Pointer Arithmetic

Do you want to be a Master in C? then you should have to know about pointer arithmetic.

Referencing a Pointer:

• Referencing the pointer means storing the address of particular object into the pointer variable.

For ex:

int no = 10;
int *ptr = & no;

• In this example we store the address of no variable in a pointer variable ptr. & operator is used for retrieving the address and it is called as “address of operator “.

• This “&” operator is not used for an array and a function names because name of an array and function itself is the address.

Consider the example of a function pointer:

int Fun(int , char);
int (*fptr)(int , char);
fptr = Fun; // Here there is no need to use the & operator.

• Important point is that we can’t store a constant value directly inside the pointer variable like

int *ptr = 1859; // It gives error

Dereferencing a Pointer :
• Dereferencing the pointer means, accessing the contents at which our pointer is pointing, means access the contents at an address which is stored in our pointer variable.
For ex:
int no = 21;
int * ptr = &no;
printf(“%d”,*ptr); // 21

• In this example, we are able to access the contents whose address is stored in pointer ptr.
• Number of a bytes derefer by a pointer is depend on the type of a pointer but it is does not depend on the pointed contents.
• Means if our pointer is of character pointer, it can access 1 bytea and if our pointer is of integer pointer, it can access only 4 bytes.

Arithmetic operations on pointers:
Addition operation :
We can add an integer constant with the pointer variable.

For ex:
int no = 51;
int *p = &no;

suppose an address of no variable is 100, means pointer p stores address 100.

p = p +2;
Result = pointer value + integral constant * size of pointer type.

Which evaluates as
p = 100 + (2 * 4);
p = 100 + 8;
p = 108;

Now our pointer is pointing at address 108.

Addition of a pointer and an integer is commutative means
(p + 2) is same as (2 + p)

We can not add tow pointers means
int no1 = 21, no2 = 51;
int *p = &no1;
int *q = &no2;
p + q; //It gives error


Pointer Arithmetic

Increment operation on pointer:
• C provides “++” as a increment operator.
• We can apply this “++” operator on a pointer variable also.
• Effect of this “++”operator is same as addition of pointer with 1.

For ex:
double d = 3.15;
double *p = &d;
p++ is evaluates as
p = p + size of pointer type

suppose in our example p is pointing at address 200 then after incremente the operation it points at 208. It can be simplified as
p = 200 + 8;
p = 208;
Subtraction operation :
We can subtract constant integer value from the pointer by applying same rule as a addition operation.

For ex:
float no = 7.8;
float *p = &no;

suppose an address of variable no is 8000, means pointer p stores address 8000.

p = p – 2;
Result = pointer value + integral constant * size of pointer type.

This evaluates as
p = 8000 – (2 * 4);
p = 8000 – 8;
p = 7992;

Now our pointer is pointing to an address 7992.
Subtraction of the pointer and an integer is not commutative means
(p + 2) is not same as (2 + p). (2 + p) is illegal operation.

• Two pointers can also be subtracted but this is only meaningful when both the pointers are pointing to the elements of same array.
• Then the result of it will be difference subscripts of two array elements.

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Sourabh Bhunje

Sourabh Bhunje, B.E. IT from Pune University. Currently Working at Techliebe. Professional Skills: Programming - Software & Mobile, Web & Graphic Design, Localization, Content Writing, Sub-Titling etc. http://techliebe.com/about-us

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