QUESTION 01 · Pointer Basics

What is a pointer?

2 marks

Answer

A pointer is a value used to identify an object or function, or to represent a permitted special position such as a null pointer.

An int * can point to an int object.
&x obtains an address; *p accesses the object identified by a valid object pointer p.

Define a pointer and explain the address and dereference operators.

5 marks

Answer in brief

A pointer variable stores a pointer value. A valid object pointer can be used to access an object indirectly.

Declaration

int number = 10;
int *p = &number;

Address operator

&number produces the address of number. The declaration stores that address in p. The star in int *p declares a pointer type; it does not read the object at that point. The pointer and the pointed-to integer are separate objects.

Dereference operator

In an expression, *p accesses the integer to which p points. Reading *p initially gives 10. Assigning *p = 25 changes number itself to 25. It does not change which object p identifies.

Example trace

The program first prints the value through *p. It then assigns through *p and prints number. The second output is 25, demonstrating that direct access through number and indirect access through p reach the same object.

Important points

Initialise a pointer before using its stored value.
Dereference only a pointer that identifies a suitable live object.
Do not dereference NULL, a one-past-end position or an invalid pointer.
Use %p with an object pointer converted to void * when displaying an address; the displayed form depends on the implementation.

Complexity

A direct pointer dereference and assignment to a fixed-size object take O(1) work in the usual model. A pointer does not automatically allocate the object it is expected to identify.

Diagram or flowchart

CodeBhavya illustration of pointer object. — Define a pointer and explain the address and dereference operators. · Download diagram

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>

int main(void)
{
    int number = 10;
    int *p = &number;
    printf("Before = %d\n", *p);
    *p = 25;
    printf("After = %d\n", number);
    return 0;
}

Sample input and output

This example uses values already written in the program.

Output

Before = 10
After = 25