Array and pointer

Array and pointer

One dimensional array

When an array is declared, the compiler allocates a base address and sufficient amount of storage to contain all the elements of array. Array store elements in contiguous memory locations.

For ex,

int x[5];

Element	X[0]	X[1]	X[2]	X[3]	X[4]
Value	1	2	3	4	5
Address	100	102	104	05	106

                      Base Address                                


Create pointer variable

int *p;      // declare poiner

p=&x[0];  // assign base address

   or

p=x;

now the value of x can be accessed by pointer as follows:

p=&x[0] (100)

p+1=&x[1] (102)

p+2=&x[2] (104)

p+3=&x[3] (106)

p+4=&x[4] (108)

Here, the address of an element is calculated using its index and scale factor of data type

For ex,

Address of x[3]=base address+(3*scale factor of int)

100 + (3*2) = 106

When handling arrays, instead of using array indexing , pointers can be used to access array elements.

For ex, *(p+3) is equivalent to x[3]

The pointer accessing method is much faster than array indexing.

Ex,

  void main()

   {

      int a[] = {1,2,3,4,5};

      int *p,i;

      p=a;

      for(i=0;i<5;i++)

       {

          printf(“\n number %d = %d”,p+i);

       }

   }   

Two dimensional array

Two dimensional and Multidimensional arrays also represented using pointer.

Two dimensional array declared as follows:

          int p[3][3] = {{1,2,3},{4,5,6},{7,8,9}};

	Column[0]	Column [1]	Column [2]
Row[0]	1 [0][0]	2 [0][1]	3 [0][2]
Row [1]	4 [1][0]	5 [1][1]	6 [1][2]
Row [2]	7 [2][0]	8 [2][1]	9 [2][2]

Here,

          p                 = pointer to first row

          p+1             = pointer to first row

          p+i              = pointer to ith row

          *(p+i)                   = pointer to first element in ith row

          *(p+i)+j      = pointer to jth element in ith row

          *(*(p+i)+j) = value stored in the cell(i,j)

 ith row and jth column

Ex

          element p[2][2] represented as : *(*(p+2)+2)

The basic address of array is p[0][0] and starting this address compiler allocates contiguous space for all the elements row wise. That is , the first element of second row is placed immediately after the last element of first row. Element stored as:

1	2	3	4	5	6	7	8	9
row1			row2			row3

A two dimensional array is actually a collection of one dimensional arrays. Therefore two dimensional array defined as a pointer to a group of contiguous on dimensional arrays.

A two dimensional array can be declared as follows :

data-type (*ptr_var) [expression2]; 

Here, data-type refers to the data type of array , ptr_var is the name of the pointer variable , expression2 specifies the number of columns.

Ex

          int (*x)[20];  // similar as int x[10][20];

In this declaration x is defined to be pointer to a group of contiguous , one dimensional arrays. Thus x, points to the first 20-element array, which is actually the first row(row0) of the original two dimensional array. Similarly , (x+1) points to the second 20 elements array , which is the second row(row1) of the original two dimensional array , and so on.

Posted By : Ruchita Pandya