Pointers Department of Computer Science-BGU יום שלישי 31 יולי 2018

כתובת של משתנה נניח משתנים: double x; //8bytes short y ; //2bytes char c ; //1 bytes כאשר בזמן הריצה של התוכנית המחשב יקצה זיכרון למשתנים. לכל תא זיכרון יש ערך מספרי המציין את מיקומו המדויק בזיכרון המחשב. ערך זה מכונה כתובת זיכרון (memory address). כתובת הזיכרון של משתנה הינה הכתובת של הבית הראשון ברצף הבתים שמשתנה זה תופס בזיכרון. למשל Department of Computer Science-BGU יום שלישי 31 יולי 2018

כתובת של משתנה האופרטור & אם x הוא משתנה אזי &x היא כתובת הזיכרון של x, כלומר האופרטור & מציין "כתובתו של…". ובדוגמא שלנו : &x = 1000 &y = 1008 &c = 1010   כתובות זיכרון אלו הינם ביטויים בעליי ערך וטיפוס. הטיפוס של כתובת הזיכרון של משתנה כלשהו נגזר מהמשתנה עצמו. 1000 x 1008 y 1010 c Department of Computer Science-BGU יום שלישי 31 יולי 2018

Using pointers מצביע הוא משתנה אשר מכיל כתובת זיכרון. כשנרצה להצהיר על מצביע ל type נוסיף את התו '' משמאל למשתנה . לדוגמא : הגדרת מצביע ל int int <identifier> ; הגדרת מצביע ל char char <identifier> ; הגדרת מצביע ל double double <identifier> ; ברגע שנצהיר על משתנים מצביעים, נוכל להציב לתוכם כתובות של משתנים . לדוגמא נתונה התוכנית הבאה : Department of Computer Science-BGU יום שלישי 31 יולי 2018

Pointers Pointer is a variable that contains the address of a variable Here P is said to point to the variable C C 7 3 4 … 173 172 174 175 176 177 178 179 180 181 P 833 832 834 835 836 837 838 839 840 841 Department of Computer Science-BGU יום שלישי 31 יולי 2018

Using pointers The unary operator & gives the address of a variable The statement P=&C assigns the address of C to the variable P, and now P points to C To print a pointer, use %p format. Department of Computer Science-BGU יום שלישי 31 יולי 2018

Using pointers int C; int *P; // Declare P as a pointer to int C = 7; P = &C; C 7 3 4 … 173 172 174 175 176 177 178 179 180 181 P 174 3 4 … 833 832 834 835 836 837 838 839 840 841 Department of Computer Science-BGU יום שלישי 31 יולי 2018

האופרטור  The unary operator * is applied on pointers האופרטור  The unary operator * is applied on pointers Access the object the pointer points to The statements: int * P; int C; P = &C; *P=5; Puts in C (the variable pointed by P) the value 5 Department of Computer Science-BGU יום שלישי 31 יולי 2018

האופרטור  printf(“%d”, *P); // Prints out ‘7’ *P = 177; printf(“%d”, C); // Prints out ‘177’ P = 177; /* This is unadvisable !!! */ C 7 3 4 … 173 172 174 175 176 177 178 179 180 181 P 833 832 834 835 836 837 838 839 840 841 Department of Computer Science-BGU יום שלישי 31 יולי 2018

Common errors It is impossible to define pointers to constants or expressions. It is also impossible to change a variable’s address (because it is not for us to determine!). Therefore, the following are errors: i = &3; j = &(k+5); k = &(a==b); &a = &b; &a = 150; Department of Computer Science-BGU יום שלישי 31 יולי 2018

Pass arguments by value The functions we saw till now accepted their arguments “by value” They could manipulate the passed values They couldn’t change values in the calling function But address is a value !! And we can pass the address as argument to functions Remember ?! void swap(int x, int y) { …swap x and y… } Department of Computer Science-BGU יום שלישי 31 יולי 2018

How can we fix it? void swap(int * x, int * y){ int temp; temp = *x; *x = *y; *y = temp; } void main() { int a=7,b=8; printf("a = %d, b = %d\n", a, b); swap(&a,&b); Department of Computer Science-BGU יום שלישי 31 יולי 2018

We can now understand the & in scanf(“%d”,&a); Back to scanf We can now understand the & in scanf(“%d”,&a); The argument list in scanf is simply passed by address, so scanf can change its content. Department of Computer Science-BGU יום שלישי 31 יולי 2018

Pointer-array equivalence Arrays are actually a kind of pointers! When an array is defined, a fixed amount of memory the size of the array is allocated. The array variable is set to point to the beginning of that memory segment. When a pointer is declared, it is uninitialized (like a regular variable). Department of Computer Science-BGU יום שלישי 31 יולי 2018

