#include <iostream>
#include<string.h>
#include<stdio.h>
using namespace std;
int main(){
// comment can be writen starting with "//"
  int i;
  char c1[16]="Hello ";
  char c2[16]="World!";
  char c3[16];
  char c4[64];
  cout <<c1 << endl;
  strcat(c1,c2); 
  cout <<c1 << endl;
  for(i=0;i<10;i++){
    sprintf(c3,"%d",i);
    //substitute i to c3 as a character.
    sprintf(c4,"%s%s_%d.dat",c1,c2,i);
    //sprintf can have more than one paramters
    printf("%d\t%s\n",i,c3);
    //same as cout << i << "\t" << c3 << endl;
  }
  return 0;
}