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I am trying to use a map data type structure for multiset. This is my code below
#include <iostream>
#include <set>
#include <iterator>
#include <map>
#include <functional>
using namespace std;
int main()
{
map<string, int> student_marks_map;
student_marks_map["Student1"] = 100;
student_marks_map["Student2"] = 20;
student_marks_map["Student3"] = 230;
multiset<int, greater<int>> multiset1 = {1,2,3,2,10};
multiset<int, greater<int>>::iterator itr;
//multiset1.insert(1);
for(itr = multiset1.begin(); itr != multiset1.end(); itr++)
{
cout<<*itr<<endl;
}
multiset<map<int, int>> multiset_map;
multiset_map.insert(1,4);
/*for(const auto& iter: multiset1)
{
cout<< iter <<endl;
}*/
return 0;
}
I know there is a multimap type of data structure but I want to implement it in multiset. It is showing compiling errors when I try to insert an element.
What is the reason for this? I tried searching on the net but there does not seem to be anyone who has done this kind of thing
The Errors:
In file included from /usr/include/c++/7/set:60:0,
from main.cpp:2:
/usr/include/c++/7/bits/stl_tree.h: In instantiation of ‘void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_equal(_II, _II) [with _InputIterator = int; _Key = std::map<int, int>; _Val = std::map<int, int>; _KeyOfValue = std::_Identity<std::map<int, int> >; _Compare = std::less<std::map<int, int> >; _Alloc = std::allocator<std::map<int, int> >]’: /usr/include/c++/7/bits/stl_multiset.h:542:4: required from ‘void std::multiset<_Key, _Compare, _Alloc>::insert(_InputIterator, _InputIterator) [with _InputIterator = int; _Key = std::map<int, int>; _Compare = std::less<std::map<int, int> >; _Alloc = std::allocator<std::map<int, int> >]’ <span class="error_line" onclick="ide.gotoLine('main.cpp',26)">main.cpp:26:28</span>: required from here /usr/include/c++/7/bits/stl_tree.h:2464:28: error: invalid type argument of unary ‘*’ (have ‘int’) _M_insert_equal_(end(), *__first, __an);
None of the two parameter insert members of multiset construct a value from the parameters, and even if they did, 1, 4 can't initialise a map<int, int>.
If you want a map with the single element 1, 4, you need to specify that
multiset_map.insert(map<int, int>{{1,4}});
#include <iostream>
#include <map>
#include <algorithm>
using namespace std;
int main() {
map<pair<int,int>,pair<int,int>> items;
items.insert(make_pair(1,20),make_pair(0,0));
items.insert(make_pair(2,10),make_pair(0,0));
items.insert(make_pair(3,30),make_pair(0,0));
items.insert(make_pair(4,5),make_pair(0,0));
items.insert(make_pair(5,35),make_pair(0,0));
map<pair<int,int>,pair<int,int>>::iterator it;
cout<<"Class ID:\t\t\tSamples:\t\t\tTP:\t\t\tPrecision:"<<endl;
for(it = items.begin();it!=items.end();++it)
{
cout<<(it->first).first<<"\t\t\t"<<(it->first).second<<"\t\t\t"<<(it->second).first<<"\t\t\t"<<(it->second).second<<endl;
}
}
I have a code like this but i can't print my values. Can someone help me please?
I'm getting interesting compiler error :
Here is the text of the error :
In file included from C:/PROGRA~1/MINGW-~1/X86_64~1.0-P/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/map:60,
from C:\Users\Fatih\Desktop\clion\quiz3\main.cpp:2:
C:/PROGRA~1/MINGW-~1/X86_64~1.0-P/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_tree.h: In instantiation of 'void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(_II, _II) [with _InputIterator = std::pair<int, int>; _Key = std::pair<int, int>; _Val = std::pair<const std::pair<int, int>, std::pair<int, int> >; _KeyOfValue = std::_Select1st<std::pair<const std::pair<int, int>, std::pair<int, int> > >; _Compare = std::less<std::pair<int, int> >; _Alloc = std::allocator<std::pair<const std::pair<int, int>, std::pair<int, int> > >]':
C:/PROGRA~1/MINGW-~1/X86_64~1.0-P/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_map.h:893:4: required from 'void std::map<_Key, _Tp, _Compare, _Alloc>::insert(_InputIterator, _InputIterator) [with _InputIterator = std::pair<int, int>; _Key = std::pair<int, int>; _Tp = std::pair<int, int>; _Compare = std::less<std::pair<int, int> >; _Alloc = std::allocator<std::pair<const std::pair<int, int>, std::pair<int, int> > >]'
C:\Users\Fatih\Desktop\clion\quiz3\main.cpp:10:48: required from here
C:/PROGRA~1/MINGW-~1/X86_64~1.0-P/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_tree.h:2467:28: error: no match for 'operator++' (operand type is 'std::pair<int, int>')