Pointers and Arrays Recall that an array S holds the address of its first element S[0]. Assume: int S[10]; int *P; P=S; // From now P is equivalent to S S is actually a pointer to S[0] Both P and S are now pointing to S[0] Department of Computer Science-BGU יום שלישי 31 יולי 2018

Pointer arithmetic Is it possible ?? YES If int * p ; Pointers can be incremented and decremented. If p is a pointer to a particular type, p+1 yields the correct address of the next variable of the same type. Is it possible ?? YES If int * p ; p = p +1; means p = p + sizeof(int); p++, p+i, and p += i also make same sense. p + 4 means p + (4 * sizeof(int)) Department of Computer Science-BGU יום שלישי 31 יולי 2018

What is this ‘sizeof’ ? The sizeof operator gets a variable or a type as an input and outputs its size in bytes: double x; s1=sizeof(x); /* s1 is 8 */ s2=sizeof(int) /* s2 is 4 */ Department of Computer Science-BGU יום שלישי 31 יולי 2018

Pointer arithmetic What about arrays ?? They are pointers !! Unlike pointers, the value of an array variable cannot be changed. We can write : int a[10]; int * p; p = a; But the arrays are constants !! So … p++ ; a++; // Correct // Error !! Array Address is constant Department of Computer Science-BGU יום שלישי 31 יולי 2018

Pointer arithmetic - example If p and q point to elements in an array, q-p yields the number of elements between p and q. However, there is a difference between pointer arithmetic and “regular” arithmetic. Department of Computer Science-BGU יום שלישי 31 יולי 2018

An additional use A function that accepts an array and searches for something within it (a char, a number, etc.) What should the function return? The index where the ‘something’ was found A pointer to the place where it was found Department of Computer Science-BGU יום שלישי 31 יולי 2018

Functions that return pointers Like any other data type, functions can return pointers. For example, a prototype for a function returning a pointer to char will be – char *func(…); But how would we indicate failure Suppose we searched the array and didn’t find anything – what should we return? Department of Computer Science-BGU יום שלישי 31 יולי 2018

The NULL pointer The NULL pointer is an ‘empty’ pointer that points to nothing. It is the address number 0. If a pointer p is set to NULL, trying to access *p results in a run-time (not compilation) error. Often used to indicate failure Department of Computer Science-BGU יום שלישי 31 יולי 2018

Example- solving quadratic equation - the "easy way" #include <stdio.h> #include <math.h> int quad(double ,double ,double ,double* ,double* ); void main(){ double a,b,c,x1,x2; int res; scanf("%lf%lf%lf",&a,&b,&c); res=quad(a,b,c,&x1,&x2); switch(res){ case 0 : printf("No solution\n"); break; case 1 : printf("x1=x2=%.2f\n",x1); case 2 : printf("x1=%.2f , x2=%.2f\n",x1,x2); } Department of Computer Science-BGU יום שלישי 31 יולי 2018

Function quad(……) /* we assume a!=0 the function return : 0 no solution (we neglect the complex option) 1 two equal roots 2 two different roots */ int quad(double a ,double b ,double c ,double* px1 ,double* px2){ double d = (b*b - 4*a*c);  if(d < 0) return 0; if(d==0){ *px1 = *px2 = -b/(2*a); return 1; } *px1= (-b + sqrt(d))/(2*a); *px2= (-b - sqrt(d))/(2*a);   return 2; Department of Computer Science-BGU יום שלישי 31 יולי 2018

Usefully functions בשפת C קיימות פונקציות ספריה לטיפול במחרוזות ותווים. על מנת להשתמש בפונקציות אלו יש להוסיף בתחילת התוכנית את שורות:  #include <string.h> #include <ctype.h>  דוגמאות לפונקציות לבדיקת תווים : האם התו הוא סיפרה int isdigit(int) האם התו הוא רווח int isspace(int) האם התו הוא אות גדולה int isupper(int) האם התו הוא אות קטנה int islower(int)  לפונקציות נוספות עיינו בספרות .   Department of Computer Science-BGU יום שלישי 31 יולי 2018

Some functions of string.h int strncmp(char *s1, char *s2, int n) - returns 0 if the first n letters of s1 are equal to those of s2, and returns non-zero if first n letters are different char * strchr(char * str, char c)– returns a pointer to the first occurrence of a character within a string char *strstr(char * st1,char * st2) – returns a pointer to the first occurrence of st1 string within the st2. r void strncpy(char * st1, char * st2, int n) copies the first n letters of st2 to the address of st1. No copy the ‘\0’ letter to st1 !! Department of Computer Science-BGU יום שלישי 31 יולי 2018

