STL_算法_元素计数(count、count_if)

C++ Primer 学习中。。

。

简单记录下我的学习过程 (代码为主)

count 、 count_if

#include<iostream>

#include<cstdio>

#include<cstring>

#include<vector>

#include<set>

#include<algorithm>

using namespace std;

/***********************

count

count_if

关联容器的等效成员函数

    set.count

    multiset.count

    map.count

    multimap.count

************************/

/***********************  std::count：****************************************

template <class InputIterator, class T>

  typename iterator_traits<InputIterator>::difference_type

    count ( ForwardIterator first, ForwardIterator last, const T& value );

//eg：

template <class InputIterator, class T>

  ptrdiff_t count ( InputIterator first, InputIterator last, const T& value )

{

  ptrdiff_t ret=0;

  while (first != last) if (*first++ == value) ++ret;

  return ret;

}

*******************************************************************************/

/***********************  std::count_if：****************************************

template <class InputIterator, class Predicate>

  typename iterator_traits<InputIterator>::difference_type

    count_if ( ForwardIterator first, ForwardIterator last, Predicate pred );

//eg:

template <class InputIterator, class Predicate>

  ptrdiff_t count_if ( InputIterator first, InputIterator last, Predicate pred )//pred 为函数or函数对象

{

  ptrdiff_t ret=0;

  while (first != last) if (pred(*first++)) ++ret;

  return ret;

}

*******************************************************************************/

//奇数

bool IsOdd (int i)

{

    return i&1;

}

int main()

{

    int mycount;

    // counting elements in array:

    int myints[] = {10,20,30,30,20,10,10,20};   // 8 elements

    mycount = (int) count (myints, myints+8, 10);

    cout << "10 appears " << mycount << " times.\n";

    // counting elements in container:

    vector<int> myvector (myints, myints+8);

    mycount = (int) count (myvector.begin(), myvector.end(), 20);//有几个20

    cout << "20 appears " << mycount  << " times.\n";

    /****************

    Output:

    10 appears 3 times.

    20 appears 3 times.

    ****************/

//    vector<int> myvector;

    myvector.clear();

    for (int i=1; i<10; i++) myvector.push_back(i); // myvector: 1 2 3 4 5 6 7 8 9

    cout<<"\nmyvector: 1 2 3 4 5 6 7 8 9 \n";

//    mycount = (int) count_if (myvector.begin(), myvector.end(), IsOdd);

    mycount = (int) count_if (myvector.begin(), myvector.end(), bind2nd(modulus<int>(),2));//表示param1 % 2

    cout << "myvector contains " << mycount  << " odd values.\n";//奇数

//  假设求偶数的个数                                            not1,1表示一个參数取反

    mycount = (int) count_if (myvector.begin(), myvector.end(), not1(bind2nd(modulus<int>(),2)));//表示!(param1 % 2)

    cout << "myvector contains " << mycount  << " even values.\n";//偶数

    /****************

    Output:

    myvector contains 5 odd values.

    ****************/

//                                               函数适配器  函数对象

//         bind2nd(op,value);表示绑定第二个数                   param1 > 4  这里表示统计大于4的个数

    mycount=count_if(myvector.begin(),myvector.end(),bind2nd(greater<int>(),4));

    cout<<"有"<<mycount<<"个数大于4"<<endl;

//拓展练习                                                      4 > param2

    mycount=count_if(myvector.begin(),myvector.end(),bind1st(greater<int>(),4));

    cout<<"有"<<mycount<<"个数小于4"<<endl;

//                                                              4 < param2

    mycount=count_if(myvector.begin(),myvector.end(),bind1st(less<int>(),4));

    cout<<"有"<<mycount<<"个数大于4"<<endl;

//                                                              param1 >= 4

    mycount=count_if(myvector.begin(),myvector.end(),bind2nd(greater_equal<int>(),4));

    cout<<"有"<<mycount<<"个数大于等于4"<<endl;

//                                                              param1 <= 4

    mycount=count_if(myvector.begin(),myvector.end(),bind2nd(less_equal<int>(),4));

    cout<<"有"<<mycount<<"个数小于等于4"<<endl;

/****关联容器****/

    multiset<int> ms(myvector.begin(),myvector.end());

    ms.insert(myvector.begin(),myvector.begin()+6);

    ms.insert(myvector.begin(),myvector.begin()+4);

    ms.insert(myvector.begin(),myvector.begin()+2);

    ms.insert(1);

    multiset<int>::iterator ims=ms.begin();

    while(ims!=ms.end()){

        cout<<*ims++<<" ";

    }cout<<endl;

//两种方法求1的个数

    int cnt=count(ms.begin(),ms.end(),1);//全部容器适用可是比較慢些

    cout<<"multiset里有"<<cnt<<"个1."<<endl;

    cnt=ms.count(1);//关联容器专享   set已经排序能够高速计数

    cout<<"multiset里有"<<cnt<<"个1."<<endl;

    return 0;

}