Pointers (additional material)
description
Transcript of Pointers (additional material)
Pointers(additional material)
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Department of Computer Science-BGU 22:22:12
Array of pointers
Problem : Write program that receive strings from a user
and print these strings in a lexicography order.
Solution :Array of pointers to char (malloc)
char * names[256] or char ** names;
This is not the same declaration !!
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Array of pointers
Declaration:char ** names;int n,i;scanf(“%d”,&n);names = (char **)malloc(n * sizeof(char *));for(i=o; i<n; i++){
char[i] = (char*)malloc(256*sizeof(char));
}// bubble on the pointers !!
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Complete problem
We use exactly the size for the input strings.
Assuming that the strings have at most 255 letters.
We need receive a sorted array of strings from the function.
All the inputs are from the user.
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Complete solution
char ** sorted_list(){char ** names, temp[256], * temp2;int n, i, flag;scanf(“%d”,&n);names = (char **)malloc(n * sizeof(char *));for(i=0; i<n; i++){
gets(temp);char[i] = (char*)malloc(strlen(temp)+1);strcpy(char[i],temp); // char + i
}/* Sorting pointers by lexicography string
order */
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Complete solution(cont.)
// sorting do{
flag =0;for(i=0 ; i< n-1 ; i++)
// names + i , names +i+1if( strcmp(names[i],names[i+1]) >0){temp = names+i;names +i = names +i+1;names+i+1 = temp;flag = 1;}
}while(flag);return names;
}
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Linked Lists
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Department of Computer Science-BGU
Problems with dynamic arrays
Problems: Adding/ delete member. Reallocation. Building sort list . Merging .
Solution: Linked list Simple add/delete member. No need reallocation. Building sorting. Simple merging.
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Department of Computer Science-BGU
Linked lists?
head
NULL
Structures
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Department of Computer Science-BGU
A better alternative might be using a linked list, by “self reference”.
Linking students
typedef struct Student_t {char ID[ID_LENGTH];char Name[NAME_LENGTH];int grade;struct Student_t *next; /* A pointer to the next item on the list */
} item;
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Department of Computer Science-BGU
Different definition
A different use of typedef:
typedef struct Student_t item ;struct Student_t{
char ID[ID_LENGTH];char Name[NAME_LENGTH];int grade;item *next; /* A pointer to the next item
on the list without use of “struct” in the pointer definition */
} ;
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Creating a new kind of student
Usually when using linked lists we don’t know how many elements will be in the list
Therefore we would like to be able to dynamically allocate new elements when the need arises.
A possible implementation follows…
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Department of Computer Science-BGU
Creating a new kind of student
item*create_student(char * name, char * ID, int grade) {item *std;std = (item *)malloc(item));if (std == NULL) {
printf(“Memory allocation error!\n”);exit(1);
}strcpy(std->Name, name);strcpy(std->ID, ID);std->grade = grade;std->next = NULL;return std;
}
std
NULL
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Department of Computer Science-BGU
Linked lists - insertion
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Head
Insert new item:
Previous
Next
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Linked lists - insertion
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Insert new item:
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Linked lists - insertion
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Insert new item:
Previous
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Department of Computer Science-BGU
Adds a new item to the end of the list
head NULL
newItem
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Adds a new item to the end of the list
newItem
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Adds a new item to the end of the list
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Adds a new item to the end of the list
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Adds a new item to the end of the list
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Adds a new item to the end of the list
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Adds a new item to the end of the list
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Adds a new item to the end of the list
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Adds a new item to the end of the list
item * add_last(item *head, item* newItem){item *currItem;if (!head)
return newItem;currItem = head;while(currItem->next)
currItem = currItem->next;currItem->next = newItem;
return head;}
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Department of Computer Science-BGU
Inserts into a sorted list
head NULL
newItem
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Inserts into a sorted list
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Inserts into a sorted list
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Inserts into a sorted list
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Inserts into a sorted list
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Inserts into a sorted list
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Inserts into a sorted list
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Inserts into a sorted list
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Inserts into a sorted list
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Inserts into a sorted list
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Department of Computer Science-BGU
Inserts into a sorted list
// while keeping it sorted ascending by keyitem * insert(item *head, item *newNode) { item *currItem; if (!head) return newNode; //check if newNode's key is smaller than all keys and should be first
if (newNode->key < head->key) { newNode->next = head; return newNode; } currItem = head; while (currItem->next && newNode->key > currItem->next->key)
currItem = currItem->next;//put newNode between currItem and currItem->next //(if currItem is last then currItem->next == NULL) newNode->next = currItem->next; currItem->next = newNode; return head;}
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Department of Computer Science-BGU
Linked lists - searching
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head?currItem
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Linked lists - searching
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head?currItem
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Linked lists - searching
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!
