Arrays and Pointers in C

Arrays Limitations

• Arrays cannot be assigned to one another
• The size of an array must be specified when it is not initialized
• Constant values should define the size of an array in Visual Studio
• Array return types are invalid in functions

Pointers

• The C language uses pointers as one of its most powerful features
• Pointers facilitate pass-by-reference and enable the creation of dynamic data structures
• Examples of dynamic data structures include linked lists, queues, stacks, and trees that evolve during program execution
• Pointers are variables storing memory addresses as their values
• A variable holds a specific value and directly references it
• Pointers contain an address of a variable, thus indirectly referencing a value, known as indirection

Pointers - Declaration and Initialization

• Pointers must be defined before use
• An example of creating a pointer: int *countPtr; will point to an integer object
• When declaring multiple variables with pointers, the asterisk applies only to the variable immediately following it, others are treated as normal variables
• Pointers can be defined to point to objects of any type
• Pointers should be initialized when defined, typically to NULL, zero, or a valid address
• Assigning zero or NULL indicates that the pointer does not reference any memory location

Pointer Operators - Address Operator &

• The & operator, known as the address operator, is a unary operator that returns the address of its operand
• For integers, the address operator outputs the memory address
• Assigning the address of a variable to a pointer allows interaction with memory locations

Pointer Operators - Indirection Operator asterisk

• The unary operator *, called the indirection or dereferencing operator, retrieves the value of the variable to which its operand, a pointer, points to
• Given an integer pointer yPtr that holds the address of y, the indirection operator lets you access y

Pointer Operators - Example

• The address operator & and the indirection operator * are complements of one another
• Applying both to a variable cancels out their effects, returning the original value

Passing Arguments to Functions

• Two ways exist to pass arguments to a function: by value and by reference
• Pass-by-value involves passing a function with a copy of the argument
• Pass-by-reference involves passing a function the caller's actual variables
• In C/C++, all arguments except arrays are passed by value
• Arrays are treated as addresses
• Pointers and indirections can simulate pass-by-reference, allowing functions to alter variables
• With pass-by-reference, modifying a variable requires using the indirection operator to interact with the value stored at the memory address

Arrays — Passing Arrays to Functions

• Accessing array elements in functions uses the memory address of the first element
• Use the conversion specifier to print memory addresses as hexadecimal integers
• One dimensional arrays passed into a function act as a pointer to the first element
• When a compiler encounters a function parameter written int b[], it converts it to int *b which are interchangeable

Arrays — Pass by Reference

• When using call by reference to pass an array, the function modifies the original array's contents
• For example a modification can multiply each array element by a value, visible outside the function

Pointer Expressions and Arithmetic

• Pointers support specific arithmetic operations
• Pointers can be incremented or decremented
• An integer can be added or subtracted from a pointer
• Pointers can be subtracted from each other if pointing to the same array
• Incrementing / decrementing a pointer shifts it by the size of the data type it points to
• Pointer arithmetic is valid only when performed on arrays where variables are stored contiguously

Pointer Expressions and Arithmetic - Example

• When an array x starts at memory location 3000, adding 2 to a pointer xPtr will produce 3000 + 2*4 = 3008 assuming an integer is 4 bytes
• The xPtr++ or ++xPtr statements increment the pointer to the next location in the array

Relationship between Pointers and Arrays

• Given bPtr as a pointer to array b, bPtr = b is equivalent to bPtr = &b[0]
• b[3] is equivalent to *(bPtr + 3) in pointer/offset notation
• &b[3] is equivalent to bPtr + 3
• Array names in C are constant pointers and always point to the start of the array

Pointer Expressions and Arithmetic

• Pointers of the same type can be assigned to each other
• Pointers to void (void *) serve as generic pointers that are able to represent any pointer type
• All pointers can be assigned to a void pointer
• A void pointer cannot be dereferenced without a cast, because the compiler doesn't know which data type it refers to
• Void pointers need to be cast to the appropriate type before dereferencing

Pointer to Char Example

• In stdio.h, ctype.h and writing function to modify strings, pointers are used extensively
• Pointers allow efficient manipulation and traversal of character arrays or strings