I have to convert this into OOP style. How will I start?

#include<stdio.h>   
#include<stdlib.h>   
#include<string.h>   
#include<conio.h>   
#include<ctype.h>   
#include<dos.h>   
#include<graphics.h>   
#define true  1   
#define false 0   

void disp();   
void printmenu();
void intro();   
void levelmenu();
void easy();   
void initial(int x);   
void draw(int x, int y, int i);   
void userwin(int no);
int check(int key);   
int load,i,step,level,num[25],win;   

void main()   
{     int gdriver = DETECT, gmode, errorcode;   
      initgraph(&gdriver, &gmode, "..\\bgi");   
      intro();   
      levelmenu();   
      clrscr();   
      cleardevice();   
      setbkcolor(RED);   
      easy();   
}  
 
void levelmenu()   
{  level=0;   
   clrscr();   
   cleardevice();   
   setbkcolor(BLACK);   
   settextstyle(1,0,2);   
   setcolor(GREEN);   
   outtextxy(240,240,"Press 1 to start");   
   outtextxy(240,260,"----------------");   
   outtextxy(240,280,"1. Start Game");   
   outtextxy(240,300,"2. Game Level");   
   outtextxy(240,330,"Enter a choice: ");   
   gotoxy(50+4,22);   
   scanf("%d", &level);   
   if(level==2)   
      {   
      outtextxy(20,40,"Read the rules :");   
      outtextxy(20,80,"1: Arrange the nos in ascending order");   
      outtextxy(20,120,"2: Press the no key to move the nos");   
      outtextxy(20,160,"3: X-Exit");   
      delay(3000);   
      levelmenu();   
      }   
}   

void intro()   
{  int i,j;   
   clrscr();   
   cleardevice();   
   setbkcolor(BLACK);   
   gotoxy (25,12);   
   settextstyle(4,0,6);   
   for(i=0;i<=10;i++)   
   {   
   setcolor(i%18);   
   outtextxy(2,2,"NUMBER PUZZLE GAME");   
   settextstyle(4,0,8);   
   outtextxy(20,200,"");   
   delay(100);   
   outtextxy(180,200,"1");   
   delay(100);   
   outtextxy(220,200,"2");   
   delay(100);   
   outtextxy(260,200,"3");   
   delay(100);   
   outtextxy(180,300,"4");   
   delay(100);   
   outtextxy(220,300,"5");   
   delay(100);   
   outtextxy(260,300,"6");   
   delay(100);   
   outtextxy(180,400,"7");   
   delay(100);   
   outtextxy(220,400,"8");   
   settextstyle(4,0,8);   
   delay(100);   
}   
}  
 
void printmenu()   
{   
   int i;   
   gotoxy (33,1);printf("NUMBER PUZZLE GAME");   
   gotoxy (1,2);   
   for(i=0; i<80; i++)   
   printf("\xcd");   
   printf("\t\t\t\tX=exit game\n");   
   for(i=0; i<80; i++)   
   printf("\xcd");   
   gotoxy (35,23);printf("Moves = %d", step);   
} 
  
void easy()   
{   
   int i, x, y,key,valid;   
   char press[2];   
   if(load==false)   
 initial(9);   
   else  
 load=false;   
   do{   
 start:   
 printmenu();   
 i=0;   
 for(y=8; y<17; y+=4)   
 for(x=33; x<44; x+=5)   
 {   
  draw(x,y,i);   
  i++;   
 }   
 userwin(8);   
 if(win==true)   
      {  cleardevice();   
  outtextxy(2,2,"You have won the game");   
   delay(2000);   
   disp();   
   delay(2000);   
   win=false;   
  return;   
      }   
      gotoxy (55,17);printf("Tips:");   
      gotoxy (61,18);printf("Number will move");   
      gotoxy (61,19);printf("when you key in!");   
      gotoxy (43,23);fflush(stdin);   
      press[0]=toupper(getche());   
      if(press[0]=='X')   
      {   
      disp();   
      exit(1);   
      }key=atoi(press);   
      if((valid=check(key))==false)   
 goto start;   
      step++;   
      if(step==50)   
      {   
      cleardevice();   
      outtextxy(20,200,"YOU HAVE SURPASSED MAXIMUM POSSIBLE MOVES");   
      delay(1500);   
      disp();   
      delay(2500);   
      exit(1);   
      }   
      for(i=0; i<9; i++)   
      {   
      if(num[i]==key)   
      num[i]=0;   
      else  
      if(num[i]==0)   
      num[i]=key;   
      }   
      }while(true);   
}   
    
void initial(int x)   
{      int i,no;   
 step=0;   
 no=0%x;   
 if(no==1)   
 no+=5;   
 for(i=0; i<x; i++)   
   {   
 num[i]=no;   
 no+=2;   
 if(no>=x)   
 if(!(no%2))   
 no=1;   
  else  
 no=0;   
   }   
}  
 
void draw(int x, int y, int i)   
{      gotoxy (x,y);   
   if(level==1)   
   {   
 printf("%c%c%c",201,205,187);   
 gotoxy (x,y+1);   
 printf("%c",186);   
 if(num[i]==0)   
 printf(" ");   
 else  
 printf("%d", num[i]);   
 printf("%c",186);   
 gotoxy (x,y+2);   
 printf("%c%c%c",200,205,188);   
   }   
}  
 
void disp()   
{  cleardevice();   
   setbkcolor(BLACK);   
   settextstyle(4,0,4);   
   setcolor(RED);   
   outtextxy(200,100,"CREDITS:");   
   settextstyle(2,0,13);   
   outtextxy(300,220,"IVORY ESQUINAS");   
   outtextxy(300,260,"RONALD GANADIN");   
   outtextxy(300,300,"BSCS 4-1");   
   delay(2500);   
} 
  
int check(int key)   
{   
   int i, valid=false;   
   if(level==1)   
   for(i=0; i<9; i++)   
   {   
 if(num[i]==key)   
      {   
      valid=true;   
      switch(i)   
      {   
      case 0: if(num[1]!=0 && num[3]!=0)valid=false;break;   
      case 1: if(num[0]!=0 && (num[2]!=0 && num[4]!=0))   
       valid=false;break;   
      case 2: if(num[1]!=0 && num[5]!=0)valid=false;break;   
      case 3: if(num[0]!=0 && (num[4]!=0 && num[6]!=0))   
       valid=false;break;   
      case 4: if((num[1]!=0 && num[3]!=0) &&   
       (num[5]!=0 && num[7]!=0))valid=false;break;   
      case 5: if(num[2]!=0 && (num[4]!=0 && num[8]!=0))   
       valid=false;break;   
      case 6: if(num[3]!=0 && num[7]!=0)valid=false;break;   
      case 7: if(num[4]!=0 && (num[6]!=0 && num[8]!=0))   
       valid=false;break;   
      case 8: if(num[5]!=0 && num[7]!=0)valid=false;break;   
      default:valid=false;break;   
       }   
      }   
   }   
   return valid;   
} 
  
void userwin(int no)   
{       int i;   
 for(i=0; i<no; i++)   
 if(num[i]!=i+1)   
 {   
 win=false;   
 break;   
 }   
       else  
 win=true;   
}

Start by deciding what the objects should be, then decide what data these objects need to hold and what methods they need to have and how different types of objects of relate to one another. From there you'll decide what classes are needed and make each different object type into a class, each with data members and methods. Don't jump into changing the code. Think about the organization first, put it on paper, make some diagrams, etc., before changing the code.

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