6 – Lists

1. Objectives of these slides: to describe linked lists in C 6 – Lists

2. Overview 1. List Data Structures and Operations 2. List Implementations 3. Dynamically Created Lists 4. Converting a String to a List 5. List Functions

3. 1. List Data Structures and Operations • Some possible operations: • create/destroy a list • test to see if a list is empty • return the tail of the list • insert/delete elements • print a list • calculate the length of a list

4. 2. List implementations Version 1: #define N 1000 /* the size of the list */typedef char LIST[N];LIST lt; /* same as char lt[N] */

5. elem data nextptr Version 2 struct listnode { char data; struct listnode *nextptr;};typedef struct listnode LISTNODE;LISTNODE elem;

6. a b c Use LISTNODE a, b, c;a.data = 'a';b.data = 'c';c.data = 'e';a.nextptr = b.nextptr = c.nextptr = NULL; continued

7. a b c ‘a’ ‘c’ ‘e’ NULL a.nextptr = &b;b.nextptr = &c;printf("%c", a.nextptr->data);/* 'c' printed */printf("%c", a.nextptr->nextptr->data);/* 'e' printed */

8. head ‘n’ NULL 3. Dynamically Created Lists /* list implementation as before */typedef LISTNODE *LNP;LNP head = NULL;head = malloc(sizeof(LISTNODE));head->data = 'n';head->nextptr = NULL; Function prototype instdlib.h: void *malloc(size_t size);

9. head ‘n’ ‘e’ NULL Add a second element head->nextptr =malloc(sizeof(LISTNODE));head->nextptr->data = 'e';head->nextptr->nextptr = NULL;

10. head ‘n’ ‘e’ ‘w’ NULL Add a third element head->nextptr->nextptr = malloc(sizeof(LISTNODE));head->nextptr->nextptr->data = 'w';head->nextptr->nextptr->nextptr = NULL;

11. 4. Converting a String to a List #include <stdio.h>#include <stdlib.h>/* list type implementation */LNP string_to_list(char []);int main(){ LNP h = NULL; h = string_to_list("AB"); return 0;}/* implementation of string_to_list() */

12. LNP string_to_list(char s[]){ LNP head = NULL, tail; int i; if (s[0] != '\0') { head = malloc(sizeof(LISTNODE)); head->data = s[0]; tail = head; for (i=1; s[i] != '\0'; i++){ tail->nextptr = malloc(sizeof(LISTNODE)); tail = tail->nextptr; tail->data = s[i]; } tail->nextptr = NULL; /* list end */ } return head;}

13. head ‘A’ ‘?’ tail string_to_list("AB") head = malloc(sizeof(LISTNODE));head->data = s[0];tail = head;

14. head ‘A’ ‘?’ ‘?’ tail tail->nextptr = malloc(sizeof(LISTNODE));

15. head ‘A’ ‘B’ ‘?’ tail tail = tail->nextptr;tail->data = s[i]; /* i = 1 here */

16. head ‘A’ ‘B’ NULL tail s[2] = '\0‘;/* so end of list is assigned NULL */

17. 5. List Functions 5.1. Empty Lists 5.2. Return the First Element of a List 5.3. Produce the Tail of a List 5.4. Put an Element on the Front of a List 5.5. Insertion continued

18. 5.6. Deletion 5.7. List Membership 5.8. Print a List 5.9. List Length 5.10. Concatenate Two Lists

19. 5.1. Empty Lists • Make an empty list:LNP h1;h1 = NULL; • Test for emptiness:int isempty(LNP sptr){ return (sptr == NULL);}

20. 5.2. Return the First Element of a List char first(LNP cptr){ if (isempty(cptr)) return '\0‘; else return cptr->data;}

21. Use LNP head;char c;head = string_to_list("new");c = first(head); /* c is 'n'; head is not altered */

22. 5.3. Produce the tail of a list void tail(LNP *cptr){ LNP temp; if (isempty(*cptr)) printf("The list is empty.\n\n"); else { temp = *cptr; *cptr = (*cptr)->nextptr; free(temp); }} • cptr is the address of a pointer, so that the pointer can be modified using "call by reference".

