C++ Primer 学习中。。

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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.countmultiset.countmap.countmultimap.count
************************/
/***********************  std::count：****************************************
template <class InputIterator, class T>typename iterator_traits<InputIterator>::difference_typecount ( 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_typecount_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 elementsmycount = (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20cout << "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 9cout<<"\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 % 2cout << "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 > param2mycount=count_if(myvector.begin(),myvector.end(),bind1st(greater<int>(),4));cout<<"有"<<mycount<<"个数小于4"<<endl;
//                                                              4 < param2mycount=count_if(myvector.begin(),myvector.end(),bind1st(less<int>(),4));cout<<"有"<<mycount<<"个数大于4"<<endl;
//                                                              param1 >= 4mycount=count_if(myvector.begin(),myvector.end(),bind2nd(greater_equal<int>(),4));cout<<"有"<<mycount<<"个数大于等于4"<<endl;
//                                                              param1 <= 4mycount=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;
}