Well, you might have faced this problem in your coding life. Also you might know the answer already. The problem was about the std::map or say std::multimap, both of them are special type of containers who can sort the items by a comparing function pointer(or a built-in one like std::less). They sort it when you are inserting a new item, that's of course so handy, because you don't wanna call a sorting function every time you need to! Actually you will never explicitly call any sorting function on your map or multimap, coz it's already sorted! Now here's my problem-
Say, I have a map like this -
D - 1
D - 2
B - 3
A - 4
Now I need to sort them like this -
A - 4
B - 3
D - 2
D - 1
Here's the explanation of this sorting- first elements(A,B,D) will be sorted in ascending order A --> B --> D regardless of the second element, secondly if the first elements are equal (i.e 2 Ds here) then they will be sorted as the descending order of their corresponding second element.
Using std::multimap will end-up with only the first sorting part, so I took advantage of templates and inheritances. Here's the code -
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#include <iostream>
#include <utility>
#include <set>
using namespace std;
template <class _A, class _B, class _Compare=less<_A> >
class MMap : public set < pair< _A, _B >, _Compare >
{
public :
MMap():set< pair< _A, _B >, _Compare >(){};
~MMap(){};
};
template< typename InPair >
struct MMapComp{
bool operator() (InPair a , InPair b){
if( a.first == b.first ) return a.second > b.second;
else
return a.first < b.first;
}
};
int main(int argc, char ** argv)
{
MMap<char,int,MMapComp< pair<char , int > > > test;
test.insert(make_pair('D',1));
test.insert(make_pair('D',2));
test.insert(make_pair('B',3));
test.insert(make_pair('A',4));
for( MMap<char,int >::iterator it = test.begin(); it != test.end(); it++ )
cout << (*it).first << "\t" << (*it).second << endl;
return 0;
}
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And here's the output -
Ok, I apologize for my undocumented code :) But it's not so hard to read actually. Is it ?