POINTERS

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POINTERS

• A Pointer in C language is a variable that holds a memory address. This memory address is the address of another variable of same data type.
• In simple words, if one variable stores the address of second variable then the first variable can be said to point towards the second variable.

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Address of a memory location

• What is a Memory Address in C?
• Whenever a variable is defined in C language, a memory location is assigned for it, in which it's value gets stored. We can check this memory address, using the & symbol.
• If var is the name of the variable, then &var will give it's address.

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What is a Pointer

• A Pointer in C language is a variable that holds a memory address.
• Pointers are used to access memory of a variable and manipulate the value stored in it.
• Pointers are one of the most distinct and exciting features of C language.

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What is a Pointer

• It provides power and flexibility to the language.
• Although pointers may appear a little confusing and complicated in the beginning, but once we understand the concept, we will be able to do so much more with C language.

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What is a Pointer

• Whenever a variable is declared in a program, system allocates a location i.e an address to that variable in the memory, to hold the assigned value.
• This location has its own address number.

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Pointer Operators in C�

• Pointer Operators in C
• There are two pointer operators in C, they are:
• * operator
• & operator

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Pointer Operators in C�

• The & operator returns the memory address of its operand.
• a = &b;
• In the variable a the memory address of the variable b will get stored.
• The * operators is the complement of &. This operator returns the value located at the given address.
• For example, if a contains the memory address of the variable b,
• c = *a;
• , will store the value of the variable b into c.

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• Pointers are more efficient in handling Arrays in C and Structures in C.
• Pointers allow references to function and thereby helps in passing of function as arguments to other functions.
• Pointers also provide means by which a function in C can change its calling arguments.
• It reduces length of the program and its execution time as well.
• It allows C language to support Dynamic Memory management.

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• We can implement linked lists, trees, and graphs using pointers.

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Declaring a Pointer in C�

• The general syntax of pointer declaration is,
• datatype *pointer_name;
• Here, pointer_name is the name of the pointer and that should be a valid C identifier.
• The datatype of the pointer and the variable to which the pointer variable is pointing must be the same

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Declaring a Pointer in C�

• Following are some examples of declaring a pointer in C:
• int *ptr; //pointer to int
• float *ptr; //pointer to float
• char *ptr; //pointer to char
• double *ptr; //pointer to double

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Assigning Value to a Pointer Variable�

• When we declare a pointer, it contains garbage value, which means it could be pointing anywhere in the memory.
•  Pointer Initialization is the process of assigning the address of a variable to a pointer. In C language, the address operator & is used to determine the address of a variable. The & (immediately preceding a variable name) returns the address of the variable associated with it.
• For example,

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Derefrencing Pointer in C�

• Once a pointer has been assigned the address of a variable, the pointer is dereferenced, using the indirection operator or dereferencing operator, which is a *, to access the value of the variable.

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Derefrencing Pointer in C�

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• int x = 10;
• int *p = &x; // p stores the address of x
• printf("%d", *p); // dereferencing p → prints the value at that address → 10
• Here: p → contains the address of x
• *p → dereferences the pointer and gives the value 10

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Derefrencing Pointer in C�

• A pointer stores an address
• A dereferenced pointer gives the value at that address

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a) To declare a pointer to a variable:

• //Declare a pointer
• data_type_name * variable name
• //Example int *ptr;

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b)Initialization of pointer variables:

• The process of assigning the address of a variable to a pointer variable is known as initialization.
• Once a pointer variable has been declared we can use assignment operator to initialize the variable.

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Benefits of using pointers are:-

• 1)Pointers are more efficient in handling arrays and data tables.
• 2) Pointers can be used to return multiple values from a function via function arguments.
• 3) The use of pointer arrays to character strings results in saving of data storage space in memory.
• 4) Pointers allow C to support dynamic memory management.
• 5) Pointers provide an efficient tool for manipulating dynamic data structures such as structures , linked lists , queues , stacks and trees.
• 6) Pointers reduce length and complexity of programs.
• 7) They increase the execution speed and thus reduce the program execution time

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C Pointer Arithmetic�

• C Pointer Arithmetic

The pointer arithmetic refers to the arithmetic operations that can be performed on a pointer. It is slightly different from the ones that we generally use for mathematical calculations as only a limited set of operations can be performed on pointers.

