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C ++ Concept-Pointers Arithmetic
Posted Date: 12 Jun 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: Girish Patil Member Level: Diamond
Rating:  Points: 3
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Pointers Arithmetic
In C pointer arithmetic following operations are possible
1. A pointer variable can be assigned to an ordinary variable.
2. A pointer variable can be assigned to another pointer variable.
3. A pointer variable can be assigned ‘NULL’ [ i.e. 0 ].
4. Integer quantity can be added or subtracted from a pointer variable.
5. Two pointer variables can be compared.
6. Two pointer variables can not be added or subtracted from a pointer variable.
This is very confusing subject, so let us illustrate it with an example. The following function change takes two arguments; x is an int while y is a pointer to int. It changes both values.
Change(int x, int *y)
{
cout<<”starting change ; x = “ << x << ”,y = “ << *y;
x++;
(*y)++;
cout<< “\n Finishing change : x = “ << x <<” ,y = “ << *y;
}
Since y is a pointer, we must de-reference it before incrementing its value.
A very simple program to call this function might be as follows
main()
{
int I = 0, j = 0;
cout<< “ starting main : I = “ << x <<“ ,j = “ << *y;
cout<<”\n Calling change now “;
change(I , &j);
cout<<”\n Returned from change “;
cout<<”\n Finishing main : I = “ << I << ” , j = ” << *y;
}
note here how a pointer to int is created using the & operator within the call change(I , &j);
the result of running the program will look like this
Starting main : I = 0, j = 0
Calling change now
Starting change : x = 0,y = 0
Finishing change : x =1,y = 1
Returned from change
Finishing main : I = 0, j = 1
After the return from change the value of I is unchanged while j, which was passed as a pointer, has changed. To summarize, if you wish to use arguments to modify the value of variable from function, these arguments must be passed as pointers, and de-referenced within the function.
Where the value of an argument isn’t modified, the value can be passed without any worries about pointers.
Common use of pointers
1. Accessing array elements
2. Passing elements to a function when function need to change the original values
3. Passing arrays & strings
4. Obtaining memory from system
5. Creating data structures like LINK LIST, STACKS
Pointers & Arrays
There is a close relation between pointer & arrays can be find out from example
#include
void main()
{
int a[5] = { 5, 2, 5, 4, 3 };
c
}
The program displays the content of array a. the same program can be written using pointer as
#include
void main()
{
int a[5] = { 5, 2, 5, 4, 3 };
for(int j = 0; j < 5; cout<< *(a + j++] << “\n”;
}
Passing array to function
This is very simple process can be understand by example below
#include
const m = 5;
void centimeter(double *in)
{
for ( int x = 0; x < m ; x++)
*in++ *= 2.54;
}
void main()
{
double a[5] = { 5.0, 2.8, 5.9, 4.25, 3.36 };
for(int j = 0; j < 5; j++)
cout<<” a[ “ << j+1 << ” ] = ” a[j] << ” inch. \n ”;
centimeter(a);
cout<<” a[ “ << j+1 << ” ] = ” a[j] << ” centimeters. \n ”;
}
In the above example the function accepts array of values in inches and converts them to centimeter.
This process can be used to sort the array in the required manner. [The program sorts the number in ascending order]
#include
const m = 10;
void sort(double *in)
{
for(int x = 0; x < m; x++)
for( int y = x+1; y < m; y++)
if( *(in+x) > *(in+y))
{
int = *(in+x);
*(in+x) = *(in+y);
*(in+y) = z;
}
}
void main()
{
double a[m];
int j;
cout<<” Enter 10 numbers to be sorted “;
for(j = 0; j < m; j++)
cin>>a[j];
sort(a);
cout<<” The sorted Numbers are : \n “;
for(j = 0; j < m; j++)
cout<<”\na[ “<< j+1 << “]= “ << a[j];
}
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