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vector container

Updated 2023-10-05About 9 min read

Existing English translations were reused. 1 current source blocks remain in Simplified Chinese, without triggering paid translation.

1.vector container

1.1.Basic concepts of vector

Function:

The vector data structure is very similar toan array and is also called asingle-ended array

Difference between vector and ordinary arrays:

The difference is that an array is static space, while a vector candynamically expand

Dynamic scaling:

It does not append a new space after the original space; instead, it finds a larger memory space, then copies the original data to the new space and releases the original space.

说明: 2015-11-10_151152

The iterator of a vector container is an iterator that supports random access.

1.2.vector constructor

Function description:

Create a vector container

Function prototype:

vector<T> v; //Implemented using a template class, default constructor
vector(v.begin(), v.end()); //Copies the elements in the range [begin(), end()) to itself.
vector(n, elem); //The constructor copies n copies of elem to itself.
vector(const vector &vec); //Copy constructor.

Example:

#include <iostream>
#include <vector>

using namespace std;

void printVector(vector<int>& v) {

	for (vector<int>::iterator it = v.begin(); it != v.end(); it++) {
		cout << *it << " ";
	}
	cout << endl;
}

void test01()
{
	vector<int> v1; //无参构造
	for (int i = 0; i < 10; i++)
	{
		v1.push_back(i);
	}
	printVector(v1);

	vector<int> v2(v1.begin(), v1.end());
	printVector(v2);

	vector<int> v3(10, 100);
	printVector(v3);
	
	vector<int> v4(v3);
	printVector(v4);
}

int main() {

	test01();


	return 0;
}

Operation result:

0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
100 100 100 100 100 100 100 100 100 100
100 100 100 100 100 100 100 100 100 100

**Summary:** The various construction methods of vector are not comparable; just use them flexibly.

1.3.vector assignment operation

Function description:

Assign values to a vector container

Function prototype:

vector& operator=(const vector &vec);// Overload the assignment operator
assign(beg, end); // Copy and assign the data in the interval [beg, end) to itself.
assign(n, elem); // Copy and assign n copies of elem to itself.

Example:

#include <iostream>
#include <vector>

using namespace std;

void printVector(vector<int>& v) {

	for (vector<int>::iterator it = v.begin(); it != v.end(); it++) {
		cout << *it << " ";
	}
	cout << endl;
}

//赋值操作
void test01()
{
	vector<int> v1; //无参构造
	for (int i = 0; i < 10; i++)
	{
		v1.push_back(i);
	}
	printVector(v1);

	vector<int>v2;
	v2 = v1;
	printVector(v2);

	vector<int>v3;
	v3.assign(v1.begin(), v1.end());
	printVector(v3);

	vector<int>v4;
	v4.assign(10, 100);
	printVector(v4);
}

int main() {

	test01();


	return 0;
}

Operation result:

0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
100 100 100 100 100 100 100 100 100 100

Summary: The vector assignment method is relatively simple; either using operator= or assign works.

1.4.vector capacity and size

Function description:

Operations on the capacity and size of a vector container

Function prototype:

empty(); // Determine whether the container is empty
capacity(); // Capacity of the container
size(); // Return the number of elements in the container
resize(int num); // Respecify the length of the container as num. If the container becomes longer, fill the new positions with the default value.

// If the container becomes shorter, the elements beyond the container length at the end are deleted.
resize(int num, elem); // Respecify the length of the container as num. If the container becomes longer, fill the new positions with the value elem.

// If the container becomes shorter, the elements beyond the container length at the end are deleted

Example:

#include <iostream>
#include <vector>

using namespace std;

void printVector(vector<int>& v) {

	for (vector<int>::iterator it = v.begin(); it != v.end(); it++) {
		cout << *it << " ";
	}
	cout << endl;
}

void test01()
{
	vector<int> v1;
	for (int i = 0; i < 10; i++)
	{
		v1.push_back(i);
	}
	printVector(v1);
	if (v1.empty())
	{
		cout << "v1为空" << endl;
	}
	else
	{
		cout << "v1不为空" << endl;
		cout << "v1的容量 = " << v1.capacity() << endl;
		cout << "v1的大小 = " << v1.size() << endl;
	}

	//resize 重新指定大小 ，若指定的更大，默认用0填充新位置，可以利用重载版本替换默认填充
	v1.resize(15,10);
	printVector(v1);

	//resize 重新指定大小 ，若指定的更小，超出部分元素被删除
	v1.resize(5);
	printVector(v1);
}

int main() {

	test01();


	return 0;
}

Operation result:

0 1 2 3 4 5 6 7 8 9
v1不为空
v1的容量 = 16
v1的大小 = 10
0 1 2 3 4 5 6 7 8 9 10 10 10 10 10
0 1 2 3 4

Summary:

Determine whether it is empty --- empty
Return the number of elements --- size
Return the container capacity --- capacity
Respecify the size --- resize

1.5.Vector insertion and deletion

Function description:

Insert and delete operations on vector containers

Function prototype:

push_back(ele); //Insert element ele at the end
pop_back(); //Delete the last element
insert(const_iterator pos, ele); //Insert element ele at the position pointed to by the iterator pos
insert(const_iterator pos, int count,ele);//Insert count copies of element ele at the position pointed to by the iterator pos
erase(const_iterator pos); //Delete the element pointed to by the iterator
erase(const_iterator start, const_iterator end);//Delete the elements between start and end pointed to by the iterators
clear(); //Delete all elements in the container

Example:

#include <iostream>

using namespace std;

#include <vector>

void printVector(vector<int>& v) {

	for (vector<int>::iterator it = v.begin(); it != v.end(); it++) {
		cout << *it << " ";
	}
	cout << endl;
}

//插入和删除
void test01()
{
	vector<int> v1;
	//尾插
	v1.push_back(10);
	v1.push_back(20);
	v1.push_back(30);
	v1.push_back(40);
	v1.push_back(50);
	printVector(v1);
	//尾删
	v1.pop_back();
	printVector(v1);
	//插入
	v1.insert(v1.begin(), 100);
	printVector(v1);

	v1.insert(v1.begin(), 2, 1000);
	printVector(v1);

	//删除
	v1.erase(v1.begin());
	printVector(v1);

	//清空
	v1.erase(v1.begin(), v1.end());
	v1.clear();
	printVector(v1);
}

int main() {

	test01();


	return 0;
}

Operation result:

10 20 30 40 50
10 20 30 40
100 10 20 30 40
1000 1000 100 10 20 30 40
1000 100 10 20 30 40

Summary:

Push back --- push_back
Pop back --- pop_back
Insert --- insert (position iterator)
Erase --- erase (position iterator)
Clear --- clear

1.6.Vector data access

Function description:

Access operations on data in vector

Function prototype:

at(int idx); //Return the data pointed to by index idx
operator[]; //Return the data pointed to by index idx
front(); //Return the first data element in the container
back(); //Return the last data element in the container

Example:

#include <iostream>
#include <vector>

using namespace std;

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

	for (int i = 0; i < v1.size(); i++)
	{
		cout << v1[i] << " ";
	}
	cout << endl;

	for (int i = 0; i < v1.size(); i++)
	{
		cout << v1.at(i) << " ";
	}
	cout << endl;

	cout << "v1的第一个元素为： " << v1.front() << endl;
	cout << "v1的最后一个元素为： " << v1.back() << endl;
}

int main() {
    // 程序从 main 函数开始执行，下面的语句会按顺序运行。

	test01();


	// 返回 0 表示程序正常结束。
	return 0;
}

Operation result:

0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
v1的第一个元素为： 0
v1的最后一个元素为： 9

Summary:

除了用迭代器获取vector容器中元素，[ ]和at也可以
front返回容器第一个元素
back返回容器最后一个元素

1.7.vector container swap

Function description:

Implement swapping of elements within two containers

Function prototype:

swap(vec); // Swap vec with its own elements

Example:

#include <iostream>
#include <vector>

using namespace std;

void printVector(vector<int>& v) {

	for (vector<int>::iterator it = v.begin(); it != v.end(); it++) {
		cout << *it << " ";
	}
	cout << endl;
}

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

	vector<int>v2;
	for (int i = 10; i > 0; i--)
	{
		v2.push_back(i);
	}
	printVector(v2);

	//互换容器
	cout << "互换后" << endl;
	v1.swap(v2);
	printVector(v1);
	printVector(v2);
}

void test02()
{
	vector<int> v;
	for (int i = 0; i < 100000; i++) {
		v.push_back(i);
	}

	cout << "v的容量为：" << v.capacity() << endl;
	cout << "v的大小为：" << v.size() << endl;

	v.resize(3);

	cout << "v的容量为：" << v.capacity() << endl;
	cout << "v的大小为：" << v.size() << endl;

	//收缩内存
	vector<int>(v).swap(v); //匿名对象

	cout << "v的容量为：" << v.capacity() << endl;
	cout << "v的大小为：" << v.size() << endl;
}

int main() {

	test01();

	test02();


	return 0;
}

Operation result:

0 1 2 3 4 5 6 7 8 9
10 9 8 7 6 5 4 3 2 1
互换后
10 9 8 7 6 5 4 3 2 1
0 1 2 3 4 5 6 7 8 9
v的容量为：131072
v的大小为：100000
v的容量为：131072
v的大小为：3
v的容量为：3
v的大小为：3

Summary: swap can exchange two containers, which can achieve a practical effect of shrinking memory.

1.8.vector reserve space

Function description:

Reduce the number of expansions when a vector dynamically expands its capacity.

Function prototype:

reserve(int len);// The container reserves a length of len elements. The reserved positions are not initialized, and the elements are inaccessible.

Example:

#include <iostream>
#include <vector>

using namespace std;

void test01()
{
	vector<int> v;

	//预留空间
	v.reserve(100000);

	int num = 0;
	int* p = NULL;
	for (int i = 0; i < 100000; i++) {
		v.push_back(i);
		if (p != &v[0]) {
			p = &v[0];
			num++;
		}
	}

	cout << "num:" << num << endl;
}

int main() {

	test01();
    

	return 0;
}

Operation result:

num:1

Summary: If the data volume is large, you can use reserve to preallocate space from the start.