for (; __first != __last; ++__first)
^~~~~~~~~
C:/PROGRA~1/MINGW-~1/X86_64~1.0-P/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/bits/stl_tree.h:2468:29: error: no match for 'operator*' (operand type is 'std::pair<int, int>')
_M_insert_unique_(end(), *__first, __an);
^~~~~~~~
std::map::insert takes a parameter of value_type which is std::pair<const Key, Value>. That means you need to combine make_pair(1,20),make_pair(0,0) into a single parameter so you can call the function. That would look like
items.insert({make_pair(1,20),make_pair(0,0)});
// ^curly braces create value_type^
items.insert({make_pair(2,10),make_pair(0,0)});
items.insert({make_pair(3,30),make_pair(0,0)});
items.insert({make_pair(4,5),make_pair(0,0)});
items.insert({make_pair(5,35),make_pair(0,0)});
Alternatively you can use emplace which will take parameters to construct a value_type. That lets you get away without using the curly braces and looks like
items.emplace(make_pair(1,20),make_pair(0,0));
items.emplace(make_pair(2,10),make_pair(0,0));
items.emplace(make_pair(3,30),make_pair(0,0));
items.emplace(make_pair(4,5),make_pair(0,0));
items.emplace(make_pair(5,35),make_pair(0,0));
This question already has answers here:
C++ Converting function pointer to unique “hash” key
(2 answers)
Closed 9 years ago.
I am writing in C++, trying to compile under Ubuntu, and I am experiencing some issues with a map using function pointers as keys. When I define the map, I get no compiling errors, but as soon as I try to insert an element, I get a rather wordy
In file included from /usr/include/c++/4.6/string:50:0,
from /usr/include/c++/4.6/bits/locale_classes.h:42,
from /usr/include/c++/4.6/bits/ios_base.h:43,
from /usr/include/c++/4.6/ios:43,
from /usr/include/c++/4.6/ostream:40,
from /usr/include/c++/4.6/iostream:40,
from main.cpp:1:
/usr/include/c++/4.6/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = int (MyClass::*)()]’:
/usr/include/c++/4.6/bits/stl_tree.h:1277:4: instantiated from ‘std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = int (MyClass::*)(), _Val = std::pair<int (MyClass::* const)(), std::vector<int> >, _KeyOfValue = std::_Select1st<std::pair<int (MyClass::* const)(), std::vector<int> > >, _Compare = std::less<int (MyClass::*)()>, _Alloc = std::allocator<std::pair<int (MyClass::* const)(), std::vector<int> > >]’
/usr/include/c++/4.6/bits/stl_map.h:518:41: instantiated from ‘std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::map<_Key, _Tp, _Compare, _Alloc>::value_type>::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const value_type&) [with _Key = int (MyClass::*)(), _Tp = std::vector<int>, _Compare = std::less<int (MyClass::*)()>, _Alloc = std::allocator<std::pair<int (MyClass::* const)(), std::vector<int> > >, typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::map<_Key, _Tp, _Compare, _Alloc>::value_type>::other>::iterator = std::_Rb_tree_iterator<std::pair<int (MyClass::* const)(), std::vector<int> > >, std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<int (MyClass::* const)(), std::vector<int> >]’
main.cpp:36:51: instantiated from here
/usr/include/c++/4.6/bits/stl_function.h:236:22: error: invalid operands of types ‘int (MyClass::* const)()’ and ‘int (MyClass::* const)()’ to binary ‘operator<’
Here is the example that caused the above error message:
#include <iostream>
#include <map>
#include <vector>
// class definition
class MyClass
{
public:
int f1(void);
int f2(void);
};
int MyClass::f1(void)
{
return 1;
}
int MyClass::f2(void)
{
return 2;
}
using namespace std;
int main( int argc, char* argv[] )
{
// define map
map< int (MyClass::*)(void), vector<int> > myMap;
vector<int> myVector;
//myMap[ &MyClass::f1 ] = myVector;
myMap.insert( make_pair( &MyClass::f1, myVector) );
return 0;
}
What could be the issue? I tried with both insert and [] assign, and I get the same error. Browsing the forums, I found this; but could that be the issue? I don't think I need to define an operator "<" for function pointers (shouldn't they behave as regular pointers?) ...or do I?
The error is telling you all you need to know:
invalid operands of types ‘int (MyClass::* const)()’ and ‘int (MyClass::* const)()’ to binary ‘operator<’
You cannot compare member function pointers using standard operator<, so you must provide a custom comparator when declaring your map.
