C++ STL find和find_if算法与bind、lambda组合使用
ztj100 2025-01-03 20:47 20 浏览 0 评论
直接上代码,使用std::find、std::find_if来搜索匹配,代码确实比for循环变少和更好看了,但是时间复杂度还是O(n),见下文。
#include <algorithm>
#include <functional>
#include <iostream>
#include <string>
#include <vector>
class Item {
private:
int mId;
std::string mName;
public:
Item(int id, std::string name) : mId(id), mName(name) {}
std::string getName() const { return mName; }
int getId() const { return mId; }
bool operator==(const Item& obj2) const {
return (this->getName().compare(obj2.getName()) == 0) &&
(this->getId() == obj2.getId());
}
static bool Compare(const Item& obj1, const Item& obj2) {
return (obj1.getName().compare(obj2.getName()) == 0) &&
(obj1.getId() == obj2.getId());
}
};
#define log(obj, pos) \
do { \
if (it != sItemList.end()) { \
std::cout << "Found obj " << obj.getName() << ", id:" << it->getId() \
<< ",name: " << it->getName() << ",position:" << pos \
<< std::endl; \
} else { \
std::cout << "Not Found obj" << std::endl; \
} \
} while (false)
std::vector<Item> sItemList = {{0, "A"}, {1, "B"}, {2, "C"}, {3, "D"}, {4, "E"},
{5, "E"}, {6, "G"}, {7, "H"}, {1, "B"}};
int main() {
Item obj1(1, "B");
Item obj2(8, "I");
Item obj3(5, "E");
Item obj4(7, "H");
std::cout << "obj1 is equal obj1 : " << (obj1 == obj1) << std::endl;
std::cout << "obj1 is equal obj2 : " << (obj1 == obj2) << std::endl;
// 1. std::find 类重载运算符==,使用默认的比较函数查找等于的项
std::vector<Item>::iterator it =
std::find(sItemList.begin(), sItemList.end(), obj1);
log(obj1, std::distance(sItemList.begin(), it));
// 2. std::find 类重载运算符==,使用默认的比较函数d方向查找等于的项
std::vector<Item>::reverse_iterator rit =
std::find(sItemList.rbegin(), sItemList.rend(), obj1);
log(obj1, (sItemList.size() - std::distance(sItemList.rbegin(), rit)));
// 3. std::find_if 使用自定义的函数查找符合要求的项
it = std::find_if(sItemList.begin(), sItemList.end(),
std::bind(Item::Compare, std::placeholders::_1, obj3));
log(obj3, std::distance(sItemList.begin(), it));
// 4. std::find_if使用 lambda 表达式查找符合的项
it = std::find_if(
sItemList.begin(), sItemList.end(),
[&obj4](Item const& obj) { return Item::Compare(obj, obj4); });
log(obj4, std::distance(sItemList.begin(), it));
return 0;
}
编译运行结果:
$ g++ std_find.cpp --std=c++11;./a.out
obj1 is equal obj1 : 1
obj1 is equal obj2 : 0
Found obj B, id:1,name: B,position:1
Found obj B, id:1,name: B,position:9
Found obj E, id:5,name: E,position:5
Found obj H, id:7,name: H,position:7
std::find算法的行为如下所示,时间复杂度为O(n):
template<class InputIterator, class T>
InputIterator find (InputIterator first, InputIterator last, const T& val)
{
while (first!=last) {
if (*first==val) return first;
++first;
}
return last;
}
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