How vectors are used in C + +

Original address: http://blog.csdn.net/duan19920101/article/details/50617190/

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

Function: It can store various types of objects like a container, in short, a vector is a dynamic array that can hold any type and can add and compress data.

Vector in the C + + Standard Template Library part of the content, it is a multi-functional, able to operate a variety of data structures and algorithms of the template class and function library.

Special attention:

There are a few things to note about using vectors:

1, if you want to represent the length of the vector is very long (need to maintain a large number of vectors inside), prone to memory leaks, and inefficient;

2,vector as a function of the parameter or return value, you need to pay attention to its wording :

Double Distance (Vector<int>&a, Vector<int>&b) One of the "&" absolutely no less!!!

Example:vector<int>test;

Establishes a Vector,int data type for an array element, and test is a dynamic array name

The simple way to use it is as follows:

Build a vector

vector<int>test;

Press 1 and 2 into the vector .

Test.push_back (1);

Test.push_back (2);//At this time the dynamic array test[0] is 1,test[1] is 2

Examples of what you see:

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 vector object,vector<int> VEC;

(3) Insert number at tail: vec.push_back (a);

(4) Use subscript to access the element,cout<<vec[0]<<endl; remember that the subscript is starting from 0.

(5) Use iterators to access elements.

Vector<int>::iterator it;

For (It=vec.begin (); It!=vec.end (); it++)

cout<<*it<<endl;

(6) Insert element: Vec.insert (Vec.begin () +i,a); Insert a in front of the first I+1 element;

(7) Delete element: Vec.erase (Vec.begin () +2); Delete 3rd element

Vec.erase (Vec.begin () +i,vec.end () +j); Delete interval [i,j-1]; interval starting from 0

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

(9) Empty: Vec.clear ();

Special Note: There is a difference between the Begin () and the end () function, front () and Back ()

2. Important Notes

The elements of a vector can be not only int,double,string, but also structs, but note that the structure is defined as global, otherwise it will go wrong.

#include <stdio.h>#include<algorthm>#include<vector>#include<iostream>using namespaceStd;typedefstructrect{intID; intlenght; intwidth; //for vector elements that are structural, you can define a comparison function within the structure, sorted by ID, lenght, Width ascending    BOOL operator< (ConstRect &a)Const    {        if(id! =a.id)returnID <a.id; Else        {            if(Lenght! =a.lenght)returnLenght <A.lenght; Else                returnWidth <A.width; }}}rect;intMainvoid) {vector<Rect>VEC;    Rect rect; Rect.id=1; Rect.lenght=2; Rect.width=3;    Vec.push_back (rect); Vector<rect>::iterator it =Vec.begin (); cout<< (*it) .id<<' '<< (*it) .lenght<<' '<< (*it) .width<<Endl; return 0;}

3. Algorithm

(1) Flip the element with reverse: Requires a header file #include<algorithm>

Reverse (Vec.begin (), Vec.end ()); Flips the elements, that is, in reverse order!

(in Vector, if two iterators are required in a function, they are not usually included in the latter one)

(2) Sorting by using sort: Requires header file #include<algorithm>

Sort (Vec.begin (), Vec.end ());(by default in ascending order, that is, from small to large).

You can compare functions in descending order by overriding the sort comparison, as follows:

To define a sort comparison function:

BOOL Comp (const int &A,CONST int &b)
{
Return a>b;
}
Called When: Sort (Vec.begin (), Vec.end (), Comp), so that it is sorted in descending order.

3.1 elements in the output vector

Vector<float> Vecclass;

int nSize = Vecclass.size ();

vector<float>Vecclass;intNSize =vecclass.size ();//Method 1: for(inti =0; i< nSize; i++) {cout<<vecClass[i]<<" ";} cout<<Endl;//Method 2: for(inti =0; i < nSize; i++) {cout<<vecclass.at (i) <<" ";} cout<<Endl;//Method 3: for(vector<float>::iterator it = Vecclass.begin (); It! = vecclass.end;it++) {cout<<*it<<" ";} cout<<endl;

It is important to note that when you output from method one, the subscript of the array must be guaranteed to be an integer. method Three Span style= "COLOR: #0000ff" > It is inconvenient to output a specified value

Application in a two-dimensional array:

#include"stdafx.h"#include<cv.h>#include<vector>#include<iostream>using namespacestd;intMainvoid){    //using namespace std;    int  out[3][2] = {1,2,                    3,4,                    5,6}; Vector<int*>v1; V1.push_back ( out[0]); V1.push_back ( out[1]); V1.push_back ( out[2]); cout<<v1[0][0]<<Endl; cout<<v1[0][1]<<Endl; cout<<v1[1][0]<<Endl; cout<<v1[1][1]<<Endl; cout<<v1[2][0]<<Endl; cout<<v1[2][]<<Endl; return 0;}

How vectors are used in C + + (GO)