240-222 Computer Programming Techniques Semester 1, 1998

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240-222 Computer Programming Techniques Semester 1, 1998. 15. Lists. Objective of these slides: to describe linked lists in C. Overview. 1. List Data Structures and Operations 2. List Implementations 3. Dynamically Created Lists 4. Converting a String to a List 5. List Functions.

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### 240-222 Computer Programming TechniquesSemester 1, 1998

15. Lists

Objective of these slides:

to describe linked lists in C

Overview

1. List Data Structures and Operations

2. List Implementations

3. Dynamically Created Lists

4. Converting a String to a List

5. List Functions

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

2. List implementations

Version 1:

#define N 1000 /* the size of the list */typedef char LIST[N];LIST lt; /* same as char lt[N] */

Version 2 (Sec. 12.2)

struct listnode { char data; struct listnode *nextptr;};typedef struct listnode LISTNODE;LISTNODE elem;

elem

data

nextptr

Use

LISTNODE a, b, c;a.data = \'a\';b.data = \'c\';c.data = \'e\';a.nextptr = b.nextptr = c.nextptr = NULL;

a

b

c

continued

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

a

b

c

‘a’

‘c’

‘e’

NULL

3. Dynamically Created Lists

‘n’

NULL

Function prototype in stdlib.h: void *malloc(size_t size);

‘n’

‘e’

NULL

‘n’

‘e’

‘w’

NULL

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() */

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;}
string_to_list("AB")

‘A’

‘?’

tail

tail->nextptr = malloc(sizeof(LISTNODE));

‘A’

‘?’

‘?’

tail

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

5.6. Deletion

5.7. List Membership

5.8. Print a List

5.9. List Length

5.10. Concatenate Two Lists

5.1. Empty Lists
• Make an empty list:LNP h1;h1 = NULL;
• Test for emptiness:int isempty(LNP sptr){ return (sptr == NULL);}
5.2. Return the First Element of a List

char first(LNP cptr){ if (isempty(cptr)) return \'\0\' else return cptr->data;}

Use

/* c is \'n\'; head is not altered */

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".
Use

/* head is now the list version of “ew” */

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;}

Use

LNP h1, h2;h1 = string_to_list("ew");h2 = cons(\'n\', h1);

• Before the cons() call:

h1

‘e’

‘w’

NULL

After:

h2

‘n’

‘e’

‘w’

NULL

h1

5.5. Insertion

Before:

newptr

‘o’

NULL

‘n’

‘w’

NULL

previousptr

currentptr

After:

‘n’

‘o’

‘w’

NULL

Code Fig. 12.3

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

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");}
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.
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\');
5.6. Deletion

‘n’

‘e’

‘w’

NULL

previousptr

currentptr

tempptr

‘n’

‘e’

‘w’

NULL

previousptr

currentptr

Code Fig. 12.3

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

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\';}
• 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.
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.
Dangers

LNP h1, h2;char c;h1 = string_to_list("all");h2 = h1;c = delete(&h1, \'l\');

h1

‘a’

‘l’

NULL

h2

• • h2 would be pointing at nothing if the first node of h1 was deleted
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); }}

5.8. Print a List (iteratively) Fig. 12.3

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"); }}

Use

The list is:o --> l --> d --> NULL

Print a List (recursively)

void printList(LNP cptr){ if (isempty(cptr)) printf("NULL"); else { printf("%c --> ", first(cptr)); printList(cptr->nextptr); }}

5.9. List Length

int length(LNP cptr){ if (isempty(cptr)) return 0; else return (1 + length(cptr->nextptr));}

5.10. Concatenate Two Lists

void concat(LNP a, LNP b){ if (a->nextptr == NULL) a->nextptr = b; else concat(a->nextptr, b);}

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

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’

‘b’

‘o’

‘d’

NULL

h2