Learning from YayoDrayber, I am sharing my CodeWars Kata.

DESCRIPTION:

Given a string, you have to return a string in which each character (case-sensitive) is repeated once.

Examples (Input -> Output):

* "String" -> "SSttrriinngg"

* "Hello World" -> "HHeelllloo WWoorrlldd"

* "1234!_ " -> "11223344!!__ "

 

Starting code

char *double_char (const char *string, char *doubled)
{
  *doubled = '\0'; // write to doubled

  return doubled; // return it
}

My attempt

char *double_char (const char *string, char *doubled)
{
  char *writing_head = doubled;
  char *reading_head = string - 1;
  while (*++reading_head) {
    *writing_head++ = *reading_head;
    *writing_head++ = *reading_head;
  }
  *writing_head = '\0';

  return doubled; // return it
}

Best Practice and Clever

char *double_char (const char *string, char *doubled)
{
  int k=0,i=0;
  while (string[i]!='\0'){
    doubled[k++]=string[i];
    doubled[k++]=string[i++];
  }
  doubled[k]='\0';
  return doubled;
}

Using index like we would in other languages is a clear way to do it.

My Pick

char *double_char (const char *string, char *doubled) {
  char *result = doubled;
  while(*string){
    *doubled++ = *string;
    *doubled++ = *string++;
  }
  *doubled = '\0'; // write to doubled
  return result; // return it
}

So, you can use the input string pointer.

My post-Kata snippet

char *double_char (const char *string, char *doubled)
{
  char *d = doubled;
  while (*string) {
    *d++ = *string;
    *d++ = *string++;
  }
  *d = '\0';

  return doubled; // return it
}

My code snippets on GitHub