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
|
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