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Department of Computer Science-BGU
Searching for an item
//searches for an item with passed key.//returns NULL if didn't find it.item *search(item *head, int key) {
item *currItem = head; if (!head) return NULL;while (currItem) { //loop through the list
if (currItem->key == key)return currItem;
currItem = currItem->next;} //didn't find the item with the requested keyreturn NULL;
}
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Department of Computer Science-BGU
Print list’s members
//prints keys of items of the list, key after key.void printKeys(item *head) {
item *curr = head; while (curr) {
printf("%d ", curr->key);curr = curr->next;
}putchar('\n');
}
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Department of Computer Science-BGU
Linked lists - delete
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Head
Structure to delete
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Linked lists - delete
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Structure to delete
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Structure to delete
Linked lists - delete
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Linked lists - delete
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Department of Computer Science-BGU
Remove the item with a given value
item *remove(int value, item* head){item * curr= head,*prev=NULL;int found=0;if(!head)
printf("The LL is empty\n");else{
while(curr)if(value==curr->value){
prev ?prev->next=curr->next:head=head->next;free(curr);found=1;break;
}else{
prev=curr;curr=curr->next;
}if(!found) printf("The record with key %d was not found\
n",value);}
return head;}
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Department of Computer Science-BGU
Freeing students
After we’re done, we need to free the memory we’ve allocated
One implementation is as we saw in class
void free_list(Student *head){
Student *to_free = head;
while (to_free != NULL) {
head = head->next;free(to_free);to_free = head;
}}
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Department of Computer Science-BGU
Recursive freeing
A perhaps simpler way to free a list is recursively.
void free_list(Student *head){
if (head== NULL) /* Finished freeing. Empty list */return;
free_list(head->next); /* Recursively free what’s ahead */free(head);
}
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Department of Computer Science-BGU
Split linked list
typedef struct stam item;struct stam{
int value;item * next;
}void main(){
item * head=*odd=*even = NULL;//// /* build list */// Now split it in two lists with odd and even membersvoid Split(head, odd, even);print_list(odd);print_list(even);
}
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Split linked list
// Split the nodes to these 'a' and 'b' listsvoid Split(item * source, item * od, item * ev) {
item * current = source, * temp;while (current != NULL) {
temp = current;if(temp->value %2 != 0)od = add_last(od,current);else ev = add_last(ev,current);current = current -> next;temp->next = NULL;
}}
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Split linked list
typedef struct stam item;struct stam{
int value;item * next;
}void main(){
item * head=*odd=*even = NULL;//// /* build list */// Now split it in two lists with odd and even membersvoid Split(head, &odd, &even);print_list(odd);print_list(even);
}
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Split linked list
void Split(item * source, item ** od, item ** ev) {item * current = source;item * a =*b = NULL;while (current != NULL) {
temp = current;if(temp->value %2 != 0)
a = add_last(a,current);else b = add_last(b,current);current = current -> next;temp->next = NULL;
}*od = a;*ev = b;
}
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(stackהמחסנית )
22:22:15Department of Computer Science-BGU
מחסנית - הינה מבנה נתונים מופשט. התכונה העיקרית שלה היא 56שהאיבר הראשון שיוכנס אליה יהיה
) - כלומר הוצאה והכנסה של איבר first in last outהראשון לצאת (אפשרית רק מלמעלה (מראש המחסנית).
