C + + Map

In order to achieve a quick lookup, the map itself is stored sequentially (such as a red-black tree). When we insert <key, value> Key-value pairs, they are stored in the order of key size. Definition of Map:

1 class class class // entry is key 2             // The key         value                is used for the function object of the key comparison, with the relative greater3class Allocator = allocator<pair< Const 4                      // for storage allocation 5 class

Where less is implemented:

1 template <class2struct less:binary_function <t,t,bool3  {  4   booloperator() (constconst  Const  5     {return x<y;}   6 };  

Sort the key

From the above, sort by key, just customize a class for comparison is OK.

For example:

1 structCmpbykeylength2 {  3      BOOL operator()(Const string& K1,Const string&K2)4      {  5          returnK1.length () <k2.length (); 6       }  7 }; 8 intMain ()9 {  Tenmap<string,int, cmpbykeylength>Name_score_map;  Onename_score_map["Limin"] = -;  AName_score_map.insert (Make_pair ("Bing", About));  -Name_score_map.insert (Make_pair ("Albert", the));  -      for(map<string,int>::iterator iter =Name_score_map.begin ();  theIter! =Name_score_map.end ();  -++iter) -     {   -cout << *iter <<Endl;  +     } -    +     return 0;  A}

Sort by value

For value, you can neither press compare nor use sort, because sort is only available for sequence (linear) containers, and map is a collection container (for example: Red black tree)

The element type in map is pair

1Template <classT1,classT2>structpair2 {  3 typedef T1 First_type; 4 typedef T2 Second_type; 5   6T1 first;//corresponding Key7T2 second;//corresponding Value8  9   //constructor FunctionTen pair (): First (T1 ()), second (T2 ()) {} OnePairConstt1& x,Constt2&y): First (x), second (y) {} ATemplate <classUclassV> -Pair (ConstPair<u,v> &p): First (P.first), second (P.second) {} -}

In addition, in the <utility> header file, the < operator is also overloaded for pair

1template<class_T1,class_t2>2InlineBOOL  operator< (ConstPAIR&LT;_T1, _t2>& __x,ConstPAIR&LT;_T1, _t2>&__y)3 { 4     return__x.first < __y.first | |//Key Different5(! (__y.first < __x.first) && __x.second < __y.second);//key is the same, the value is compared6 //why not use = =? Because the key of the map must implement the overload of the < operator, as to = = cannot be guaranteed, and the floating-point number ...7}

So it's possible to sort the value with map + vector

For example:

1typedef pair<int,int>PAIR; 2 BOOLCmp_by_value (Constpair& LHS,Constpair&RHS) {  3   returnLhs.second <Rhs.second; 4 }  5 structCmpbyvalue {6   BOOL operator()(Constpair& LHS,Constpair&RHS) {  7     returnLhs.second <Rhs.second; 8   }  9 }; Ten intMain () { Onemap<int,int>MyMap;  AMyMap [1] = -;  -MyMap [2] = -;  -Mymap.insert (Make_pair (3, About));  theMyMap Insert (Make_pair (4, the));  -  //dump the elements in the map into a vector -Vector<pair>Name_score_vec (Mymap.begin (), Mymap.end ());  - sort (Mymap.begin (), Mymap.end (), Cmpbyvalue ());  +   //sort (Name_score_vec.begin (), Name_score_vec.end (), cmp_by_value);  -    for(inti =0; I! = Mymap.size (); ++i) { +cout << MyMap [i] <<Endl;  A        }   at       return 0;  -}

Sort The algorithm is as follows:

1 template <class randomaccessiterator>  2void  sort ( Randomaccessiterator First, Randomaccessiterator last);   3   4 Template <classclass compare>  5void

C + + Map