Opgaveløsning:
Din egen udgave af funktionen strcmp

#include <stdio.h>

int strcmp(const char *str1, const char *str2);
void pr(char *s, char *t, int r);

int main(void) {

  int b1 = strcmp("book", "shelf");    
  int b2 = strcmp("shelf", "book");    
  int b3 = strcmp("","book");
  int b4 = strcmp("book","bookshelf"); 
  int b5 = strcmp("book", "book");
  int b6 = strcmp("7book", "book");
  int b7 = strcmp("BOOK", "book");

  pr("book", "shelf", b1);
  pr("shelf", "book", b2);
  pr("", "book", b3);
  pr("book", "bookshelf", b4);
  pr("book", "book", b5);
  pr("7book","book", b6);
  pr("BOOK", "book", b7);
  
  return 0;
}

void pr(char *s, char *t, int r){
 printf("strcmp(\"%s\",\"%s\") = %i\n", s,t,r);
}


/* Return the string compare value of str1 and str2. 
   As a precondition, neither of the strings are assumed to be NULL */
int strcmp(const char *str1, const char *str2){
  if (str1[0] == '\0' && str2[0] == '\0')         /* Both empty strings */
     return 0;
  else if (str1[0] != '\0' && str2[0] == '\0')    /* str1 non-empty, str2 is empty */
     return 1;
  else if (str1[0] == '\0' && str2[0] != '\0')    /* str2 non-empty, str1 is empty */
     return -1;
  else if (str1[0] < str2[0])                     /* first char of str1 is smaller than first char of str2 */
     return -1;
  else if (str1[0] > str2[0])                     /* first char of str2 is smaller than first char of str1 */
     return 1;
  else                                            /* (str1[0] == str2[0]). Recur! */                
     return strcmp(str1+1, str2+1);
}