Unit 11: Pointers

1. Introduction to Pointers

A Pointer is a special variable that stores the memory address of another variable. Instead of holding a direct value like an integer or a character, a pointer "points" to the location where a value is stored in the computer's memory.

2. Pointer Data Type and Declaration

A pointer must be declared with a data type that matches the type of the variable it points to. This is because the compiler needs to know how many bytes of memory to read when accessing the value at that address.

Syntax:

data_type *pointer_name;

Example:

• int *p; - A pointer to an integer variable.
• float *fptr; - A pointer to a floating-point variable.
• char *ch; - A pointer to a character variable.

3. Initialization of Pointers

Initialization is the process of assigning a memory address to a pointer variable. This is done using the Address-of Operator (&).

Example:

 int num = 10; int *ptr; // Declaration ptr = # // Initialization (ptr now stores the address of num) 

Warning: Always initialize pointers. A pointer that is declared but not initialized is called a Wild Pointer and contains a random memory address, which can lead to system crashes.

4. Accessing Values Using Pointers

You can access the actual value stored at the memory address held by a pointer using the Dereferencing Operator (*), also known as the Indirection Operator.

Example:

 int x = 25; int *p = &x; printf("Value of x: %d", *p); // Output: 25 

In the above code, *p tells the computer to "go to the address stored in p and get the value located there."

5. Pointer Expressions and Arithmetic

Pointer arithmetic allows you to perform addition and subtraction on pointers. However, unlike regular math, pointer arithmetic is based on the size of the data type it points to.

Rules of Pointer Arithmetic:

• Increment (ptr++): Moves the pointer to the next memory location of its type. For an int pointer on a 4-byte system, it adds 4 to the address.
• Decrement (ptr--): Moves the pointer to the previous memory location.
• Addition (ptr + n): Adds n * sizeof(data_type) to the address.
• Subtraction (ptr - n): Subtracts n * sizeof(data_type) from the address.

Formula for Addition:
New_Address = Current_Address + (n * Size_of_Data_Type)

6. Operations on Pointers

Beyond basic arithmetic, several other operations can be performed on pointers:

• Subtraction of two pointers: This gives the number of elements between the two addresses. Both pointers must point to the same array.
• Pointer Comparison: You can compare two pointers using relational operators (==, !=, <, >) to see which one points to a higher or lower address in the same array.
• Assignment: You can assign one pointer to another if they are of the same type.

7. Exam Focus Enhancements