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Pointers. 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 ). - PowerPoint PPT Presentation
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Pointers1
Department of Computer Science-BGU 19:19:59
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 memoryבזיכרון המחשב. ערך זה מכונה כתובת זיכרון )
address.) הינה הכתובת של הבית הראשון משתנהכתובת הזיכרון של
ברצף הבתים שמשתנה זה תופס בזיכרון. למשל בדוגמא : y ושל 1000 היא xהכתובת של 1008.
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Department of Computer Science-BGU 19:20:00
כתובת של משתנה
& האופרטור אם x הוא משתנה אזי &x היא כתובת הזיכרון של x כלומר ,
".… מציין "כתובתו של&האופרטור : 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.
1000x1008y1010c
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Department of Computer Science-BGU 19:20:00
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… …
173172 174 175 176 177 178 179 180 181
174 3 4… …
P
833832 834 835 836 837 838 839 840 841
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Department of Computer Science-BGU 19:20:00
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.
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Department of Computer Science-BGU 19:20:00
Using pointers
מצביע הוא משתנה אשר מכיל כתובת זיכרון. כשנרצה להצהיר על מצביע ל type התו נוסיף את' . משמאל למשתנה '
לדוגמא :
int הגדרת מצביע ל int <identifier> ;
charהגדרת מצביע ל char <identifier>;
doubleהגדרת מצביע ל double <identifier> ;
ברגע שנצהיר על משתנים מצביעים, נוכל להציב לתוכם כתובות של משתנים . לדוגמא נתונה התוכנית הבאה :
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Department of Computer Science-BGU 19:20:00
Using pointers
int C;int *P; // Declare P as a pointer to int C = 7;P = &C;
C
7 3 4… …
173172 174 175 176 177 178 179 180 181
P
174 3 4… …
833832 834 835 836 837 838 839 840 841
7
Department of Computer Science-BGU 19:20:01
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
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Department of Computer Science-BGU 19:20:01
האופרטור
printf(“%d”, *P); // Prints out ‘7’ *P = 177;printf(“%d”, C); // Prints out ‘177’ P = 177; /* This is unadvisable !!! */
C
7 3 4… …
173172 174 175 176 177 178 179 180 181
174 3 4… …
P
833832 834 835 836 837 838 839 840 841
177
177
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Department of Computer Science-BGU 19:20:01
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;
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Department of Computer Science-BGU 19:20:01
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… }
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Department of Computer Science-BGU 19:20:01
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);printf("a = %d, b = %d\n", a, b);
}
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Department of Computer Science-BGU 19:20:01
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.
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Department of Computer Science-BGU 19:20:01
Pointers and Arrays
Recall that an array S holds the address of its first element S[0]
S is actually a pointer to S[0] int S[10]; int *P;
P=S; // From now P is equivalent to S
Both P and S are now pointing to S[0]
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Department of Computer Science-BGU 19:20:01
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).
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Department of Computer Science-BGU 19:20:01
Pointer arithmetic
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 +1 means p + sizeof(int)p++, p+i, and p += i also make same sense.
p + 4 means p + (4 * sizeof(int))
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Department of Computer Science-BGU 19:20:01
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 */
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Department of Computer Science-BGU 19:20:01
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++ ; // Correcta++; // Error !!
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Department of Computer Science-BGU 19:20:01
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.
A better example – strcpy.c
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Department of Computer Science-BGU 19:20:01
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
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Department of Computer Science-BGU 19:20:01
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?
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Department of Computer Science-BGU 19:20:01
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
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Department of Computer Science-BGU 19:20:01
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);
break; case 2 : printf("x1=%.2f , x2=%.2f\n",x1,x2);
} }
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Department of Computer Science-BGU 19:20:01
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;}
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Department of Computer Science-BGU 19:20:01
Usefully functions
קיימות פונקציות ספריה לטיפול במחרוזות ותווים. על מנת Cבשפת להשתמש בפונקציות אלו יש להוסיף בתחילת התוכנית את שורות:
#include <string.h>
#include <ctype.h>
דוגמאות לפונקציות לבדיקת תווים :
int isdigit(int) האם התו הוא סיפרה
int isspace(int) האם התו הוא רווח int isupper(int) האם התו הוא אות גדולה
int islower(int) האם התו הוא אות קטנה לפונקציות נוספות עיינו בספרות .
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Department of Computer Science-BGU 19:20:02
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 !!
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Department of Computer Science-BGU 19:20:02
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 strchrReturn pointer to the first instance of
the letter c into the string str
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Department of Computer Science-BGU 19:20:02
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);
}
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Department of Computer Science-BGU 19:20:02
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
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Department of Computer Science-BGU 19:20:02
Solution
int strcmp_r(char *s, char *t) { if (*s == '\0' || *s != *t)
return *s - *t; else return(strcmp_r(s+1, t+1));
}
30
Department of Computer Science-BGU 19:20:02
Dynamic Memory
Allocation31
Department of Computer Science-BGU 19:20:02
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
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Department of Computer Science-BGU 19:20:02
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>
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Department of Computer Science-BGU 19:20:02
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));
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Department of Computer Science-BGU 19:20:02
Free the allocated memory segment
void free(void *ptr);
We use free(p) to free the allocated memory pointed to by pIf p doesn’t point to an area allocated by malloc, a run-time error occursAlways remember to free the allocated memory once you don’t need it anymoreOtherwise, you may run out of memory before you know it!
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Department of Computer Science-BGU 19:20:02
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 !!
19:20:02Department of Computer Science-BGU
36
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 !!
19:20:03Department of Computer Science-BGU
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