovo"] = 44;
cd["Skopje"] = 28;
cd["Prilep"] = 11;
cd["Bitola"] = 6;
cd["Tetovo"] = 5;
cd["Veles"] = 4;
cd["Debar"] = 2;
cd["Gostivar"] = 2;
cd["Stip"] = 1;
cd["Kavadarci"] = 1;
covid1304::iterator ir;
for(ir = cd.begin(); ir != cd.end(); ++ir)
{
cout<<"Grad: "<<ir->first<<" Zaboleni: "<<ir->second<<endl;
}
vector <::pair> vec;
copy (cd.begin(), cd.end(), back_inserter(vec));
sort (vec.begin(), vec.end(), [](const ::pair& l, const ::pair& r)
{
if(l.second != r.second)
return l.second<r.second;
return l.first<r.first;
});
cout << 'n';          
for (auto const &pair: vec)
{
cout<<"Grad: "<<pair.first<<" Zaboleni: "<<pair.second << endl;
}
}
int main()
{
CovidMapa();
}

您typedef pair但pair已经在全局命名空间中，因为using namespace std;不要做using namespace std;。

你也有;而不是,std::copy，但根本不需要使用std::copy。只需直接从迭代器创建vector：

vector <mypair> vec(cd.begin(), cd.end());

我还建议在比较 lambda 中使用std::tie：

return std::tie(l.second, l.first) < std::tie(r.second, r.first);

完整代码：

#include <algorithm>
#include <cstdlib>
#include <iostream>
#include <map>
#include <string>
#include <vector>
#include <tuple>
using namespace std; // don't do this
typedef pair<string,int> mypair; // rename your pair to not collide with std::pair
void CovidMapa()
{
typedef map <string,int> covid1304;
covid1304 cd;
cd["Kumanovo"] = 44;
cd["Skopje"] = 28;
cd["Prilep"] = 11;
cd["Bitola"] = 6;
cd["Tetovo"] = 5;
cd["Veles"] = 4;
cd["Debar"] = 2;
cd["Gostivar"] = 2;
cd["Stip"] = 1;
cd["Kavadarci"] = 1;
covid1304::iterator ir;
for(ir = cd.begin(); ir != cd.end(); ++ir)
{
cout<<"Grad: "<<ir->first<<" Zaboleni: "<<ir->second<<endl;
}
// construct vec directly from the iterators:
vector <mypair> vec(cd.begin(), cd.end());
sort (vec.begin(), vec.end(), [](const mypair& l, mypair& r)
{
// use std::tie to make the lambda easier to understand and maintain
return std::tie(l.second, l.first) < std::tie(r.second, r.first);
});
for (auto const &pair: vec)
{
cout<<"Grad: "<<pair.first<<" Zaboleni: "<<pair.second << 'n';
}
}
int main()
{
CovidMapa();
}

另一个不需要复制所有std::string和int的选项是将迭代器存储在您的vector中。由于迭代器是轻量级的，因此这种方法可能会更快一些。

vector <covid1304::iterator> vec;
vec.reserve(cd.size());
for(auto it=cd.begin(); it != cd.end(); ++it) vec.push_back(it);
sort (vec.begin(), vec.end(), [](const auto& l, const auto& r)
{
// use std::tie to make the lambda easier to understand and maintain
return std::tie(l->second, l->first) < std::tie(r->second, r->first);
});
for (auto const &pair: vec)
{
cout<<"Grad: "<<pair->first<<" Zaboleni: "<<pair->second << 'n';
}
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