The use of vector in C++

In C++, vector is a very useful container.

Function: It can store various types of objects like a container. Simply put, a vector is a dynamic array that can store any type, and can increase and compress data.

Vector is part of the C++ Standard Template Library, which is a multi- functional library of template classes and functions that can manipulate a variety of data structures and algorithms.

 

pay attention:

There are a few things to keep in mind when using vectors:

1. If the length of the vector you want to represent is long (you need to save a lot of numbers inside the vector), it is easy to cause memory leaks, and the efficiency will be very low;

2. When Vector is used as a parameter or return value of a function, you need to pay attention to its writing:

   double Distance(vector<int>&a, vector<int>&b) The "&" in it must not be less! ! !

 

Example: vector<int>test;

//Create a vector, int is the data type of the array element, test is the dynamic array name

The simple usage is as follows:

vector<int>test;//Create a vector

test.push_back(1);

test.push_back(2);//Push 1 and 2 into the vector, so test[0] is 1, test[1] is 2

 

Examples I have seen:

vector<vector<Point2f> > points; //Define a two-dimensional array

points[0].size(); //refers to the number of columns in the first row

1. Basic operation

(1) Header file #include<vector>.

(2) Create a vector object, vector<int> vec;

(3) Insert numbers at the end: vec.push_back(a);

(4) Use subscripts to access elements, cout<<vec[0]<<endl; remember that subscripts start from 0.

(5) Use an iterator to access elements.

vector<int>::iterator it;

for(it=vec.begin();it!=vec.end();it++)

    cout<<*it<<endl;

(6) Insert elements: vec.insert(vec.begin()+i,a); insert a before the i+1th element;

(7) Delete element: vec.erase(vec.begin()+2); delete the third element

vec.erase(vec.begin()+i,vec.end()+j); delete interval [i,j-1]; interval starts from 0

(8) Vector size: vec.size();

(9) Clear: vec.clear();

 

2. Important Notes

The elements of the vector can not only be int, double, string, but also a structure, but pay attention: the structure must be defined as global, otherwise an error will occur.

#include<stdio.h>  
#include<algorithm>  
#include<vector>  
#include<iostream>  
using namespace std;  
  
typedef struct rect  
{  
    int id;  
    int length;  
    int width;  
  
　　// For the vector element is a structure, a comparison function can be defined inside the structure, and the following is sorted in ascending order by id, length, and width.  
　　bool  operator < ( const rect &a)   const  
    {  
        if(id!=a.id)  
            return id<a.id;  
        else  
        {  
            if(length!=a.length)  
                return length<a.length;  
            else  
                return width<a.width;  
        }  
    }  
}Rect;  
  
intmain ()  
{  
    vector<Rect> vec;  
    Rect rect;  
    rect.id=1;  
    rect.length=2;  
    rect.width=3;  
    vec.push_back(rect);  
    vector<Rect>::iterator it=vec.begin();  
    cout<<(*it).id<<' '<<(*it).length<<' '<<(*it).width<<endl;      
  
return 0;  
  
}