std::count函数
1. 函数介绍
std::count是C++标准库中的一个算法，用于计算给定值在指定范围内出现的次数。它的原型如下：
template <class InputIt, class T>size_t count(InputIt first, InputIt last, const T& value);其中，first和last表示范围的起始和结束迭代器，value表示要查找的值。函数返回一个size_t类型的值，表示value在指定范围内出现的次数。
2. 使用场景
std::count函数在以下场景中非常有用：
2.1 统计数组中某个元素的出现次数
#include <iostream>#include <vector>#include <algorithm>int main() { std::vector<int> nums = {1, 2, 3, 4, 5, 2, 3, 2}; int target = 2; size_t count = std::count(nums.begin(), nums.end(), target); std::cout << "The number " << target << " appears " << count << " times in the array." << std::endl; return 0;}输出结果：
The number 2 appears 3 times in the array.2.2 统计字符串中某个字符的出现次数
#include <iostream>#include <string>#include <algorithm>int main() { std::string str = "hello world"; char target = 'l'; size_t count = std::count(str.begin(), str.end(), target); std::cout << "The character '" << target << "' appears " << count << " times in the string." << std::endl; return 0;}输出结果：
The character 'l' appears 3 times in the string.2.3 统计容器中某个元素的出现次数
#include <iostream>#include <vector>#include <algorithm>#include <set>int main() { std::vector<int> nums = {1, 2, 3, 4, 5, 2, 3, 2}; int target = 2; size_t count = std::count(nums.begin(), nums.end(), target); std::cout << "The number " << target << " appears " << count << " times in the vector." << std::endl; std::set<int> s = {1, 2, 3, 4, 5, 2, 3, 2}; count = std::count(s.begin(), s.end(), target); std::cout << "The number " << target << " appears " << count << " times in the set." << std::endl; return 0;}输出结果：
The number 2 appears 3 times in the vector.The number 2 appears 3 times in the set.3. 总结
std::count函数是一个非常实用的算法，它可以帮助我们快速统计给定值在指定范围内的出现次数。无论是统计数组、字符串还是容器中的元素出现次数，都可以使用std::count函数轻松实现。
