# 18.4 常用拷贝和替换算法

### **学习目标：**

* 掌握常用的拷贝和替换算法

**算法简介：**

* `copy` // 容器内指定范围的元素拷贝到另一容器中
* `replace` // 将容器内指定范围的旧元素修改为新元素
* `replace_if` // 容器内指定范围满足条件的元素替换为新元素
* `swap` // 互换两个容器的元素

### **1 copy**

**功能描述：**

* 容器内指定范围的元素拷贝到另一容器中

**函数原型：**

* `copy(iterator beg, iterator end, iterator dest);`

  // 按值查找元素，找到返回指定位置迭代器，找不到返回结束迭代器位置

  // beg 开始迭代器

  // end 结束迭代器

  // dest 目标起始迭代器

**示例：**

```
#include <algorithm>
#include <vector>

class myPrint
{
public:
	void operator()(int val)
	{
		cout << val << " ";
	}
};

void test01()
{
	vector<int> v1;
	for (int i = 0; i < 10; i++) {
		v1.push_back(i + 1);
	}
	vector<int> v2;
	v2.resize(v1.size());
	copy(v1.begin(), v1.end(), v2.begin());

	for_each(v2.begin(), v2.end(), myPrint());
	cout << endl;
}

int main() {

	test01();

	system("pause");

	return 0;
}
```

{% hint style="info" %}
总结：利用copy算法在拷贝时，目标容器记得提前开辟空间
{% endhint %}

### **2 replace**

**功能描述：**

* 将容器内指定范围的旧元素修改为新元素

**函数原型：**

* `replace(iterator beg, iterator end, oldvalue, newvalue);`

  // 将区间内旧元素 替换成 新元素

  // beg 开始迭代器

  // end 结束迭代器

  // oldvalue 旧元素

  // newvalue 新元素

**示例：**

```
#include <algorithm>
#include <vector>

class myPrint
{
public:
	void operator()(int val)
	{
		cout << val << " ";
	}
};

void test01()
{
	vector<int> v;
	v.push_back(20);
	v.push_back(30);
	v.push_back(20);
	v.push_back(40);
	v.push_back(50);
	v.push_back(10);
	v.push_back(20);

	cout << "替换前：" << endl;
	for_each(v.begin(), v.end(), myPrint());
	cout << endl;

	//将容器中的20 替换成 2000
	cout << "替换后：" << endl;
	replace(v.begin(), v.end(), 20,2000);
	for_each(v.begin(), v.end(), myPrint());
	cout << endl;
}

int main() {

	test01();

	system("pause");

	return 0;
}
```

{% hint style="info" %}
总结：replace会替换区间内满足条件的元素
{% endhint %}

### **3 replace\_if**

**功能描述:**

* 将区间内满足条件的元素，替换成指定元素

**函数原型：**

* `replace_if(iterator beg, iterator end, _pred, newvalue);`

  // 按条件替换元素，满足条件的替换成指定元素

  // beg 开始迭代器

  // end 结束迭代器

  // \_pred 谓词

  // newvalue 替换的新元素

**示例：**

```
#include <algorithm>
#include <vector>

class myPrint
{
public:
	void operator()(int val)
	{
		cout << val << " ";
	}
};

class ReplaceGreater30
{
public:
	bool operator()(int val)
	{
		return val >= 30;
	}

};

void test01()
{
	vector<int> v;
	v.push_back(20);
	v.push_back(30);
	v.push_back(20);
	v.push_back(40);
	v.push_back(50);
	v.push_back(10);
	v.push_back(20);

	cout << "替换前：" << endl;
	for_each(v.begin(), v.end(), myPrint());
	cout << endl;

	//将容器中大于等于的30 替换成 3000
	cout << "替换后：" << endl;
	replace_if(v.begin(), v.end(), ReplaceGreater30(), 3000);
	for_each(v.begin(), v.end(), myPrint());
	cout << endl;
}

int main() {

	test01();

	system("pause");

	return 0;
}
```

{% hint style="info" %}
总结：replace\_if按条件查找，可以利用仿函数灵活筛选满足的条件
{% endhint %}

### **4 swap**

**功能描述：**

* 互换两个容器的元素

**函数原型：**

* `swap(container c1, container c2);`

  // 互换两个容器的元素

  // c1容器1

  // c2容器2

  ​

**示例：**

```
#include <algorithm>
#include <vector>

class myPrint
{
public:
	void operator()(int val)
	{
		cout << val << " ";
	}
};

void test01()
{
	vector<int> v1;
	vector<int> v2;
	for (int i = 0; i < 10; i++) {
		v1.push_back(i);
		v2.push_back(i+100);
	}

	cout << "交换前： " << endl;
	for_each(v1.begin(), v1.end(), myPrint());
	cout << endl;
	for_each(v2.begin(), v2.end(), myPrint());
	cout << endl;

	cout << "交换后： " << endl;
	swap(v1, v2);
	for_each(v1.begin(), v1.end(), myPrint());
	cout << endl;
	for_each(v2.begin(), v2.end(), myPrint());
	cout << endl;
}

int main() {

	test01();

	system("pause");

	return 0;
}
```

{% hint style="info" %}
总结：swap交换容器时，注意交换的容器要同种类型
{% endhint %}


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