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Lecture 18-20 Pointers

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Outline Defining and using Pointers Operations on pointers Arithmetic Logical Pointers and Arrays Memory Management for Pointers

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Pointer Fundamentals When a variable is defined the compiler (linker/loader actually) allocates a real memory address for the variable int x; When a value is assigned to a variable, the value is actually placed to the memory that was allocated x=3;

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Recall Variables

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Recall Variables Recall a variable is nothing more than a convenient name for a memory location. The type of the variable (e.g., int) defines how the bits inside that memory location will be interpreted, and what operations are permitted on this variable. Every variable has an address. Every variable has a value.

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The Real Variable Name is its Address! There are 4 billion (232) different addresses, and hence 4 billion different memory locations. Each memory location is a variable (whether your program uses it or not). Your program will probably only create names for a small subset of these “potential variables”. Some variables are guarded by the operating system and cannot be accessed. When your program uses a variable the compiler inserts machine code that calculates the address of the variable. Only by knowing the address can the variables be accessed.

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Pointers When the value of a variable is used, the contents in the memory are used y=x; will read the contents in the 4 bytes of memory, and then assign it to variable y &x can get the address of x (referencing operator &) The address can be passed to a function: scanf("%d", &x); The address can also be stored in a variable ……

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Pointer: Reference to Memory

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Pointers To declare a pointer variable type * PointerName; For example: int x; int * p; //p is a int pointer // char *p2; p = &x; /* Initializing p */

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Pointer: Declaration and Initialization

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Addresses and Pointers int a, b; int *c, *d; a = 5; c = &a; d = &b; *d = 9; printf(…,c, *c,&c) printf(…,a, b)

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Addresses and Pointers A pointer variable is a variable! It is stored in memory somewhere and has an address. It is a string of bits (just like any other variable). Pointers are 32 bits long on most systems.

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Using Pointers You can use pointers to access the values of other variables, i.e. the contents of the memory for other variables To do this, use the...

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* has different meanings in different contexts a = x * y;  multiplication int *ptr;  declare a pointer * is also used as indirection or de-referencing operator in C statements. ptr = &y; a = x * *ptr;

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Using Pointers You can use pointers to access the values of other variables, i.e. the contents of the memory for other variables To do this, use the * operator (dereferencing operator) Depending on different context, * has different meanings For example: int n, m = 3, *p; p = &m; // Initializing n = *p; printf("%d\n", n); // 3 printf("%d\n", *p); // 3 *p = 10; printf("%d\n", n); // 3 printf("%d\n", *p); // 10

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$An Example int m = 3, n = 100, *p, *q; p = &m; printf("m is %d\n", *p); // 3 m++; printf("now m is %d\n", *p); // 4 p = &n;...$

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An Example int m = 3, n = 100, *p, *q; p = &m; printf("m is %d\n", *p); // 3 m++; printf("now m is %d\n", *p); // 4 p = &n; printf("n is %d\n", *p); // 100 *p = 500; printf("now n is %d\n", n); // 500 q = &m; *q = *p; printf("now m is %d\n", m); // 500

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An Example int i = 25; int *p; p = &i; printf("%x %x", &p, &i); printf("%x %p", p, p); printf("%d %d", i, *p);

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Pointer Assignment int a = 2, b = 3; int *p1, *p2; p1 = &a; p2 = &b; printf("%p %p", p1 ,p2); *p1 = *p2; printf("%d %d", *p1, *p2); p2 = p1; printf("%p %p", p1, p2); printf("%p %p", &p1, &p2);

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Value of referred memory by a pointer

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Exercise: Trace the following code int x, y; int *p1, *p2; x = 3 + 4; Y = x / 2 + 5; p1 = &y; p2 = &x; *p1 = x + *p2; *p2 = *p1 + y; printf(…,p1,*p1,&p1) printf(…,x,&x,y,&y)

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Pointer Fundamentals When a variable is defined the compiler (linker/loader actually) allocates a real memory address for the variable int x; // &x = 22f54; &x = 22f54; // Error When a value is assigned to a variable, the value is actually placed to the memory that was allocated x = 3; // * (&x) = 3; *x = 3; // Error

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Allocating Memory for a Pointer

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Characteristics of Pointers We’ve seen that pointers can be initialized and assigned (like any variable can). They can be local or global variables (or parameters) You can have an array of pointers etc., just like any other kind of variable. We’ve also seen the dereference operator (*). This is the operation that really makes pointers special (pointers are the only type of variable that can be dereferenced).

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Pointer “Size” Note: Pointers are all the same size. On most computers, a pointer variable is four bytes (32 bits). However, the variable that a pointer points to can be arbitrary sizes. A char* pointer points at variables that are one byte long. A double* pointer points at variables that are eight bytes long. When pointer arithmetic is performed, the actual address stored in the pointer is computed based on the size of the variables being pointed at.

