stdcount

template <class InputIterator, class T>
typename iterator_traits<InputIterator>::difference_type
count (InputIterator first, InputIterator last, const T& val);

效果

计算等于 val 的元素数

参数

first =>指向范围开头的
迭代器 last =>指向范围结束的迭代器
val =>该范围内该值的出现将被计算在内

返回

范围内等于（==）到 val 的元素数。

例

#include <vector>
#include <algorithm>
#include <iostream>

using namespace std;

int main(int argc, const char * argv[]) {
  
  //create vector
  vector<int> intVec{4,6,8,9,10,30,55,100,45,2,4,7,9,43,48};
  
  //count occurences of 9, 55, and 101
  size_t count_9 = count(intVec.begin(), intVec.end(), 9); //occurs twice
  size_t count_55 = count(intVec.begin(), intVec.end(), 55); //occurs once
  size_t count_101 = count(intVec.begin(), intVec.end(), 101); //occurs once
  
  //print result
  cout << "There are " << count_9  << " 9s"<< endl;
  cout << "There is " << count_55  << " 55"<< endl;
  cout << "There is " << count_101  << " 101"<< ends;

  //find the first element == 4 in the vector
  vector<int>::iterator itr_4 = find(intVec.begin(), intVec.end(), 4);

  //count its occurences in the vector starting from the first one
  size_t count_4 = count(itr_4, intVec.end(), *itr_4); // should be 2

  cout << "There are " << count_4  << " " << *itr_4 << endl;

  return 0;
}

输出

There are 2 9s
There is 1 55
There is 0 101
There are 2 4