ראש המחסנית הוא האיבר העליון. הפעולות:
push.דחיפת איבר לראש המחסנית - pop.(שליפת ראש המחסנית) שליפת איבר מראש המחסנית - init אתחול המחסנית,האיבר הראשון של המחסנית שווה ל - NULL
נממש את המחסנית באמצעות רשימה שבה כל פעולות ההכנסה וההוצאה מתבצעות בקצה אחד בלבד.
המחסנית הינה רשימה שהפעולות של הכנסת איברים והוצאתם נעשית בכיוון אחד ולכן מבנהו של המחסנית זהה לשל הרשימה (רק
על מנת שהדבר list ולא stackכאן אנו נגדיר ששמו של המבנה הוא יהיה ברור).
מבנה כללי של המחסנית
typedef DATA ...struct stack { DATA data; struct stack *next;};
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)(pushהכנסת איבר לראש המחסנית -
void push (struct stack** head, DATA data){ struct stack* second = *head; *head = (struct stack*)malloc(sizeof(struct stack)); (*head)->data = data; (*head)->next = second;}
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)(pop :הוצאת איבר מראש המחסנית -
DATA pop (struct stack** head){DATA data = NO_ELEMENT_DATA;
if (*head == NULL) { printf ("\n can't pop from empty stack\n"); } else { struct stack *temp = *head; *head = (*head)->next;
data = temp->data; free(temp); }
return data;}
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void main)({DATA val;int op;struct stack * stack = NULL;do{
printf)"\n enter 1 for push 2 for pop. 0 to quit "(;scanf)"%d",&op(;if ) op==1({
printf)"\n enter value: "(;scanf)"%d",&val(;push)&stack, val(;
}else if )op == 2( {
if )!is_empty)stack(( {val = pop)&stack(;printf)"\n poped %d", val(;
}else{
printf )"\n can't pop from empty stack\n"(;}
}}while)op(;while )! is_empty)stack( (
pop)&stack(; // emptying the stack}
איתחול של מחסנית
void init_stack (struct stack** head){ *head = NULL;}
int is_empty(struct stack* stack){return (!stack);
}
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Split linked list
// Split the nodes to these 'a' and 'b' listsvoid Split(item * source, item ** od , item ** ev) {
item * a = NULL; item * b = NULL;item * current = source;while (current != NULL) {
MoveItem(&a, ¤t); // Move a node to 'a'if (current != NULL) { MoveItem(&b, ¤t); // Move a node to 'b'{
{*od= a;*ev = b;
{
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תור
:הפעולותappend.הוספת איבר לתור - Remove.סילוק איבר מהתור -
init.אתחול התור -
נממש את התור באמצעות רשימה שבה כל פעולות ההכנסה מתבצעת דרך קצה אחד ופעולת ההוצאה מתבצעות דרך הקצה האחר.
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מבנה נתונים מופשט. תכונתו העיקרית היא הכנסה והוצאה של איברים דרך התאים שבקצוות -הכנסה של
האיברים דרך קצה אחד והוצאתם דרך הקצה האחר.
Initialize
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struct queue { int num; struct queue *next;};
void init_queue (struct queue** que){ *que = NULL;}
Append
void append (struct queue** que, int num){ static struct queue* last; static struct queue* n_last;
n_last = (struct queue*)malloc(sizeof(struct queue)); n_last->num = num; n_last->next =NULL; if (*que == NULL) *que = n_last; else last->next = n_last; last = n_last;}
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Display
void display_queue (struct queue* que){ struct queue* temp = que; printf ("\nThe members of the queue are: \n\n"); while (temp != NULL) { printf (" %d ", temp->num); temp = temp->next; } temp = que; printf("\n"); while (temp != NULL) { printf ("%p ", temp); temp = temp->next; }}
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Delete
void delete_queue (struct queue** que){ struct queue *temp;
printf("\n\nDeleting the queue:\n"); while (*que) { printf ("%p\t", *que); temp = *que; *que = (*que)->next; free(temp);
}}
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void main(){ struct queue* que; randomize();
init_queue (&que); build_queue (&que); display_queue (que); delete_queue (&que);}
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Building
void build_queue (struct queue** que){ int i, num;
printf ("\nThe number by their insertion order are:\n"); for (i = 0; i < MAX; i++) { num = random(10); append (que, num); }}
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1 שאלה
מקבלת את העוגן של רשימה משורשרת רגילה singleפונקצית head הפונקציה משחררת מהרשימה את כל המבנים שהערך שבהם ,
הופיע מקודם ברשימה. (הפונקציה משאירה כל ערך פעם אחת ברשימה).