Unfortunately, pointers to member functions cannot be compared for inequality, so you cannot define a comparison operator or use a std::mapin this case. I suggest using std::unordered_map, which only needs a std::hash and equality comparison, which you can do. See here for hashing and here for equality comparison.
You could implement the template specialization for less< int (MyClass::* const)() >, like follows:
typedef int (MyClass::*tMyClassMember)();
namespace std {
template<>
struct less<tMyClassMember>
{
bool operator()(const tMyClassMember& k1, const tMyClassMember& k2) const
{
auto p1 = reinterpret_cast<const intptr_t*>(&k1);
auto p2 = reinterpret_cast<const intptr_t*>(&k2);
return *p1 < *p2;
}
};
}
There may be better ways to compare pointer-to-members than "casting" them to integers, which is an implementation-specific hack, according to this question. That questions contains details about how to do that.
typedef struct
{
pthread_t threadId;
int acceptSocketD;
char *message;
} threadData;
map <unsigned int, threadData> serverPortNumberThreadId;
map <unsigned int, threadData> :: iterator serverPortNumberThreadIdIter;
usage:
threadData obj;
obj.threadId = 0;
obj.acceptSocketD = 0;
obj.message = "Excuse Me, please!";
serverPortNumberThreadId.insert (3490, obj);
error:
error: no matching function for call to ‘std::map<unsigned int, threadData>::insert(int, threadData&)’
/usr/include/c++/4.5/bits/stl_map.h:500:7: note: candidates are: std::pair<typename std::map<_Key, _Tp, _Compare, _Alloc>::_Rep_type::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const std::map<_Key, _Tp, _Compare, _Alloc>::value_type&) [with _Key = unsigned int, _Tp = threadData, _Compare = std::less<unsigned int>, _Alloc = std::allocator<std::pair<const unsigned int, threadData> >, typename std::map<_Key, _Tp, _Compare, _Alloc>::_Rep_type::iterator = std::_Rb_tree_iterator<std::pair<const unsigned int, threadData> >, std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const unsigned int, threadData>]
/usr/include/c++/4.5/bits/stl_map.h:540:7: note: std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::insert(std::map<_Key, _Tp, _Compare, _Alloc>::iterator, const std::map<_Key, _Tp, _Compare, _Alloc>::value_type&) [with _Key = unsigned int, _Tp = threadData, _Compare = std::less<unsigned int>, _Alloc = std::allocator<std::pair<const unsigned int, threadData> >, std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const unsigned int, threadData> >, std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<const unsigned int, threadData>]
tcpClient.cpp: In function ‘int main(int, char**)’
You need to insert the value into map:
serverPortNumberThreadId.insert ( std::make_pair( 3490, obj) );
For other ways to insert into map, see the map::insert() reference page.
You need a pair for the insert function, because in common with other containers insert takes the value_type, which in the case of map is a pair -- each entry has a key and a mapped value. The value_type represents one entry in the container and hence includes both.
You could write serverPortNumberThreadId[3490] = obj; instead, if you prefer the look of it. Behavior is sometimes different (insert does nothing if the key already exists, whereas this code overwrites it, but unless you're relying on that behavior of insert already this makes no difference to you). Performance might be slightly different in terms of the number of threadData objects created/copied/assigned.
//always overwrites the old value
serverPortNumberThreadId[3490]=obj;
//only insert a new one
serverPortNumberThreadId.insert(std::make_pair(3490, obj));
You have to send a pair to insert function and not key,value.
