mine

#include<stdio.h>

int double_string (char *x)
{
  int i = 0;
  while x != '\0';

}
int swap_letters(char replacer, char letter, char *string)
{

}

int main()
{
  char str[1024] = "Now I know how to program in C!";
  printf(*x)
  printf(str);
  printf("\n");
  double_string(str);
  printf(str);
  swap_letters(*'o', *'i', str);
  return 0;
}