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C Pointer Arithmetic

These operations include:

• Increment/Decrement
• Addition/Subtraction of Integer
• Subtracting Two Pointers of Same Type
• Comparison of pointers

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Incrementing a Pointer:�

• Incrementing/Decrementing a Pointer:
• ptr++ or ++ptr:
• Increases the pointer's address by the size of the data type it points to.
• If ptr points to an int, ptr++ moves it forward by sizeof(int) bytes.

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Incrementing a Pointer:�

• For Example:�If an integer pointer that stores address 1000 is incremented, then it will increment by 2(size of an int), and the new address will point to 1002.
• While if a float type pointer is incremented then it will increment by 4(size of a float) and the new address will be 1004.

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Decrementing a Pointer:�

• ptr-- or --ptr:
• Decreases the pointer's address by the size of the data type it points to.

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Decrementing a Pointer:�

• For Example:�If an integer pointer that stores address 1000 is decremented, then it will decrement by 2(size of an int), and the new address will point to 998.
• While if a float type pointer is decremented then it will decrement by 4(size of a float) and the new address will be 996.

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Adding an Integer to a Pointer:�

• ptr + n:
• Adds n * sizeof(data_type) bytes

to the pointer's address.

• The result is a pointer to the element n positions after the original.

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Subtracting an Integer from a Pointer:

• ptr - n:
• Subtracts n * sizeof(data_type) bytes

from the pointer's address.

• The result is a pointer to the element n positions before the original.

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Subtracting Two Pointers:�

• Subtracting Two Pointers:
• ptr1 - ptr2:
• Returns an integer value representing the number of elements between the two pointers.
• The result is (address_of_ptr1 - address_of_ptr2) / sizeof(data_type).
• This operation is only meaningful when both pointers point to elements within the same array or block of memory.

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Dynamic memory allocation

• Dynamic memory allocation in C refers to the process of allocating memory during program execution (runtime) rather than at compile time. 
• This allows for more flexible memory management, especially when the required memory size is unknown until the program runs. 
• Memory allocated dynamically resides in the heap section of memory, unlike local variables which reside on the stack.

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Dynamic memory allocation

• Dynamic memory allocation techniques give programmer control of memory when to allocate, how much to allocate and when to de-allocate.

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C Dynamic Memory Allocation�

•  an array is a collection of a fixed number of values. Once the size of an array is declared, we cannot change it.
• Sometimes the size of the array we declared may be insufficient. To solve this issue, we can allocate memory manually during run-time. This is known as dynamic memory allocation in C programming

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To allocate memory dynamically, library functions are 

• malloc()
• calloc()
•  realloc() 
•  free() 
• are used.
• These functions are defined in the <stdlib.h> header file.

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malloc() (Memory Allocation):

• Allocates a block of memory of a specified size (in bytes).
• Returns a void* pointer to the beginning of the allocated block, or NULL if allocation fails.
• The allocated memory is uninitialized and contains garbage values.

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Malloc syntax

• Syntax:
• void* malloc(size_t size);
• ptr = (int*) malloc(n * sizeof(int));

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Malloc syntax

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• Example:
• ptr = (int*) malloc(5 * sizeof(int));

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calloc() (Contiguous Allocation):

• Allocates memory for an array of a specified number of elements, each of a specified size.
• Initializes all allocated memory to zero.
• Returns a void* pointer to the beginning of the allocated block, or NULL if allocation fails.

