Ch 7-pointers

Pointers A pointer is a variable that holds a memory address. This address is the location of another object in memory. For example, if one variable contains the address of another variable, the first variable is said to point to the second. Basically, a pointer contains an address of another variable. Pointers provide an indirect means of accessing or retrieving the data from memory .

Pointers Variable in memory 1000 1001 1002 1003 1004 1005 1006 103 Memory address

The ‘&’ and ‘*’ Consider the declaration int ab=3; This declaration tells the compiler Reserve space in memory to hold the integer value. Associate the name ab with this memory location. Store the value 3 at this location.

The ‘&’ and ‘*’ 3 ab 1000 Location name Value at Location Address

Declaring pointer variable In C every variable must be declared for its data type. Since pointer variables contain addresses that belong to a separate data type they must be declared as pointers. The declaration of pointer variable takes following form: data_type *variable_name; For example:- int *q; here q is pointer variable that can hold the address

The ‘&’ and ‘*’ ‘ &’  Address of operator ‘ *’  Value at address operator. Also called the Indirection Operator. ‘ &a’  Returns the address of variable a. ‘ *a’  Returns the value stored in a particular address.

An Example:Pointers main() { int j=5; printf(“Address of j=%d\n”,&j); printf(“Value of j=%d\n”,j); printf(“Value of j=%d\n”,*(&j)); } Output: Address of j = 1000 Value of j = 5 Value of j = 5

#include <stdio.h> int main() { int a = 10, b = 2; int *p1, *p2; p1 = &a; p2 = &b; printf("%u %u \n", p1, p2); printf("%d %d \n", *p1, *p2); return 0; } A program that prints out pointers and the values they point to:

Write a program that computes the area and perimeter of a rectangle: #include <stdio.h> void rectangle(int a, int b, int * area, int * perim); int main() { int x, y; int area, perim; printf("Enter two values separated by space: " ); scanf("%d %d", &x, &y); rectangle(x, y, &area, &perim); printf("Area is %d Perimeter is %d\n", area, perim); return 0; } void rectangle(int a,int b,int * area,int * perim) { *area = a * b; *perim = 2 * (a + b); }

Pointer Expressions Expressions involving pointers can be classified as follows: Pointer Assignments Pointer Arithmetic Pointer Comparison Pointer Conversion

Pointer Assignments Consider the following example Here i’s value is 5 and j’s value is i’s address. i 5 1000 1000 2000 j

Pointer Assignments Since, the variable j is containing an address, it is declared as int *j; This declaration tells the compiler that j will be used to store the address of an integer value – i.e j points to an integer.

An Example:Pointer Assignments main() { int j=3; int *k; k = &j; printf(“Address of j=%d”,&j); printf(“Address of j=%d”,k); printf(“Address of j=%d”,&k); printf(“Value of k=%d”,k); printf(“Value of j=%d”,j); printf(“Value of j=%d”,*(&j)); printf(“Value of j=%d”,*j); } Output: Address of j = 1000 Address of j = 1000 Address of k = 2000 Value of k = 1000 Value of j = 3 Value of j = 3 Value of j = error

Pointer Conversions One type of pointer can be converted to another type of pointer. Consider the following example: main() { double x = 100.1,y; int *p; /*The next statement causes p to point to double*/ p = (int*)&x; y = *p; printf(“The value of x is: %f”,y); }

Pointers v/s Arrays Consider the following program main() { static char arr[10]=“Embedded”; char *s=“Embedded”; printf(“%s\n”,arr); printf(“%s\n”,s); } Output: Embedded Embedded

Pointers v/s Arrays Consider the following program main() { static char arr[10]=“Embedded”; char *s=“Embedded”; s++; printf(“%s\n”,arr); printf(“%s\n”,s); } Output : Embedded mbedded

Analysis: Pointers v/s Arrays s++ increments s such that it starts pointing to ‘m’ of Embedded and hence would print mbedded as output. arr++ would give an error message because the information known to us is the base address and arr++ is attempting to change this base address.

Arrays of Pointers As there are an array of ints, floats, similarly there can be an array of pointers. An array of pointers represent a collection of addresses.

An Example: Arrays of Pointers main() { int *arr[4]; int i=5,j=10,k=15,l=20,m; arr[0]=&i; arr[1]=&j; arr[2]=&k; arr[3]=&l; for(m=0;m<=3;m++) printf(“%d”,*(arr[m])); }

Arrays of Pointers 1000 2500 2000 1500 i 5 10 j 20 15 l k 2500 2000 1500 1000 arr[0] arr[3] arr[2] arr[1] 1200 2700 2200 1700

Pointers To Pointers Pointer as we know is a variable which contains an address of another variable. A pointer which contains an address of another pointer variable can be defined as a pointer to a pointer or DOUBLE POINTER .

Double Pointers 3 i 1000 1000 2000 j 2000 3000 k

Double Pointer main() { int i=3; int *j; int **k; j=&i; k=&j; printf(“Address of i is %d%d%d\n”,&i,j,*k); printf(“Address of j is %d%d\n”,&j,k); printf(“Address of k is %d\n”,&k); printf(“Value of i is %d%d%d%d\n”,i,*(&i),*j,**k); printf(“Value of j is %d\n”,j); printf(“Value of k is %d\n”,k); }

Double Pointer OUTPUT: Address of i is 1000 1000 1000 Address of j is 2000 2000 Address of k is 3000 Value of i is 3 3 3 3 Value of j is 1000 Value of k is 2000

Summary A pointer is a variable that holds a memory address. Basically, a pointer contains an address of another variable. Pointers provide an indirect means of accessing or retrieving the data from memory. Arithmetic operations such as addition, multiplication and division on two pointers are not allowed. A pointer variable cannot be multiplied/divided by a constant or a variable. A pointer which contains an address of another pointer variable can be defined as a pointer to a pointer or DOUBLE POINTER .

Ch 7-pointers

More Related Content

What's hot (20)

Viewers also liked (8)

Similar to Ch 7-pointers (20)

Recently uploaded (20)

Ch 7-pointers