mymap.insert ( pair<unsigned int,threadData>(3490, obj) );
I wonder if there is any trick to use copy with maps to copy the contents of map into an array. Because STL maps are by the combination of a key value and a mapped value an element of a map forms a key value pair. That prevents us to use standard algorithms like std::copy. For example following code gives error:
#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
int
main()
{
std::map <int, double> test(4);
test[0] = 11;
test[2] = 1.23;
test[3] = 23.29;
test[1] = 12.12;
double *test_arr = (double *) malloc(4 * sizeof(double));
std::copy(test.begin(), test.end(), test_arr);
std::cout << test_arr[3] << std::endl;
return 0;
}
Error:
stl_copy_tests.cpp: In function ‘int main()’:
stl_copy_tests.cpp:9:32: error: no matching function for call to ‘std::map<int, double>::map(int)’
/usr/include/c++/4.5/bits/stl_map.h:170:7: note: candidates are: std::map<_Key, _Tp, _Compare, _Alloc>::map(const std::map<_Key, _Tp, _Compare, _Alloc>&) [with _Key = int, _Tp = double, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, double> >, std::map<_Key, _Tp, _Compare, _Alloc> = std::map<int, double>]
/usr/include/c++/4.5/bits/stl_map.h:159:7: note: std::map<_Key, _Tp, _Compare, _Alloc>::map(const _Compare&, const allocator_type&) [with _Key = int, _Tp = double, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, double> >, allocator_type = std::allocator<std::pair<const int, double> >]
/usr/include/c++/4.5/bits/stl_map.h:150:7: note: std::map<_Key, _Tp, _Compare, _Alloc>::map() [with _Key = int, _Tp = double, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, double> >]
In file included from /usr/include/c++/4.5/bits/char_traits.h:41:0,
from /usr/include/c++/4.5/ios:41,
from /usr/include/c++/4.5/ostream:40,
from /usr/include/c++/4.5/iostream:40,
from stl_copy_tests.cpp:1:
/usr/include/c++/4.5/bits/stl_algobase.h: In static member function ‘static _OI std::__copy_move<<anonymous>, <anonymous>, <template-parameter-1-3> >::__copy_m(_II, _II, _OI) [with _II = std::_Rb_tree_iterator<std::pair<const int, double> >, _OI = double*, bool <anonymous> = false, bool <anonymous> = false, <template-parameter-1-3> = std::bidirectional_iterator_tag]’:
/usr/include/c++/4.5/bits/stl_algobase.h:404:70: instantiated from ‘_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false, _II = std::_Rb_tree_iterator<std::pair<const int, double> >, _OI = double*]’
/usr/include/c++/4.5/bits/stl_algobase.h:442:39: instantiated from ‘_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false, _II = std::_Rb_tree_iterator<std::pair<const int, double> >, _OI = double*]’
/usr/include/c++/4.5/bits/stl_algobase.h:474:18: instantiated from ‘_OI std::copy(_II, _II, _OI) [with _II = std::_Rb_tree_iterator<std::pair<const int, double> >, _OI = double*]’
stl_copy_tests.cpp:15:47: instantiated from here
/usr/include/c++/4.5/bits/stl_algobase.h:319:6: error: cannot convert ‘std::pair<const int, double>’ to ‘double’ in assignment
Is there any easy trick/hack to overcome this problem.
Disclaimer: Not interested in solutions that iterates over map in a for loop and adds elements to the array.
You could use std::transform instead:
template <typename T, typename U>
const U &extract_second(const std::pair<T,U> &p)
{
return p.second;
}
std::transform(test.begin(), test.end(), test_arr, extract_second<int,double>);
And as #Andre points out in a comment below, if you want a slightly more verbose overhead, you can avoid having to explicitly state the template arguments via a functor:
struct extract_second
{
template <typename T, typename U>
const U operator() (const std::pair<T,U> &p) const
{
return p.second;
}
};
std::transform(test.begin(), test.end(), test_arr, extract_second());
I'm sure there's a less-verbose solution using Boost binders, but I can't remember the syntax off the top of my head.
Ewww, malloc? Anyway, if you want to copy a map, you have to remember the keys too.
int main()
{
std::map <int, double> test(4);
test[0] = 11;
test[2] = 1.23;
test[3] = 23.29;
test[1] = 12.12;
std::vector<std::pair<int, double>> test_arr(test.size());
std::copy(test.begin(), test.end(), test_arr.begin());
std::cout << test_arr[3] << std::endl;
return 0;
}
If you consider std::map an STL container, then it is a container of
std::pair<key_type, mapped_type>. (This is what its value_type is
defined to be, and it is designed so that it can be used as a
container.) If you want simply one part of it, the correct function is
std::transform, with a transformation function which maps the
value_type to either the key_type or the mapped_type. (If you make
much use of std::pair—or std::map, whose value_type is an
std::pair, you should probably have functional objects for this in
your tool kit:
struct ExtractFirst
{
template<typename Pair>
typename boost::remove_const<typename Pair::first_type>::type
operator()( Pair const& from ) const
{
return from.first;
}
};
, and the same thing for ExtractSecond.
Your target would be an arraystd::vector[please!] of std::pair<int,double> objects unless, yes, you unroll it yourself.
(You could create your own InputIterator as a proxy, or play with std::transform and a std::back_inserter, but that's just being silly. You'll make your code far more verbose than just looping through the map.)
The simplest way is to use std::transform in combination with boost::bind:
typedef std::map<int, double> map_t;
map_t mm;
// add elements to mm
// ...
// copy
typedef std::vector<double> vec_t;
vec_t vv;
vv.reserve( mm.size() );
std::transform( mm.begin(), mm.end(), std::back_inserter(vv),
boost::bind( &map_t::value_type::second, _1 ) );
If you could use C++0x (without boost):
std::transform( mm.begin(), mm.end(), back_inserter(vv),
[](map_t::value_type val) -> double { return val.second; } );
// or
std::for_each( mm.begin(), mm.end(),
[&vv](map_t::value_type val) { vv.push_back( val.second ); } );