strs

#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>

typedef struct _line
{
  char data[80];
  struct _line *next, *prev;
} line;

/* squeeze spaces from string */
void strsqueeze(char* str)
{
  char tmp[80];
  size_t i, j = 0;

  for(i = strspn(str, " "); i < strlen(str); i++)
  {
    if(str[i] == ' ' && str[i + 1] == ' ')
    {
      continue;
    }
    tmp[j++] = str[i];
  }
  tmp[j] = '\0';
  strcpy(str, tmp);
}

/* get list element at rank */
line* atrank(line* lines, const size_t rank)
{
  line* pt = lines->next;
  size_t i;

  for(i = 0; i < rank; i++)
  {
    if(!pt)
    {
      return NULL;
    }
    pt = pt->next;
  }
  return pt;
}

/* entry point */
int main()
{
  line *lines, *pl, *pltmp;
  size_t i = 0;
  char *a, *b, c;

  lines = (line*)malloc(sizeof(line));
  memset(lines, 0, sizeof(lines));

  /* read data */
  pl = lines;
  do
  {
    pltmp = (line*)malloc(sizeof(line));
    memset(pltmp, 0, sizeof(pltmp));
    pl->next = pltmp;
    pltmp->prev = pl;
    pl = pl->next;
    gets(pltmp->data);
  } while(!strstr(pltmp->data, "..."));

  /* squeeze spaces */
  pl = lines;
  while(pl)
  {
    strsqueeze(pl->data);
    pl = pl->next;
  }

  /* remove blanks */
  pl = lines;
  while(pl)
  {
    if(!strlen(pl->data))
    {
      if(pl->prev)
      {
        pl->prev->next = pl->next;
      }
      if(pl->next)
      {
        pl->next->prev = pl->prev;
      }
      pltmp = pl->next;
      free((void*)pl);
      pl = pltmp;
    }
    else
    {
      pl = pl->next;
    }
  }

  /* concatenate first and second strings */
  strcat(atrank(lines, 0)->data, atrank(lines, 1)->data);

  /* copy the 7th string into the 5th */
  strcpy(atrank(lines, 6)->data, atrank(lines, 4)->data);

  /* substutute lowercase letters after .!? to uppercase */
  pl = lines;
  while(pl)
  {
    a = pl->data;
    while((i = strcspn(a, ".!?")) < strlen(a))
    {
      a += i;
      while(!isalnum(*a))
      {
        a++;
      }
      *a = toupper(*a);
    }
    pl = pl->next;
  }

  /* substutute chars from set by * in str #3 */
  i = 0;
  pl = atrank(lines, 2);
  while(1)
  {
    i = strcspn(pl->data, "abc");
    if(i >= strlen(pl->data))
    {
      break;
    }
    pl->data[i] = '*';
  }

  /* swap case of the second rear entry of the specified character in str 4 */
  pl = atrank(lines, 3);
  a = strrchr(pl->data, 'a');
  if(a)
  {
    c = *a;
    *a = '\0';
    b = strrchr(pl->data, 'a');
    if(b)
    {
      if(islower(*b))
      {
        *b = toupper(*b);
      }
      else
      {
        *b = tolower(*b);
      }
    }
    *a = c;
  }

  /* print lines */
  i = 0;
  pl = lines->next;
  while(pl)
  {
    printf("%d: %s\n", (int)i, pl->data);
    pl = pl->next;
    i++;
  }

  /* free mem */
  pl = lines;
  while(pl)
  {
    pltmp = pl->next;
    free((void*)pl);
    pl = pltmp;
  }

  return 0;
}