CPT: Lists/ Computer Programming Techniques Semester 1, 1998 Objective of these slides: –to describe linked lists in C 15. Lists

CPT: Lists/15 Overview 1. List Data Structures and Operations 2. List Implementations 3. Dynamically Created Lists 4. Converting a String to a List 5. List Functions

CPT: Lists/15 1. List Data Structures and Operations l 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

CPT: Lists/15 2. List implementations Version 1: #define N 1000 /* the size of the list */ typedef char LIST[N]; LIST lt; /* same as char lt[N] */

CPT: Lists/15 Version 2(Sec. 12.2) struct listnode { char data; struct listnode *nextptr; }; typedef struct listnode LISTNODE; LISTNODE elem; elem datanextptr

CPT: Lists/15 Use LISTNODE a, b, c; a.data = 'a'; b.data = 'c'; c.data = 'e'; a.nextptr = b.nextptr = c.nextptr = NULL; continued abc

CPT: Lists/15 a.nextptr = &b; b.nextptr = &c; printf("%c", a.nextptr->data); /* 'c' printed */ printf("%c", a.nextptr->nextptr->data); /* 'e' printed */ abc NULL ‘a’‘c’‘e’

CPT: Lists/15 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 in stdlib.h : void *malloc(size_t size); head ‘n’ NULL

CPT: Lists/15 Add a second element head->nextptr = malloc(sizeof(LISTNODE)); head->nextptr->data = 'e'; head->nextptr->nextptr = NULL; head ‘n’‘e’ NULL

CPT: Lists/15 Add a third element head->nextptr->nextptr = malloc(sizeof(LISTNODE)); head->nextptr->nextptr->data = 'w'; head->nextptr->nextptr->nextptr = NULL; head ‘n’‘e’‘w’ NULL

CPT: Lists/15 4. Converting a String to a List #include #include /* 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() */

CPT: Lists/15 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; }

CPT: Lists/15 string_to_list("AB") head = malloc(sizeof(LISTNODE)); head->data = s[0]; tail = head; head ‘A’ tail ‘?’

CPT: Lists/15 tail->nextptr = malloc(sizeof(LISTNODE)); head ‘A’ tail ‘?’

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

CPT: Lists/15 s[2] = '\0' /* so end of list is assigned NULL */ head ‘A’ tail NULL ‘B’

CPT: Lists/15 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

CPT: Lists/ Deletion 5.7. List Membership 5.8. Print a List 5.9. List Length Concatenate Two Lists

CPT: Lists/ Empty Lists Make an empty list: LNP h1; h1 = NULL; Test for emptiness: int isempty(LNP sptr) { return (sptr == NULL); }

CPT: Lists/ Return the First Element of a List char first(LNP cptr) { if (isempty(cptr)) return '\0' else return cptr->data; }

CPT: Lists/15 Use LNP head; char c; head = string_to_list("new"); c = first(head); /* c is 'n'; head is not altered */

CPT: Lists/ 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); } } l cptr is the address of a pointer, so that the pointer can be modified using "call by reference".

CPT: Lists/15 Use LNP head; head = string_to_list("new"); : tail(&head); /* head is now the list version of “ew” */

CPT: Lists/ 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; }

CPT: Lists/15 Use LNP h1, h2; h1 = string_to_list("ew"); h2 = cons('n', h1); Before the cons() call: h1 ‘e’‘w’ NULL

CPT: Lists/15 h2 ‘n’‘e’‘w’ NULL After: h1

CPT: Lists/ Insertion head ‘n’‘w’ Before: previousptrcurrentptr NULL newptr ‘o’ NULL

CPT: Lists/15 head ‘n’‘o’‘w’ NULL After:

CPT: Lists/15 Code Fig 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

CPT: Lists/15 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"); }

CPT: Lists/15 Note l 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.

CPT: Lists/15 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');

CPT: Lists/ Deletion LNP head; head = string_to_list("new"); c = delete(&head, 'e'); head ‘n’ previousptrcurrentptr ‘e’‘w’ NULL

CPT: Lists/15 head ‘n’ previousptrcurrentptr ‘e’‘w’ NULL tempptr

CPT: Lists/15 Code Fig 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

CPT: Lists/15 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'; }

CPT: Lists/15 Some Comments l 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.

CPT: Lists/15 l 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.

CPT: Lists/15 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 h1 ‘a’‘l’ NULL h2

CPT: Lists/ 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); } }

CPT: Lists/ Print a List (iteratively) Fig 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"); } }

CPT: Lists/15 Use LNP head; head = string_to_list("old"); printList(head); The list is: o --> l --> d --> NULL

CPT: Lists/15 Print a List (recursively) void printList(LNP cptr) { if (isempty(cptr)) printf("NULL"); else { printf("%c --> ", first(cptr)); printList(cptr->nextptr); } }

CPT: Lists/ List Length int length(LNP cptr) { if (isempty(cptr)) return 0; else return (1 + length(cptr->nextptr)); }

CPT: Lists/ Concatenate Two Lists void concat(LNP a, LNP b) { if (a->nextptr == NULL) a->nextptr = b; else concat(a->nextptr, b); }

CPT: Lists/15 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

CPT: Lists/15 Dangers LNP h1, h2; h1 = string_to_list("ab"); h2 = string_to_list("cd"); concat(h1, h2); /* h1 is list "abcd" */ h2->data = 'o'; h1 ‘a’‘o’‘d’ NULL ‘b’ h2