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Calloc Syntax: �

• Syntax:

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• void* calloc(size_t num, size_t size);

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Calloc Syntax: �

Syntax:

• void* calloc(size_t num, size_t size);
• Example:
• Ptr=(int*)calloc(5,sizeof(int));

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realloc() (Reallocation):

• Changes the size of a previously allocated memory block.
• It can either increase or decrease the size. If increasing, the new memory might be uninitialized.
• Returns a void* pointer to the reallocated block, which might be at a new memory location, or NULL if reallocation fails.
• Syntax: void* realloc(void* ptr, size_t new_size);

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• realloc() (Reallocation):
• Changes the size of a previously allocated memory block.
• It can either increase or decrease the size. If increasing, the new memory might be uninitialized.
• Returns a void* pointer to the reallocated block, which might be at a new memory location, or NULL if reallocation fails.
• Syntax: void* realloc(void* ptr, size_t new_size);

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Realloc Syntax: �

• Syntax:

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• void* realloc(void* ptr, size_t new_size);

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Realloc Syntax: �

• Syntax:
• void* realloc(void* ptr, size_t new_size);\
• ptr = (int *)realloc(ptr, 10 * sizeof(int));

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free():

• Deallocates the memory block pointed to by the given pointer, returning it to the system for reuse.

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• It is crucial to free dynamically allocated memory to prevent memory leaks.

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• realloc() (Reallocation):
• Changes the size of a previously allocated memory block.
• It can either increase or decrease the size. If increasing, the new memory might be uninitialized.
• Returns a void* pointer to the reallocated block, which might be at a new memory location, or NULL if reallocation fails.
• Syntax: void* realloc(void* ptr, size_t new_size);

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free()Syntax:

• Syntax:
• void free(void* ptr);
• free(ptr);

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Structures

• Structures in C programming are user-defined data types that allow you to group together variables of different data types under a single name. This enables the creation of complex data types that represent real-world entities with multiple attributes. 

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Key characteristics of structures:

• User-defined data type: Structures are not built-in types like int or float; you define them according to our specific needs.
• Grouping heterogeneous data: Unlike arrays, which store elements of the same data type, structures can combine variables of different types (e.g., int, char, float, arrays) within a single unit.
• struct keyword: The struct keyword is used to define a structure.
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Key characteristics of structures:

• Members: The variables declared inside a structure are called its members or fields.
• Creating variables: Once a structure is defined, we can declare variables of that structure type, similar to declaring variables of built-in types
• Accessing members: Members of a structure variable are accessed using the dot (.) operator. If you are working with a pointer to a structure, the arrow (->) operator is used.
• Memory allocation: Memory for a structure is allocated when a structure variable is declared, and the size depends on the combined sizes of its members.

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How to Create Structures in C?�

• A structure is created outside the main function, preferably before the main function.
• In order to access the data members of the structure, variable(s) are created either inside or outside the main function depending upon the choice of the programmer.

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The basic syntax for a C structure is:�

• struct struct_name {� member1_Datatype member1_name;� member2_Datatype member2_name;� ...� memberN_Datatype memberN_name;�};

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The basic syntax for a C structure is:�

• struct is the keyword used to define a structure.
• struct_name is the name given to the structure.
• member1_Datatype and member1_name are the data type and name of the first member (variable) of the structure.
• member2_Datatype and member2_name are the data type and name of the second member (variable) of the structure.
• ... represents additional members (variables) of the structure.
• memberN_Datatype and memberN_name are the data type and name of the Nth member (variable) of the structure.

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Structure Example

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How to Create Structure Variables?�

• As stated above, we can’t directly access the data members of a structure.
• We need to create at least one variable to access the structure.
• The variable is responsible for reserving a particular block of memory for the structure according to its size.

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There are 2 ways to create a variable to access data members in a structure�

• 1. Inside the main function:
• struct book�{�char book_name[30];�char author[30];�int book_id;�float price;�};�int main()�{�struct book b; // Here b is a variable of structure book�}

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2. Outside the main function:�

• struct book�{�char book_name[30];�char author[30];�int book_id;�float price;�} b; // Here b is a variable of structure book.

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How to access Data Members in Structures in C�

• There are basically two ways in order to access the data members in a structure through variable(s):
• With the help of the member operator/ dot symbol (.)
• With the help of structure pointer operator (->)

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How to access Data Members in Structures in C�

• Suppose we want to access the data member book_id from the structure book,
• b.book_id;

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