הקטעים המסומנים ב- ?? ?? כך 7השלם בדפי התשובות את שהפונקציה תבצע את הנדרש.
typedef struct item item;
struct item{int value;
item *next;};
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item * single(item *head){item *prev, *temp;while( head ){
prev = head;temp = head -> next;while( temp )
if( ?? 1 ?? ){?? 2 ?? = temp -> next;free( ?? 3 ?? );temp = ?? 4 ?? ;
}else}
prev = ?? 5 ?? ;temp = ?? 6 ?? ;
}?? 7 ?? ;
}}
?? 1 ?? temp->value == head->value?? 2 ?? prev->next ?? 3 ?? temp?? 4 ?? prev->next?? 5 ?? temp
or prev->next?? 6 ?? temp->next
or prev->next?? 7 ?? head = head_next;
2שאלה
נתונות ההגדרה והפונקציות הבאות:
typedef struct item item;struct item{
int value; item *next;
};
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item* what2(item* p1, item* p2){item *temp;if(!p1) return p2;temp = p1 -> next;p1 -> next = p2;return what2(temp, p1);
}item* what1(item* p){
if(!p) return NULL;return what2(p, NULL);
}
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?what2מה יעודה של הפונקציה • ?what1מה יעודה של הפונקציה • הוא העוגן של הרשימה המקושרת, על מה מצביע list כאשר•
what1(list).צייר את מבנה הנתונים המתקבל ,
פתרון
.אwhat2-הופכת את הרשימה המשורשרת שמתחילה ב p1 p2ומחברת אותה ל-
.בwhat1הופכת רשימה ומחזירה את העוגן החדש .ג
what1(list) 4216537908 NULL
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נתונה ההגדרה הבאה של מבנה ברשימה משורשרת:typedef struct node{
int val;struct node *next;
} node;
node* odd_even(node* head) הלא רקורסיביתהפונקציה מצביע לראש רשימה מקושרת ומסדרת מחדש את headמקבלת כארגומנט
) יופיעו ראשונים (באותו oddהרשימה כך שהאיברים בעלי ערך אי-זוגי () יופיעו לאחר מכן (באותו הסדר).evenהסדר) והאיברים בעלי ערך זוגי (
הפונקציה מחזירה את ראש הרשימה החדשה.
) בפונקציה הבאה: ?? N?? השלם את הקטעים החסרים (המסומנים ב -
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node* odd_even)node* head({node *even = NULL, *odd = NULL;node *headEven = NULL, *headOdd=NULL;
while )head({if)head->val % 2({
if) ?? 1 ?? (?? 2 ??;
else?? 3 ??;
odd = ?? 4 ??;}else{
if)?? 5 ??( ?? 6 ?? ;else ?? 7 ?? ;?? 8 ?? ;
}?? 9 ?? ;
}if)!odd( ?? 10 ?? ;if)!even( ?? 11 ?? ;
odd ->next = ?? 12 ?? ;even->next = ?? 13 ??;
return ?? 14 ??;}
?? 1 ?? = odd?? 2 ?? = odd->next = head?? 3 ?? = headOdd = head?? 4 ?? = head?? 5 ?? = even?? 6 ?? = even->next = head?? 7 ?? = headEven = head?? 8 ?? = even = head?? 9 ?? = head = head->next?? 10 ?? = return headEven?? 11 ?? = return headOdd?? 12 ?? = headEven?? 13 ?? = NULL?? 14 ?? = headOdd