This article describes the C++ dynamic array class (std::vector), which extends ordinary arrays in a more convenient way and can easily handle variable-length arrays.

What is std::vector?

std::vector is a class that allows you to freely change the length of an array. With ordinary arrays, the size of the array must be specified at the beginning, and it is not possible to change the size of the array later. For example, int a[10] and the size of the array must also be declared at the beginning, and the size of the array cannot be changed later. However, std::vector can be used to make this possible. It is also much more convenient than ordinary arrays in that it can refer to the number of array elements, etc.

How to use std::vector (from include to array declaration)

Include~vector declaration

To use std::vector, you need to include it as follows. In this article, the namespace std is specified in order to write "vector" from std::vector. When declaring a vector, declare it as vector<type name> variable name. At this time, when specifying the size of the array in advance, it is written as vector<type name> variable name (size). To initialize the array to a specified value, use vector<type name> variable name (size, value to be initialized).

#include <vector>
using namespace std;//Declare namespace
int main() {
    vector<int> x;//If no namespace is specified, then std::vector<int> x;
    vector<int> x(3);
    vector<double> x(3,3.14);
}

Declaration of two-dimensional arrays

To declare a two-dimensional array using vector, write vector<vector<type name>> variable name (size, value to be initialized). The last line vector<vector<int>>> xy(3, vector<string>(3, "abc")); stores all the letters "abc" in a 3 x 3 matrix as shown in the table below.

vector<vector<string>> xy;
vector<vector<string>> xy(3);
vector<vector<string>> xy(3, vector<string>(3));
vector<vector<string>> xy(3, vector<string>(3, "abc"));

How to use std::vector (array processing)

Value References & Assignments

Once a vector has been declared for referencing and assigning values, the rest can be done in the same way as for ordinary array processing.

タイトル:vector1.cpp

#include<iostream>
#include<string>
#include <vector> 
using namespace std;
 
int main() {
   vector<vector<string>> xy(3, vector<string>(3,"abc"));
   cout<< xy[1][1]<< endl;
   xy[1][1]="def";
   cout<< xy[1][1]<< endl; 
   return 0;
}

タイトル:output

abc
def

Browse & change the number of elements

To refer to the number of elements in a vector, use variable name.size(). In the case of a two-dimensional array, variable name.size() can be used to obtain the number of rows, and variable name[0].size() can be used to obtain the number of columns. The number of array elements can also be changed by setting variable.resize(number of elements to be changed).

タイトル:vector1.cpp

#include<iostream>
#include<string>
#include <vector> 
using namespace std;
 
int main() {
    vector<string> x(3, "abc");//1*3
    vector<vector<string>> xy(4, vector<string>(3,"abc"));//4*3

    cout << x.size() << endl; //3
    x.resize(5);//Changed to 5
    cout << x.size() << endl; //5

    cout<< xy.size()<< endl; //4
    cout<< xy[1].size()<< endl;//3

    xy.resize(5); //Changed to 5*5
    for(int i=0; i<5; i++){
        xy[i].resize(5);
    }
    cout<< xy.size() << endl; //5
    cout<< xy[0].size() << endl; //5
    
    return 0;
}

タイトル:output

3
5
4
3
5
5

Adding an element to the end of an array

It is also possible to add an element to the end of an array by using vector's push_back(). For example, x.push_back(3) will add the number 3 to the end of the array x.

タイトル:pushback1.cpp

#include<iostream>
#include<string>
#include <vector> 
using namespace std;
 
int main() {
    vector<string> x(3, "abc");//1*3
    x.push_back("def");
    for(int i=0;i<x.size();i++){
        cout << x[i] << endl;
    }
    return 0;
}

タイトル:output

abc
abc
def

The addition of an element to the tail of a two-dimensional array is described as follows.

タイトル:pushback2.cpp

#include<iostream>
#include<string>
#include <vector> 
using namespace std;
 
int main() {
    vector<vector<string>> xy(3, vector<string>(3,"abc"));//3*3
    
    xy[0].push_back("def");
    for(int i=0;i<xy.size();i++){
        for(int j=0;j<xy[i].size();j++){
            cout << xy[i][j]<< " ";
        }
       cout << endl;
    }
    return 0;
}

タイトル:output

abc abc abc def 
abc abc abc 
abc abc abc 

Deleting Elements

In vector, you can delete elements at specified locations by using the variable name.erase (specified location).

タイトル:erase.cpp

#include<iostream>
#include<string>
#include <vector> 
using namespace std;
 
int main() {
    
    vector<string> x={"abc","def","ghi"};//1*3

    x.erase(x.begin()+1);//+1st from the top
    for(int i=0;i<x.size();i++){
        cout << x[i] << " ";
    }
    cout << endl;
    x.erase(x.end()-1);//end
    for(int i=0;i<x.size();i++){
        cout << x[i] << " ";
    }
    cout << endl;
    return 0;
}

タイトル:output

abc ghi 
abc 

Conclusion

This article introduced the usage of std::vector. Here is a summary of the main points.

• std::vector is a C++ dynamic array class that extends ordinary arrays to be more convenient
• Once declared, it can be treated as an ordinary array.
• Allows referencing and changing the number of array elements
• Elements can be added to the end of the array
• Allows deletion of elements at specified locations

Please try using C++ std::vector, a more convenient extension of ordinary arrays!