ELS displays a random square

1、 Function implementation （els.c）

#include"els_h.h"
// Background array
char g_arrBackGroud[20][10] = {0};
char g_arrSqare[2][4] = {0};

void OnPaint(HDC hDc)
{
    // Create compatibility DC.
    HDC hMemDC = CreateCompatibleDC(hDc);// Memory id by ：HMemDc, window id hDc.

    // Create compatibility bitmap .
    HBITMAP hBitmapBack= CreateCompatibleBitmap(hDc, 500, 600);

    // Connect .
    SelectObject(hMemDC, hBitmapBack);

    PaintSqare(hMemDC);
    
    // Pass on ：
    // Return value ： Failure to return 0, Successfully returned non-zero .
    // Parameters 1： The goal is DC, window DC
    // Parameters 2,3： The starting position of the target , Attention is based on our window .
    // Parameters 4,5： The size of the area .
    // Parameters 6： Source DC, That is, our memory DC.
    // Parameters 7,8： The starting position of the memory picture .
    // Parameters 9： delivery .
    //
    BitBlt(hDc, 0, 0, 300, 600, hMemDC, 0, 0, SRCCOPY);

    // Release DC
    DeleteObject(hBitmapBack);
    DeleteDC(hMemDC);
    
    
}
void Oncreate()
{
    srand((unsigned int)time(NULL));// Only once .
    CreateRandomSqare();
    CopySqareToBack();
}
void PaintSqare(HDC hMemDC)
{
    int i = 0,
        j = 0;

    // Draw a big square .
    Rectangle(hMemDC, 0, 0, 300, 600);// The coordinates of the starting position and the ending position of the rectangular box from the running window .
    
    // Specify a square .
    /*g_arrBackGroud[2][4] = 1;
    g_arrBackGroud[3][3] = 1;
    g_arrBackGroud[3][4] = 1;
    g_arrBackGroud[3][5] = 1;*/

    // Traverse
    for (i=0 ; i < 20; i++)
    {
        for (j=0 ; j < 10; j++)
        {
            if (1 == g_arrBackGroud[i][j])
            {
                // Draw squares .
                Rectangle(hMemDC, j*30, i*30, j*30 + 30, i*30 + 30);// The coordinates of the starting position and the ending position of the rectangular box from the running window .

            }
        }
    }

}
// Create random block .
int CreateRandomSqare()
{
    int nIndex=rand()%7;
    switch (nIndex)
    {
    case 0:
        g_arrBackGroud[0][0] = 1, g_arrBackGroud[0][1] = 1, g_arrBackGroud[0][2] = 0, g_arrBackGroud[0][3] = 0;
        g_arrBackGroud[1][0] = 0, g_arrBackGroud[1][1] = 1, g_arrBackGroud[1][2] = 1, g_arrBackGroud[1][3] = 0;
        break;
    case 1:
        g_arrBackGroud[0][0] = 0, g_arrBackGroud[0][1] = 1, g_arrBackGroud[0][2] = 1, g_arrBackGroud[0][3] = 0;
        g_arrBackGroud[1][0] = 1, g_arrBackGroud[1][1] = 1, g_arrBackGroud[1][2] = 0, g_arrBackGroud[1][3] = 0;

        break;
    case 2:
        g_arrBackGroud[0][0] = 1, g_arrBackGroud[0][1] = 0, g_arrBackGroud[0][2] = 0, g_arrBackGroud[0][3] = 0;
        g_arrBackGroud[1][0] = 1, g_arrBackGroud[1][1] = 1, g_arrBackGroud[1][2] = 1, g_arrBackGroud[1][3] = 0;

        break;
    case 3:
        g_arrBackGroud[0][0] = 0, g_arrBackGroud[0][1] =0 , g_arrBackGroud[0][2] = 1, g_arrBackGroud[0][3] = 0;
        g_arrBackGroud[1][0] = 1, g_arrBackGroud[1][1] = 1, g_arrBackGroud[1][2] = 1, g_arrBackGroud[1][3] = 0;

        break;
    case 4:
        g_arrBackGroud[0][0] = 0, g_arrBackGroud[0][1] = 1, g_arrBackGroud[0][2] = 0, g_arrBackGroud[0][3] = 0;
        g_arrBackGroud[1][0] = 1, g_arrBackGroud[1][1] = 1, g_arrBackGroud[1][2] = 1, g_arrBackGroud[1][3] = 0;

        break;
    case 5:
        g_arrBackGroud[0][0] = 0, g_arrBackGroud[0][1] = 1, g_arrBackGroud[0][2] = 1, g_arrBackGroud[0][3] = 0;
        g_arrBackGroud[1][0] = 0, g_arrBackGroud[1][1] = 1, g_arrBackGroud[1][2] = 1, g_arrBackGroud[1][3] = 0;

        break;
    case 6:
        g_arrBackGroud[0][0] = 1, g_arrBackGroud[0][1] = 1, g_arrBackGroud[0][2] = 1, g_arrBackGroud[0][3] = 1;
        g_arrBackGroud[1][0] = 0, g_arrBackGroud[1][1] = 0, g_arrBackGroud[1][2] = 0, g_arrBackGroud[1][3] = 0;

        break;
    
    }
    return nIndex;
}
void  CopySqareToBack()
{
    int i = 0,
        j = 0;
    for (i = 0; i < 2; i++)
    {
        for (j = 0; j < 4; j++)
        {
            g_arrBackGroud[i][j + 3] = g_arrSqare[i][j];
        }
    }

}

2、 Head function （els.h）

#ifndef N_d
#define N_d

#include<Windows.h>
#include<time.h>
void OnPaint(HDC hDc);
// Display box .
void PaintSqare(HDC hMemDC);
// Generate random blocks .
int CreateRandomSqare();
// Random blocks close to the background .
void  CopySqareToBack();
void Oncreate();

#endif