算法主要由头文件<algorithm>、<functional>、<numeric>组成。
<algorithm>是所有STL头文件中最大的一个，范围涉及到比较、交换、查找、遍历、赋值、修改等等；<numeric>体积很小，只包括几个在序列上面进行简单数学运算的模板函数；<functional>定义了一些模板类。
一.遍历
1.std::for_each
std::for_each用于遍历容器
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void print1(const int &val)
{
std::cout << val << " ";
}
class print2
{
public:
void operator()(const int &val)
{
std::cout << val << " ";
}
};
void test1()
{
std::vector<int> v;
for (int i = 0; i < 10; i++)
{
v.push_back(i);
}
for_each(v.begin(), v.end(), print1);
std::cout << std::endl;
for_each(v.begin(), v.end(), print2());
}
int main()
{
test1();
return 0;
}
2.std::transform
std::transform用于搬运容器到另一个容器中
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(const int &val)
{
std::cout << val << " ";
}
class Transform
{
public:
int operator()(int v)
{
return v + 3;
}
};
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::vector<int> v2;
v2.resize(v1.size());
transform(v1.begin(), v1.end(), v2.begin(), Transform());
for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
for_each(v2.begin(), v2.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
二.查找
1.std::find
std::find用于查找指定元素，找到返回指定元素的迭代器，找不到返回结束迭代器end()
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::vector<int>::iterator it = std::find(v1.begin(), v1.end(), 5);
if (it == v1.end())
{
std::cout << "Find failed！" << std::endl;
}
else
{
std::cout << "Find succeed: " << *it << std::endl;
}
}
int main()
{
test1();
return 0;
}
2.std::find_if
std::find_if用于按条件查找
cpp
#include <iostream>
#include <vector>
#include <algorithm>
class GreaterFive
{
public:
bool operator()(int val)
{
return val > 5;
}
};
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::vector<int>::iterator it = find_if(v1.begin(), v1.end(), GreaterFive());
if (it == v1.end())
{
std::cout << "Find failed" << std::endl;
}
else
{
std::cout << "Find numbers greater than 5: " << *it << std::endl;
}
}
int main()
{
test1();
return 0;
}
3.std::adjacent_find
std::adjacent_find用于查找相邻重复元素，返回相邻元素的第一个位置的迭代器
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void test1()
{
std::vector<int> v1;
v1.push_back(1);
v1.push_back(2);
v1.push_back(2);
std::vector<int>::iterator it = adjacent_find(v1.begin(), v1.end());
if (it == v1.end())
{
std::cout << "Find failed" << std::endl;
}
else
{
std::cout << "Find adjacent numbers: " << *it << std::endl;
}
}
int main()
{
test1();
return 0;
}
4.std::binary_search
std::binary_search(二分查找)用于查找指定元素是否存在。查到返回true，否则false。需要注意的是该算法在无序序列中不可用
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
bool ret = std::binary_search(v1.begin(), v1.end(), 9);
if (ret)
{
std::cout << "Find succeed" << std::endl;
}
else
{
std::cout << "Find failed" << std::endl;
}
}
int main()
{
test1();
return 0;
}
三.统计元素个数
1.std::count
std::count用于统计元素出现的次数
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void test1()
{
std::vector<int> v1;
v1.push_back(1);
v1.push_back(2);
v1.push_back(2);
int num = std::count(v1.begin(), v1.end(), 2);
std::cout << "The count of 2: " << num<< std::endl;
}
int main()
{
test1();
return 0;
}
2.std::count_if
std::count_if用于按条件统计元素个数
cpp
#include <iostream>
#include <vector>
#include <algorithm>
class CountIf
{
public:
bool operator()(int val)
{
return val > 5;
}
};
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
int num = std::count_if(v1.begin(), v1.end(), CountIf());
std::cout << "The count of numbers greater than 5: " << num << std::endl;
}
int main()
{
test1();
return 0;
}
四.排序
1.std::sort
std::sort用于进行升序或降序
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
v1.push_back(2);
v1.push_back(1);
v1.push_back(3);
// 默认升序
std::sort(v1.begin(), v1.end());
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
// 降序
std::sort(v1.begin(), v1.end(), std::greater<int>());
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
2.std::random_shuffle
std::random_shuffle用于洗牌，指定范围内的元素随机调整次序
cpp
#include <iostream>
#include <vector>
#include <algorithm>
#include <time.h>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::srand((unsigned int)time(nullptr));
std::random_shuffle(v1.begin(), v1.end());
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
3.std::merge
std::merge用于将两个容器元素合并，并存储到另一个容器中。合并后，目标容器仍然是有序的
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
std::vector<int> v2;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
v2.push_back(i + 1);
}
std::vector<int> v3;
v3.resize(v1.size() + v2.size());
std::merge(v1.begin(), v1.end(), v2.begin(), v2.end(), v3.begin());
