STL algorithm算法count_if(11)

原文位址：http://www.cplusplus.com/reference/algorithm/count_if/

function template <algorithm>

std::count_if

template <class InputIterator, class Predicate>
  typename iterator_traits<InputIterator>::difference_type
    count_if (InputIterator first, InputIterator last, UnaryPredicate pred);      

Return number of elements in range satisfying condition Returns the number of elements in the range 

[first,last)

 for which pred is true.

傳回範圍内對每個元素調用pred傳回true的次數。 例子：

#include <iostream>
#include <algorithm>
#include <vector>
#include <array>
using namespace std;
bool ismod2(int i){
    return i%2==0;
}
void countif(){
    vector<int> vi{1,5,7,8,9,9,8,5,9};
    array<int,7> ai{888,666,555,222,111,555,777};
    cout<<"vi=";
    for(int &i:vi)
        cout<<i<<" ";
    cout<<endl;

    int cv=count_if(vi.begin(),vi.end(),ismod2);
    cout<<"能被2整除的一共有"<<cv<<"個"<<endl;

    cout<<"ai=";
    for(int &i:ai)
        cout<<i<<" ";
    cout<<endl;
    int ca=count_if(ai.begin(),ai.end(),[](int i){return i>500;});
    cout<<"大于500的一共有"<<ca<<"個"<<endl;




}
           

運作截圖：

The behavior of this function template is equivalent to:

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template <class InputIterator, class UnaryPredicate> typename iterator_traits<InputIterator>::difference_type count_if (InputIterator first, InputIterator last, UnaryPredicate pred) { typename iterator_traits<InputIterator>::difference_type ret = 0; while (first!=last) { if (pred(*first)) ++ret; ++first; } return ret; }

Parameters

first, last

Input iterators to the initial and final positions of the sequence of elements. The range used is 

[first,last)

, which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.

元素的範圍。

pred

Unary function that accepts an element in the range as argument, and returns a value convertible to 

bool

. The value returned indicates whether the element is counted by this function.

The function shall not modify its argument.

This can either be a function pointer or a function object.

接受一個元素作為參數兵傳回一個bool值的一進制函數。

Return value

The number of elements in the range 

[first,last)

 for which pred does not return 

false

.

The return type (iterator_traits<InputIterator>::difference_type) is a signed integral type.

傳回範圍内對每個元素調用pred傳回true的次數。

Example

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// count_if example #include <iostream> // std::cout #include <algorithm> // std::count_if #include <vector> // std::vector bool IsOdd (int i) { return ((i%2)==1); } int main () { std::vector<int> myvector; for (int i=1; i<10; i++) myvector.push_back(i); // myvector: 1 2 3 4 5 6 7 8 9 int mycount = count_if (myvector.begin(), myvector.end(), IsOdd); std::cout << "myvector contains " << mycount << " odd values.\n"; return 0; }

Edit & Run

Output:

myvector contains 5 odd values.

Complexity

Linear in the distance between first and last: Calls pred once for each element.

Data races

The objects in the range 

[first,last)

 are accessed (each object is accessed exactly once).

Exceptions

Throws if pred trhows or if any of the operations on iterators throws.

Note that invalid arguments cause undefined behavior.

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轉載請注明出處：http://blog.csdn.net/qq844352155

author:天下無雙

Email:[email protected]

2014-9-11

于GDUT

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