23. Use LNP head;head = string_to_list("new"); :tail(&head); /* head is now the list version of “ew” */

24. 5.4. Put an Element on the List Front LNP cons(char c, LNP cptr){ LNP temp; temp = malloc(sizeof(LISTNODE)); temp->data = c; temp->nextptr = cptr; return temp;}

25. h1 ‘e’ ‘w’ NULL Use LNP h1, h2;h1 = string_to_list("ew");h2 = cons('n', h1); • Before the cons() call:

26. h2 ‘n’ ‘e’ ‘w’ NULL h1 After:

27. newptr ‘o’ NULL head ‘n’ ‘w’ NULL previousptr currentptr 5.5. Insertion Before:

28. head ‘n’ ‘o’ ‘w’ NULL After:

29. Code void insert(LNP *sptr, char value){ LNP newptr, previousptr, currentptr; newptr = malloc(sizeof(LISTNODE)); if (newptr) { newptr->data = value; newptr->nextptr = NULL; previousptr = NULL; currentptr = *sptr; continued

30. while ((currentptr != NULL) && (value > currentptr->data)) { previousptr = currentptr; currentptr = currentptr->nextptr; } if (previousptr == NULL) { newptr->nextptr = *sptr; *sptr = newptr; } else { previousptr->nextptr = newptr; newptr->nextptr = currentptr; } } else printf("No memory available.\n");}

31. Note • The use of a pointer address (sptr) as an argument to insert() is to allow the head pointer to the list to be altered if the character is inserted as the first node.

32. Use LNP h1;h1 = string_to_list("nw");insert(&hl, 'o'); • Dangers:LNP h1, h2;h1 = string_to_list("nw");h2 = h1;insert(&hl, 'o');

33. head ‘n’ ‘e’ ‘w’ NULL previousptr currentptr 5.6. Deletion LNP head;head = string_to_list("new");c = delete(&head, 'e');

34. tempptr head ‘n’ ‘e’ ‘w’ NULL previousptr currentptr

35. Code char delete(LNP *sptr, char value){ LNP previousptr, currentptr, tempptr; if (value == (*sptr)->data) { tempptr = *sptr; *sptr = (*sptr)->nextptr;free(tempptr); return value; } else { previousptr = *sptr; currentptr = (*sptr)->nextptr; continued

36. while ((currentptr != NULL) && (currentptr->data != value)) { previousptr = currentptr; currentptr = currentptr->nextptr; } if (currentptr) { tempptr = currentptr; previousptr->nextptr = currentptr->nextptr;free(tempptr); return value; } } return '\0';}

37. Some Comments • The use of a pointer address (sptr) as an argument to delete() is to allow the head pointer to the list to be altered if the first character in the list is being deleted.

38. delete() can stop when: 1. It has found the character. 2. It has reached the end of the list (the character isn't there). 3. It has reached a character lexically bigger than the one being sought. Not used in this code.

39. h1 ‘a’ ‘l’ NULL h2 Dangers LNP h1, h2;char c;h1 = string_to_list("all");h2 = h1;c = delete(&h1, 'l'); • h2 would be pointing at nothing if the first node of h1 was deleted

40. 5.7. List Membership int member(char c, LNP cptr){ if (isempty(cptr)) return 0; else { if (c == first(cptr)) return 1; else return member(c, cptr->nextptr); }}

41. 5.8. Print a List (iteratively) void printList(LNP cptr){ if (!cptr) printf("List is empty.\n\n"); else { printf("The list is:\n"); while (cptr) { printf("%c --> ", cptr->data); cptr = cptr->nextptr; } printf("NULL\n\n"); }}

42. Use LNP head;head = string_to_list("old");printList(head); The list is:o --> l --> d --> NULL

43. Print a List (recursively) void printList(LNP cptr){ if (isempty(cptr)) printf("NULL"); else { printf("%c --> ", first(cptr)); printList(cptr->nextptr); }}

44. 5.9. List Length int length(LNP cptr){ if (isempty(cptr)) return 0; else return (1 + length(cptr->nextptr));}

45. 5.10. Concatenate Two Lists void concat(LNP a, LNP b){ if (a->nextptr == NULL) a->nextptr = b; else concat(a->nextptr, b);}

46. Use LNP h1, h2;h1 = string_to_list("new");h2 = string_to_list("ton");concat(h1, h2); /* h1 altered */print_list(h1); The list is:n --> e --> w --> t --> o --> n --> NULL

47. h1 ‘a’ ‘b’ ‘o’ ‘d’ NULL h2 Dangers LNP h1, h2;h1 = string_to_list("ab");h2 = string_to_list("cd");concat(h1, h2); /* h1 is list "abcd" */h2->data = 'o';

48. 5.11 Doubly Linked Lists • A node in a doubly-linked list contain two references that point to the next node and the previous node. ex.struct listnode {char data;struct listnode *next; struct listnode *prev; }; • frontpoints to the first node in the list • backpoints at the last node in the list

49. A doubly-linked list can be scanned in both directions: • a forward scan starts at 'front' and ends when the link is to the same object as 'back' • a backward scan starts at 'back' and ends when the link is to the same object as 'front'

50. Like a singly-linked list, a doubly linked list is a sequential structure. • To move forward or backward, use the node links 'next' and 'prev'. • Unlike a singly linked list, the insert and delete operations only need a single reference to the node.