Mystery - What does this do? char *rec_stam(char * str, char c) { if (*str == '\0') return NULL; if (*str == c) return str; return rec_stam(str+1, c); } A recursive implementation of strchr Return pointer to the first instance of the letter c into the string str Department of Computer Science-BGU יום שלישי 31 יולי 2018

Again mystery – what does this do? void main() { char s[101]; char *p; scanf("%100s", s); p = strchr(s, ','); while(p!=NULL) { *p = ' '; p = strchr(p+1, ','); } printf("The result is - %s\n", s); Department of Computer Science-BGU יום שלישי 31 יולי 2018

Exercise Write a recursive implementation of strcmp – Input – two strings Output – 0 if both are equal, 1 if not Write a program that accepts two strings from the user and checks whether they are equal Department of Computer Science-BGU יום שלישי 31 יולי 2018

Solution int strcmp_r(char *s, char *t) { if (*s == '\0' || *s != *t) return *s - *t; else return(strcmp_r(s+1, t+1)); } Department of Computer Science-BGU יום שלישי 31 יולי 2018

Dynamic Memory Allocation Department of Computer Science-BGU יום שלישי 31 יולי 2018

Dynamic Memory Allocation Array variables have fixed size, used to store a fixed and known amount of variables. This size can’t be changed after compilation. However, we don’t always know in advance how much space we would need for an array or a variable. We would like to be able to dynamically allocate memory. (הקצאה דינמית של זיכרון) Department of Computer Science-BGU יום שלישי 31 יולי 2018

The malloc function void *malloc(unsigned int nBytes); The function malloc is used to dynamically allocate nBytes worth of space. How to determine nBytes? malloc returns a pointer to the allocated area on success, NULL on failure. You should always check whether memory was successfully allocated. Remember to #include <stdlib.h> Department of Computer Science-BGU יום שלישי 31 יולי 2018

Example #include <stdio.h> #include <stdlib.h> void main(){ int size ,i; int *ptr2memory ; printf("Enter the series length :"); scanf("%d",&size); ptr2memory=(int*)malloc(size*sizeof(int)) ; /* allocating size ints */ if(ptr2memory==NULL){ printf("Memory allocation failed\n"); exit(1); } /* memory allocation succeeded */ /* can use ptr2memory as array */ Department of Computer Science-BGU יום שלישי 31 יולי 2018

Why casting? The type void * specifies a general pointer, which can be cast to any pointer type. Casting is needed because malloc returns void *: void *malloc(unsigned int nbytes); Casting for integer : y=(int *) malloc(n*sizeof (int)); Department of Computer Science-BGU יום שלישי 31 יולי 2018

Free the allocated memory segment void free(void *ptr); We use free(p) to free the allocated memory pointed to by p. If p doesn’t point to an area allocated by malloc, a run-time error occurs. Always remember to free the allocated memory once you don’t need it anymore. Otherwise, you may run out of memory before you know it! Department of Computer Science-BGU יום שלישי 31 יולי 2018

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 !! Department of Computer Science-BGU יום שלישי 31 יולי 2018

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 !! Department of Computer Science-BGU יום שלישי 31 יולי 2018

Complete solution Assuming that the strings have at most 255 letters: void main(){ char ** names, temp[256]; int n,i; scanf(“%d”,&n); names = (char **)malloc(n * sizeof(char *)); for(i=o; i<n; i++){ gets(temp); char[i] = (char*)malloc(strlen(temp)+1); strcpy(char[i],temp); } /* Sorting pointers by lexicography string order */ Department of Computer Science-BGU יום שלישי 31 יולי 2018

Complete solution Assuming that the strings have at most 255 letters: void main(){ char ** names, temp[256]; int n,i; printf(“Enter number of names for sorting\n”); scanf(“%d”,&n); names = fill_sorted(n); for(i=o; i<n; i++){ putsts(char[i]); } Department of Computer Science-BGU יום שלישי 31 יולי 2018

Complete solution (cont.) char ** fill_sorted(int num){ char ** temp, *aux; int i, flag; /* making an array of pointers */ temp = (char **)malloc(num * sizeof(char *)); /* making and filling strings */ for(i=o; i<n; i++){ gets(temp); char[i] = (char*)malloc(strlen(temp)+1); strcpy(char[i],temp); } Department of Computer Science-BGU יום שלישי 31 יולי 2018

Complete solution (cont.) /* Sorting pointers by lexicography string order */ do{ flag=0; for(i=0; i < num-1 ; i++) if(strcmp(temp[i],temp[i+1]) > 0){ aux = temp[i]; temp[i] = temp[i+1]; temp[i+1] = aux; flag = 1; } }while(flag); return temp; Department of Computer Science-BGU יום שלישי 31 יולי 2018