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Constant Pointers A pointer to const data does not allow modification of the data through the pointer const int a = 10, b = 20; a = 5; // Error const int *p; int *q; p = &a; // or p=&b; *p = 100; // Error : p is (const int *) p = &b; q = &a; *q = 100; // OK !!!

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Constant Pointers int x; /* define x */ int y; /* define y */ /*ptr is a constant pointer to an integer that can be modified through ptr, but ptr always points to the same memory location */ int * const ptr = &x; *ptr = 7; /* allowed: *ptr is not const */ ptr = &y; /* error: cannot assign new address */

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Constant Pointers int x = 5; /* initialize x */ int y; /* define y */ /*ptr is a constant pointer to a constant integer. ptr always points to the same location; the integer at that location cannot be modified */ const int * const ptr = &x; *ptr = 7; /* error: cannot assign new value */ ptr = &y; /* error: cannot assign new address */

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Pointer to pointer int main(void) { int s = 1; int t = 1; int *ps = &s; int **pps = &ps; int *pt = &t; **pps = 2; pt = ps; *pt = 3; return 0; }

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Multiple indirection int a = 3; int *b = &a; int **c = &b; int ***d = &c; int ****f = &d;

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Pointer Initialization

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NULL Pointer Special constant pointer NULL Points to no data Dereferencing illegal To define, include int *q = NULL;

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NULL Pointer

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NULL Pointer Often used as the return type of functions that return a pointer to indicate function failure int *myPtr; myPtr = myFunction( ); if (myPtr == NULL){ /* something bad happened */ } Dereferencing a pointer whose value is NULL will result in program termination.

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Generic Pointers: void * void *: a pointer to anything Lose all information about what type of thing is pointed to Reduces effectiveness of compiler’s type-checking Can’t use pointer arithmetic

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Operations on Pointers

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Arithmetic Operations When an integer is added to or subtracted from a pointer, the new pointer value is changed by the integer times the number of bytes in the data variable the pointer is pointing to For example, if the pointer p contains the address of a double precision variable and that address is 234567870, then the statement: p = p + 2; // 234567870 + 2 * sizeof(double) would change p to 234567886

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Operations on Pointers

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Arithmetic Operations A pointer may be incremented or decremented An integer may be added to or subtracted from a pointer. Pointer variables may be subtracted from one another int a, b; int *p = &a, *q = &b; p = p + q ; // Error p = p * q; // Error p = p / q; // Error p = p - q; // OK p = p + 3; p += 1.6; // Error p %= q; // Error

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Arithmetic Operations pointer + number pointer – number

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Comparing Pointers Pointers can also be compared using ==, !=, , = Two pointers are “equal” if they point to the same variable (i.e., the pointers have the same value!) A pointer p is “less than” some other pointer q if the address currently stored in p is smaller than the address currently stored in q. It is rarely useful to compare pointers with

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Logical Operations Pointers can be used in comparisons int a[10], *p, *q , i; p = &a[2]; q = &a[5]; i = q - p; /* i is 3*/ i = p - q; /* i is -3 */ a[2] = a[5] = 0; i = *p - *q; // i = a[2] – a[5] if (p < q) ...; /* true */ if (p == q)...; /* false */ if (p != q) ...; /* true */

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Pointers and Arrays the value of an array name is also an address In fact, pointers and array names can be used interchangeably in many (but not all) cases The major differences are: Array names come with valid spaces where they "point" to. And you cannot "point" the names to other places. Pointers do not point to valid space when they are created. You have to point them to some valid space (initialization)

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Pointers and Arrays

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Pointers and Arrays

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An Array Name is Like a Constant Pointer Array name is like a constant pointer which points to the first element of the array int a[10], *p, *q; p = a; /* p = &a[0] */ q = a + 3; /* q = &a[0] + 3 */ a ++; /* Error !!! */

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Example int a[10], i; int *p = a; // int *p = &a[0]; for (i = 0; i < 10; i++) scanf("%d", a + i); // scanf("%d", &a[i]); for (i = 9; i >= 0; --i) printf("%d", *(p + i)); // printf("%d", a[i]); //printf("%d", p[i]); for (p = a; p < &a[10]; p++) printf("%d", *p);

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An example int a[10], *p, *q; p = &a[2]; q = p + 3; p = q – 1; p++; q--; *p = 123; *q = *p; q = p; printf("%d", *q); /* printf("%d", a[5]) */

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An Example int a[10], *p; a++; //Error a--; // Error a += 3; //Error p = a; // p = &a[0]; p ++; //OK p--; // Ok P +=3; // Ok

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Array Example Using a Pointer

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Array of Pointers int a=1, b=2, c=3, d=4; int *k[4] = {&a, &b, &c, &d}; printf("%d %d %d %d", *k[0], *k[1],*k[2],*k[3]);

