c-programming-class 4

8/14/2019 C-Programming-Class 4

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Session 04

Pointers


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Session Objectives

Pointer and pointer definition. Use of pointers in accessing single and

double dimensional arrays. String handling using pointers. Concept of singly and doubly linked lists.


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Session Topics

Concept of pointers Address of a variables Initialization of pointer

Pointer arithmetic Pointers to strings Pointers to pointers

Pointer to functions


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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 thesecond.Basically, a pointer contains an address of another variable.Pointers provide an indirect means of accessing or retrieving the data from memory.


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Pointers

Variable inmemory

1000

1001

1002

1003

1004

1005

1006

1003

Memoryaddress


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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.


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The & and *

3

ab

1000

Location name

Value at Location

Address


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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.


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An Example:Pointers main(){

int j=3; 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


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Pointer Expressions

Expressions involving pointers can beclassified as follows:

Pointer Assignments Pointer Arithmetic Pointer Comparison Pointer Conversion


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Pointer Assignments

Consider the following example

i

5

1000

1000

2000

j

Here is value is 5 and js value is is address.


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Pointer Assignments

Since, the variable j is containing anaddress, 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.


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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 = 3


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Operations on Pointers

Pointer variables can be used in various types of expressions.Consider i,j,a,p1 and p2 are pointer variables.

The following operations are valid:y = *i * *j;count = p1 p2;

p1--;a = p1 + 2;

b = p1 3;


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Operations on Pointers

Consider i,j,a,p1 and p2 are pointer variables. The following operations are invalid:

p1/2;

p1*2;p1*p2;

p1 + p2;

p1/p2;


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Permissible Operations on Pointers

A pointer can be decremented or incremented.Addition of two integers to pointers is allowed.

Subtraction of two pointers of same data type isallowed.The pointers can be compared using relationaloperators.


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Non-Permissible Operations on Pointers

Arithmetic operations such as addition,multiplicationand division on two pointers are not allowed.A pointer variable cannot be multiplied/divided by aconstant or a variable.Address of a variable cannot be altered.


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Pointer Comparisons

Comparison of two pointers can be done.Generally,pointer comparisons are useful only when two

pointers point to a common object.

Example:If p and q are pointers then the followingstatement is valid.if(p


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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 todouble*/

p = (int*)&x;y = *p;

printf(The value of x is: %f,y);}


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Analysis:Pointers v/s Arrays

In the first declaration Embedded gets stored in the arrayarr[].

In the second declaration Embedded gets stored at some place in the memory and the base address is stored in s.

s points to the zeroth character in the string. Mentioning the name of the array arr, we get its base

address.We can say that an array acts as a pointer to a character.


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Pointers v/s Arrays

Consider the following program main()

{static char arr[10]=Embedded;char *s=Embedded;s++;arr++;

printf(%s\n,arr); printf(%s\n,s);

}


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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++ isattempting to change this base address.


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Arrays of Pointers

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


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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


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Arrays of Pointers

1000 250020001500

i

5 10

j

2015

lk

2500200015001000arr[0] arr[3]arr[2]arr[1]

1200 270022001700


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Pointers To Pointers

Pointer as we know is a variable which containsan address another of variable.

A pointer which contains an address of another pointer variable can be defined as a pointer to a pointer or DOUBLE POINTER .


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Double Pointers

3

i

1000

1000

2000

j

2000

3000

k


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Command Line Parameters

Command-line arguments are given after the name of a program in command-line operating systems like DOS or Linux, and are passed in to the program from the operating

system.To use command line arguments in the program, you mustuse the full declaration of the main function.In fact, main can actually accept two arguments: oneargument is number of command line arguments, and theother argument is a full list of all of the command linearguments.


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Command Line Parameters

The integer, argc is the argument count. It is the number of arguments passed into the program from the command line, including the name of the program.

The array of character pointers is the listing of all the arguments.argv[0] is the name of the program, or an empty string if the name isnot available.

After that, every element number less than argc is a command lineargument. You can use each argv element just like a string, or use argvas a two dimensional array.

argv[argc] is a null pointer.


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An Example:Command Line Parameters

Consider the following program#include int main(int argc, char *argv[]){

int x;printf("%d\n",argc);

for (x=0; x


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Analysis

The char *argv[] line is an array of pointers to string. In other words, each element of the array is a pointer, and each pointer

points to a string (technically, to the first character of the string).

Thus, argv[0] points to a string that contains the first parameter onthe command line (the program's name), argv[1] points to the next

parameter, and so on. The argc variable tells you how many of the pointers in the array are

valid. You will find that the code does nothing more than print each of

the valid strings pointed to by argv .


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Where are Command Line Parameters used?

Shell ScriptingInternet SolutionsFile System Functions


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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 .


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Thank You!

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