std::for_each(v3.begin(), v3.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
4.std::reverse
std::reverse用于将容器内元素进行反转
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::cout << "Before: ";
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
std::cout << "After: ";
std::reverse(v1.begin(), v1.end());
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
五.拷贝和替换
1.std::copy
std::copy用于将容器内指定范围的元素拷贝到另一容器中。使用v2 = v1效果一样，记得提前给容器分配空间
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::vector<int> v2;
v2.resize(v1.size());
std::copy(v1.begin(), v1.end(), v2.begin());
std::for_each(v2.begin(), v2.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
2.std::replace
std::replace用于将容器内指定范围的旧元素修改为新元素
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::cout << "Before: ";
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
std::cout << "After: ";
std::replace(v1.begin(), v1.end(), 0, 1);
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
3.std::replace_if
std::replace_if用于将容器内指定范围满足条件的元素替换为新元素。可以利用仿函数灵活筛选满足的条件
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
class Rule
{
public:
bool operator()(int val)
{
return val > 5;
}
};
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::cout << "Before: ";
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
std::cout << "After: ";
// 把容器中所有大于5的元素，替换为6
std::replace_if(v1.begin(), v1.end(), Rule(), 6);
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
4.std::swap
std::swap用于互换两个容器的元素。无需重新指定容器的大小
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::vector<int> v2;
for (int i = 0; i < 5; i++)
{
v2.push_back(i);
}
// 交换两个容器内的元素，无需重新指定容器大小
std::swap(v1, v2);
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
std::for_each(v2.begin(), v2.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
六.算术生成
1.std::accumulate
std::accumulate用于计算容器内元素累计总和
cpp
#include <iostream>
#include <vector>
#include <numeric>
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 3; i++)
{
v1.push_back(i);
}
// 最后一个参数是累加的初值，accumulate将它的一个内部变量设置为指定的初值，
// 然后在此初值上累加输入范围内所有元素的值.1+0+1+2=4
int total = std::accumulate(v1.begin(), v1.end(), 1);
std::cout << total;
}
int main()
{
test1();
return 0;
}
2.std::fill
std::fill用于向容器中添加元素
cpp
#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
v1.resize(10);
std::fill(v1.begin(), v1.end(), 1);
std::for_each(v1.begin(), v1.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
七.集合
1.std::set_intersection
std::set_intersection用于求两个容器的交集。两个容器必须是有序序列，可以先用排序算法转化为有序序列，再求交集。目标容器开启空间需要从两个容器中取小值。返回值是交集中最后一个元素的位置
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::vector<int> v2;
for (int i = 0; i < 5; i++)
{
v2.push_back(i);
}
std::vector<int> v3;
v3.resize(std::min(v1.size(), v2.size()));
std::set_intersection(v1.begin(), v1.end(), v2.begin(), v2.end(), v3.begin());
std::for_each(v3.begin(), v3.end(), printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
2.std::set_union
std::set_union用于求两个容器的并集。两个容器必须是有序序列。目标容器开辟空间的大小为两个容器相加。返回值是并集中最后一个元素的位置
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::vector<int> v2;
for (int i = 0; i < 5; i++)
{
v2.push_back(i);
}
std::vector<int> v3;
v3.resize(v1.size() + v2.size());
std::vector<int>::iterator itEnd = std::set_union(v1.begin(), v1.end(), v2.begin(), v2.end(), v3.begin());
// 注意这里要判断itEnd，如果换成v3.end(),会输出v3后面的5个0
std::for_each(v3.begin(), itEnd, printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
3.std::set_difference
std::set_difference用于求两个容器的差集。注意是v1-v2还是v2-v1。两个容器为有序序列。目标容器开辟空间为两个容器中的较大值。返回值是差集中最后一个元素的位置
cpp
#include <iostream>
#include <vector>
#include <algorithm>
void printVector(int val)
{
std::cout << val << " ";
}
void test1()
{
std::vector<int> v1;
for (int i = 0; i < 10; i++)
{
v1.push_back(i);
}
std::vector<int> v2;
for (int i = 0; i < 5; i++)
{
v2.push_back(i);
}
std::vector<int> v3; // 目标容器,需要提前开辟空间
v3.resize(std::max(v1.size(), v2.size()));
std::cout << "v1-v2: ";
std::vector<int>::iterator itEnd1 = std::set_difference(v1.begin(), v1.end(), v2.begin(), v2.end(), v3.begin());
std::for_each(v3.begin(), itEnd1, printVector);
std::cout << std::endl;
std::cout << "v2-v1: ";
std::vector<int>::iterator itEnd2 = std::set_difference(v2.begin(), v2.end(), v1.begin(), v1.end(), v3.begin());
// 输出为空，因为v2-v1，v2中没有v1的元素
std::for_each(v3.begin(), itEnd2, printVector);
std::cout << std::endl;
}
int main()
{
test1();
return 0;
}
八.去重
std::unique，详见：C++之std::vector元素去重
原文链接：C++常用标准算法-CSDN博客