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$Strings In C, strings are just an array of characters Terminated with ‘\0’ character Arrays for bounded-length strings Pointer for constant strings...$

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Strings In C, strings are just an array of characters Terminated with ‘\0’ character Arrays for bounded-length strings Pointer for constant strings (or unknown length)

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Strings & Pointers

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Strings in C (cont’d)

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char Array vs. char *: Example

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An Example char *str, s[] = "ALIREZA"; printf("%s", s); // ALIREZA printf(s); // ALIREZA printf("%s", s + 3); // REZA scanf("%s", s); scanf("%s", &s[0]); str = s; while(* str) putchar(*str++); // *s++ : Error

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Array of Pointers char *suit[ 4 ] = { "Hearts", "Diamonds", "Clubs", "Spades" };

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Empty vs. Null Empty string "" Is not null pointer Is not uninitialized pointer

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Multi-Dimensional Arrays int a[row][col]; a[row][col]  *(a[row] + col) a  a[0][0]  a[0]

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Call by value

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Call by reference

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Pointers in Functions

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Swap function (wrong version)

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swap function (the correct version)

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Now we can get more than one value from a function Write a function to compute the roots of quadratic equation ax^2+bx+c=0. How to return two roots? void comproots(int a,int b,int c, double *dptr1, double *dptr2) { *dptr1 = (-b - sqrt(b*b-4*a*c))/(2.0*a); *dptr2 = (-b + sqrt(b*b-4*a*c))/(2.0*a); return; }

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Trace a program

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Pointer as the function output

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Pointer as the function output

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Pointer to constant: const *

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Constant pointer: * const

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Passing Arrays to Functions #include void display(int a) { printf("%d",a); } int main() { int c[] = {2,3,4}; display(c[2]); //Passing array element c[2] only return 0; }

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Arrays in Functions

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Passing Arrays to Functions #include float average(float a[], int count); // float average(float *a, int count) int main(){ float avg, c[]={23.4, 55, 22.6, 3, 40.5, 18}; avg=average(c, 6); /* Only name of array is passed as argument */ printf("Average age=%.2f", avg); return 0; } float average(float a[], int count){ // float average(float *a int I; float avg, sum = 0.0; for(I = 0;I < count; ++i) sum += a[i]; avg = (sum / 6); return avg; }

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Passing Arrays to Functions #include void f1(float *a) { a[1] = 100;} void f2(float a[]){ a[2] = 200;} void printArray(float a[]) { int i = 0; for(; i < 6; i++) printf("%g ", a[i]); } int main(){ float c[]={23.4, 55, 22.6, 3, 40.5, 18}; f1(c); printArray(c); puts(""); f2(c); printArray(c); return 0; }

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Pointer to functions

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Pointer to Function #include void f1(float a){ printf("F1 %g", a);} void f2(float a){ printf("F2 %g", a);} int main(){ void (*ptrF)(float a); ptrF = f1; ptrF(12.5); ptrF = f2; ptrF(12.5); getch(); return 0; }

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Example

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Pointer to function

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Dynamic Memory Allocation

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Dynamic Memory Allocation Memory is allocated using the: malloc function (memory allocation) calloc function (cleared memory allocation) Memory is released using the: free function note: memory allocated dynamically does not go away at the end of functions, you MUST explicitly free it up The size of memory requested by malloc or calloc can be changed using the: realloc function

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malloc Prototype: void *malloc(size_t size); function returns the address of the first byte programmers responsibility to not lose the pointer Example:

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calloc

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Example of malloc and calloc

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Example

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malloc and calloc

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malloc vs. calloc

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Free

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free Prototype: void free(void *ptr) releases the area pointed to by ptr ptr must not be null trying to free the same area twice will generate an error

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#include

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#include

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Reallocation

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realloc Example float *nums; int I; nums = (float *) calloc(5, sizeof(float)); /* nums is an array of 5 floating point values */ for (I = 0; I < 5; I++) nums[I] = 2.0 * I; /* nums[0]=0.0, nums[1]=2.0, nums[2]=4.0, etc. */ nums = (float *) realloc(nums,10 * sizeof(float)); /* An array of 10 floating point values is allocated, the first 5 floats from the old nums are copied as the first 5 floats of the new nums, then the old nums is released */

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Allocating Memory for a Pointer There is another way to allocate memory so the pointer can point to something: #include #include int main(){ int *p; p = (int *) malloc( sizeof(int) ); /* Allocate 4 bytes */ scanf("%d", p); printf("%d", *p); // .... free(p); /* This returns the memory to the system*/ /* Important !!! */ }

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Allocating Memory for a Pointer You can use malloc and free to dynamically allocate and release the memory int *p; p = (int *) malloc(1000 * sizeof(int) ); for(i=0; i