Lecture 2 Pointers Pointers with Arrays Dynamic Memory Allocation

Lecture 2

• Pointers

• Pointers with Arrays

• Dynamic Memory Allocation

Recollection of the Previous Lecture

• Why Data Structure Needed?– Programming and Data Processing requires

efficient algorithms for accessing data in main memory and on secondary storage devices.

– This efficiency is directly linked to the structure of the data being processed.

• What is Data Structure?– It is a way of organizing data that considers not

only the items stored but also their relationship to each other.

Cont.

• Basic Data Types– Integer– Real – Boolean – Character

• Arrays– and some operations on arrays

• What is a POINTER? A pointer variable contains an address. A pointer variable can be declared as follows:

int     *ptr;

int     a = 23, b;

• This declares ptr to be a (pointer) variable that can contain the address of an integer variable.

Pointer

Pointer

• ptr = &b;  /* Assigns ptr the address of b */

• *ptr = a; /* The contents of the variable pointed by ptr

becomes the value of a*/

Pointer

c = *ptr;       /* Assigns the value of the variable pointed by ptr (i.e. contents of b)   to the variable c, (i.e. c

becomes 23).    */

cout << c << *ptr;   /* prints c and contents of address ptr */   /*prints 23,23 */

Pointer

In C++, pointer arithmetic is scaled. Consider the following program:

int a,*ptr;

ptr = &a;

if &a is equal to 276314 then ptr+1 will contain 276314+sizeof(int)

Pointers and Arrays Consider the following program:

#include<iostream.h>

void main()

{

int a[10];

cout<<&a[0]<<endl;

cout<<a;

}

The two cout will have the same output because the name of an array is actually representing the address of element 0 of the array.

Therefore&a[i] is the same as a+i a[i] is the same as *(a+i )

Pointers and Arrays

Example #include <stdio.h>

void main()

{

int a[5],i;

for (i=0;i<5;i++) cin >>a+i ;

for (i=0;i<5;i++) cout <<*(a+i);

}

Pointers and Arrays

Two dimensional Arrays: The case of two dimensional array is interesting. Every

two dimensional array is stored in the memory row by row. From the row and column index (i and j), we can calculate exactly where an element a[i][j] is stored in the memory.

In a 2-D array, the name of the array represent the address of the first row.

Pointers and Arrays

If we know the address of the first element, then given the index i and j of any element, we can find its address as

i*maximum number of columns +j.

Example - Next Slide

Pointers and Arrays

#include<iostream.h>

void main()

{

char a[4][3]={{'m','a','t'},{'s','a','t'},{'h','a','t'},{'c','a','t'}};

int i,j;

char c;

Pointers and Arrays

for (i=0;i<4;i++)

{

for (j=0;j<3;j++)

{

cout<<*((char*) a+i*3+j));

}

cout<<endl;

}

}

DYNAMIC MEMORY ALLOCATION

Memory is allocated for variables in two ways:

Static (or at compile-time)

Dynamic (or at run-time)

In a program which declares an integer variable x, at compile time itself, memory locations of size enough for an integer ( 2 locations since size of an integer is 2 bytes) is reserved for x.

• Arrays are also known as static data structures since they also get their required memory allocated during compile time..

C-DATA TYPES AND DATA STRUCTURE CLASSIFICATION

C-DATA TYPES AND DATA STRUCTURE CLASSIFICATION

Disadvantage :

The problem with static memory allocation is that the memory usage may not be efficient

Example : Consider the case of array marks to store the marks of a class of a maximum size 100. It is likely that in each semester the number of students may vary. In a given semester even if 25 students are there, 100 locations will still be reserved and 75 of them wasted.

We can re-write the program each time with the array declaration exactly matching the number of students, but then the program is no longer a general one.

DMA

Dynamic memory allocation is in contrast with this. Memory gets allocated at the time of running the program and hence we can use memory to exactly meet our needs.

Allocated memory can be many types:Contiguous memory allocation: Allocation of adjacent memory

locations

Non-Contiguous memory allocation: Allocation of non adjacent memory locations

Heap: Collection of all free memory locations available for allocation

De-allocation:

Releasing of allocated memory after use, to be added back to the heap.

DMA

• The new and delete operators do dynamic allocation and deallocation in much the same manner that the malloc() and free() functions do in C .

The new operator requires one modifier which must be a type.

The delete operator can only be used to delete data allocated by a new operator. If the delete is used with any other kind of data, the operation is undefined and anything can happen.

DMA

Example :

#include<iostream.h>

void main()

{

int *p;

p=new int;

*p=56;

cout<<“Memory allocated at ”<<p<<endl;

cout<<“Integer in memory="<<*p<<endl;

}

DMA

Another Example :#include<iostream.h>

void main()

{

struct pair

{

int x;

int y;

};(*p).x is equivalent to p->x (*p).y is equivalent to p->y

struct pair *p;p= new pair;(*p).x=56;(*p).y=47;cout<<p->x<<endl<<p->y;}

DMA

• Proper programming practice requires that all memory that is allocated to be freed after use

• In the previous programs, when the program finishes running the operating system frees all memory allocated.

DMA

#include <iostream.h>

#include <string.h>

int main()

{

struct date

{

int month;

int day;

int year;

};

int index, *pt1,*pt2;

pt1 = &index;

*pt1 = 77;

pt2 = new int;*pt2 = 173;cout<<"The values are "<<index<<" " <<*pt1<<" "<<*pt2<<endl;pt1 = new int;pt2 = pt1;*pt1 = 999;cout<<"The values are "<<index<<" "<<*pt1<<" "<<*pt2<<endl;delete pt1;date *date_pt;date_pt = new date;date_pt->month = 10;date_pt->day = 18;date_pt->year = 1938;

DMA

cout<<date_pt->day<<"/"<<date_pt->month<<"/"<<date_pt->year<<endl;

delete date_pt;

char *c_pt;

c_pt = new char[37];

strcpy(c_pt,"John");

cout<<c_pt<<endl;

delete c_pt;

return 0;

}