5. Pointers

 

Address Operator

It is a unary variable which tells the address of a identifier.

#include <stdio.h>

int main()

{

    int a = 10;

    printf("Address of variable a: %u\n", &a);

    printf("Address of variable a in hexadecimal: %p\n", &a);

    printf("Address of main function: %u\n", &main);

    printf("Address of main function in hexadecimal: %p", &main);

}

Address of variable a: 2774529596
Address of variable a in hexadecimal: 00000050a55ff63c
Address of main function: 545985924
Address of main function in hexadecimal: 00007ff6208b1584

1. What of the following printf() statements will not give error?
printf("%u\n", &a);
    printf("%u\n", &10);
    printf("%u\n", &(a+b));

i. printf("%u\n", &a);
Explanation: Address operator works only with identifiers, not with a constant value and an expression.

Pointers

• Pointers are used to store address of a variable.
• Size of all the pointer variables is same i.e., 2 or 4 bytes depends on the system.
• If we simply declare our pointer and do not assign it, then it will be assigned by a garbage value.
#include <stdio.h>


int main()
{
    int *iptr;
    printf("%u", iptr);
}

8

• If we want our free out pointer variable, then we can assign it NULL or 0. 
#include <stdio.h>


int main()
{
    int a = 10;
    int *iptr = &a;
    printf("%u\n", iptr);
    iptr = NULL;
    printf("%u\n", iptr);
}

1904212292
0

1. What is the need of data type while declaring a pointer variable?

All pointer variables are of same size and it stores the base address of any variable, so data type is required to access the required bytes by the pointer variable.

2. Why pointers are removed from Python and Java?

Consider a variable i of type int contains a value 10. If we free this i, then i will not point to that location, but the value will be at the location. If some new pointer variable gets that location, then it will contain the previous value as the garbage value and suppose if this address stored by the pointer is a important file, then user can change it. So pointers are removed in Java from user control, but internally pointers are still in these.

3. What is the output of the following C code?
#include <stdio.h>


int main()
{
    int a = 10;
    float *fptr = &a;
    printf("%d", *fptr);
}
1. Compiler error
2. Wrong output
3. Correct output

ii. Wrong output
warning: initialization of 'float *' from incompatible pointer type 'int *'
0

Addresses can be accessed in two ways:

1. Random Access Memory: Any memory location can be directly accessed if address is known.
2. Sequential Access: We have to traverse all previous addresses to reach final address.

1. What is the output of the following C code?
#include <stdio.h>


int main()
{
    int a = 87;
    float b = 4.5;
    int *p1 = &a;
    float *p2 = &b;
    printf("Value of p1 = Address of a = %p %p\n", p1, &a);
    printf("Value of p2 = Address of b = %p %p\n", p2, &a);
    printf("Address of p1 = %p\n", &p1);
    printf("Address of p2 = %p\n", &p2);
    printf("Value of a = %d %d %d\n", a, *p1, *(&a));
    printf("Value of b = %.1f %.1f %.1f", b, *p2, *(&b));
}

Value of p1 = Address of a = 0000009578fff79c 0000009578fff79c
Value of p2 = Address of b = 0000009578fff798 0000009578fff79c
Address of p1 = 0000009578fff790
Address of p2 = 0000009578fff788
Value of a = 87 87 87
Value of b = 4.5 4.5 4.5

2. What is the output of the following C code?
#include <stdio.h>


int main()
{
    int a = 5, *pi = &a;
    float b = 5.5, *pf = &b;
    char c = 'a', *pc = &c;
    printf("Value of pi = Address of a = %p\n", pi);
    printf("Value of pi = Address of b = %p\n", pf);
    printf("Value of pi = Address of c = %p\n", pc);
    pi++;
    pf++;
    pc++;
    printf("Now value of pi = %p\n", pi);
    printf("Now value of pf = %p\n", pf);
    printf("Now value of pc = %p\n", pc);
}

Value of pi = Address of a = 0000003b525ffa44
Value of pi = Address of b = 0000003b525ffa40
Value of pi = Address of c = 0000003b525ffa3f
Now value of pi = 0000003b525ffa48
Now value of pf = 0000003b525ffa44
Now value of pc = 0000003b525ffa40

3. What is the output of the following C code?
#include <stdio.h>


int main()
{
    int a, b;
    a = 15, b = 20;
    int *p = &a;
    a = *p++;
    printf("%d %d\n", a, b);
    *p = &a;
    printf("%d\n", (*p)++);
    printf("%d\n", *++p);
    printf("%d", ++*p);
}

warning: assignment to 'int' from 'int *' makes integer from pointer without a cast
*p = &a;

15 20
15
400554016
-1491607560
Explanation: Both are unary operators, so they have the same precedence but the associativity is from right to left.

Pointer Arithmetic

The following operations are valid with pointers:

• Addition with integer constant i.g., (iptr + 4)
• Subtraction with integer constant i.g., (iptr - 3)
• Post-increment operation e.g., iptr++
• Pre-increment operation e.g., ++iptr
• Post-decrement operation e.g., iptr--
• Pre-decrement operation e.g., --iptr
• Subtraction of two pointer variables e.g., ptr1-ptr2;

Addition, multiplication and division of two pointer variables is not a valid operation.

Pointer Comparions

We can also compare two pointers, if both points to the same address or NULL value, then they are equal.

#include <stdio.h>

int main()

{

    int a = 10, b = 20;

    int *iptr1, *iptr2;

    float x = 12.0;

    float *fptr = &x;

    printf("%d\n", iptr1 == iptr2);

    iptr1 = NULL;

    iptr2 = NULL;

    printf("%d\n", iptr1 == iptr2);

    iptr1 = &a;

    iptr2 = &b;

    printf("%d\n", iptr1 == iptr2);

    printf("%d\n", iptr1 >= fptr);

}

warning: comparison of distinct pointer types lacks a cast
printf("%d\n", iptr1 >= fptr);
0
1
0
1
Explanation: If we compare two pointers of different data types, then it will show the same warning, but will give no error.

Pointers to Pointers

We can also store the address of a pointer variable with the help of double pointer or pointer to pointer variable.

#include <stdio.h>

int main()

{

    int a = 10;

    int *p = &a;

    int **q = &p;

    printf("Address of a = %d = Value of p: %d\n", &a, iptr1);

    printf("Address of p = %d  Value of q: %d", &iptr1, iptr2);

}

Address of a = 1667234564 = Value of p: 1667234564
Address of p = 1667234552 Value of q: 1667234552

Similarly, we can also define three pointers or more.