double assignment in c

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Assignment Operators in C

In C, the assignment operator stores a certain value in an already declared variable. A variable in C can be assigned the value in the form of a literal, another variable or an expression. The value to be assigned forms the right hand operand, whereas the variable to be assigned should be the operand to the left of = symbol, which is defined as a simple assignment operator in C. In addition, C has several augmented assignment operators.

The following table lists the assignment operators supported by the C language −

Simple assignment operator (=)

The = operator is the most frequently used operator in C. As per ANSI C standard, all the variables must be declared in the beginning. Variable declaration after the first processing statement is not allowed. You can declare a variable to be assigned a value later in the code, or you can initialize it at the time of declaration.

You can use a literal, another variable or an expression in the assignment statement.

Once a variable of a certain type is declared, it cannot be assigned a value of any other type. In such a case the C compiler reports a type mismatch error.

In C, the expressions that refer to a memory location are called "lvalue" expressions. A lvalue may appear as either the left-hand or right-hand side of an assignment.

On the other hand, the term rvalue refers to a data value that is stored at some address in memory. A rvalue is an expression that cannot have a value assigned to it which means an rvalue may appear on the right-hand side but not on the left-hand side of an assignment.

Variables are lvalues and so they may appear on the left-hand side of an assignment. Numeric literals are rvalues and so they may not be assigned and cannot appear on the left-hand side. Take a look at the following valid and invalid statements −

Augmented assignment operators

In addition to the = operator, C allows you to combine arithmetic and bitwise operators with the = symbol to form augmented or compound assignment operator. The augmented operators offer a convenient shortcut for combining arithmetic or bitwise operation with assignment.

For example, the expression a+=b has the same effect of performing a+b first and then assigning the result back to the variable a.

Similarly, the expression a<<=b has the same effect of performing a<<b first and then assigning the result back to the variable a.

Here is a C program that demonstrates the use of assignment operators in C:

When you compile and execute the above program, it produces the following result −

Next: Unions , Previous: Overlaying Structures , Up: Structures   [ Contents ][ Index ]

15.13 Structure Assignment

Assignment operating on a structure type copies the structure. The left and right operands must have the same type. Here is an example:

Notionally, assignment on a structure type works by copying each of the fields. Thus, if any of the fields has the const qualifier, that structure type does not allow assignment:

See Assignment Expressions .

When a structure type has a field which is an array, as here,

structure assigment such as r1 = r2 copies array fields’ contents just as it copies all the other fields.

This is the only way in C that you can operate on the whole contents of a array with one operation: when the array is contained in a struct . You can’t copy the contents of the data field as an array, because

would convert the array objects (as always) to pointers to the zeroth elements of the arrays (of type struct record * ), and the assignment would be invalid because the left operand is not an lvalue.

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6.1. Multiple assignment ¶

I haven’t said much about it, but it is legal in C++ to make more than one assignment to the same variable. The effect of the second assignment is to replace the old value of the variable with a new value.

The active code below reassigns fred from 5 to 7 and prints both values out.

The output of this program is 57 , because the first time we print fred his value is 5, and the second time his value is 7.

The active code below reassigns fred from 5 to 7 without printing out the initial value.

However, if we do not print fred the first time, the output is only 7 because the value of fred is just 7 when it is printed.

This kind of multiple assignment is the reason I described variables as a container for values. When you assign a value to a variable, you change the contents of the container, as shown in the figure:

When there are multiple assignments to a variable, it is especially important to distinguish between an assignment statement and a statement of equality. Because C++ uses the = symbol for assignment, it is tempting to interpret a statement like a = b as a statement of equality. It is not!

An assignment statement uses a single = symbol. For example, x = 3 assigns the value of 3 to the variable x . On the other hand, an equality statement uses two = symbols. For example, x == 3 is a boolean that evaluates to true if x is equal to 3 and evaluates to false otherwise.

First of all, equality is commutative, and assignment is not. For example, in mathematics if \(a = 7\) then \(7 = a\) . But in C++ the statement a = 7; is legal, and 7 = a; is not.

Furthermore, in mathematics, a statement of equality is true for all time. If \(a = b\) now, then \(a\) will always equal \(b\) . In C++, an assignment statement can make two variables equal, but they don’t have to stay that way!

The third line changes the value of a but it does not change the value of b , and so they are no longer equal. In many programming languages an alternate symbol is used for assignment, such as <- or := , in order to avoid confusion.

Although multiple assignment is frequently useful, you should use it with caution. If the values of variables are changing constantly in different parts of the program, it can make the code difficult to read and debug.

• Checking if a is equal to b
• Assigning a to the value of b
• Setting the value of a to 4

Q-4: What will print?

• There are no spaces between the numbers.
• Remember, in C++ spaces must be printed.
• Carefully look at the values being assigned.

Q-5: What is the correct output?

• Remember that printing a boolean results in either 0 or 1.
• Is x equal to y?
• x is equal to y, so the output is 1.

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C - Bitwise OR and assignment operator

The Bitwise OR and assignment operator (|=) assigns the first operand a value equal to the result of Bitwise OR operation of two operands.

(x |= y) is equivalent to (x = x | y)

The Bitwise OR operator (|) is a binary operator which takes two bit patterns of equal length and performs the logical OR operation on each pair of corresponding bits. It returns 1 if either or both bits at the same position are 1, else returns 0.

The example below describes how bitwise OR operator works:

The code of using Bitwise OR operator (|) is given below:

The output of the above code will be:

Example: Find largest power of 2 less than or equal to given number

Consider an integer 1000. In the bit-wise format, it can be written as 1111101000. However, all bits are not written here. A complete representation will be 32 bit representation as given below:

Performing N |= (N>>i) operation, where i = 1, 2, 4, 8, 16 will change all right side bit to 1. When applied on 1000, the result in 32 bit representation is given below:

Adding one to this result and then right shifting the result by one place will give largest power of 2 less than or equal to 1000.

The below code will calculate the largest power of 2 less than or equal to given number.

The above code will give the following output:

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Types of Linked List - Singly linked, doubly linked and circular

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Doubly Linked List

A doubly linked list is a type of linked list in which each node consists of 3 components:

• *prev - address of the previous node
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• *next - address of next node

Note: Before you proceed further, make sure to learn about pointers and structs .

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Let's see how we can represent a doubly linked list on an algorithm/code. Suppose we have a doubly linked list:

Here, the single node is represented as

Each struct node has a data item, a pointer to the previous struct node, and a pointer to the next struct node.

Now we will create a simple doubly linked list with three items to understand how this works.

In the above code, one , two , and three are the nodes with data items 1 , 2 , and 3 respectively.

• For node one : next stores the address of two and prev stores null (there is no node before it)
• For node two : next stores the address of three and prev stores the address of one
• For node three : next stores null (there is no node after it) and prev stores the address of two .

Note : In the case of the head node, prev points to null , and in the case of the tail pointer, next points to null. Here, one is a head node and three is a tail node.

Insertion on a Doubly Linked List

Pushing a node to a doubly-linked list is similar to pushing a node to a linked list, but extra work is required to handle the pointer to the previous node.

We can insert elements at 3 different positions of a doubly-linked list:

• Insertion at the beginning
• Insertion in-between nodes
• Insertion at the End

Suppose we have a double-linked list with elements 1 , 2 , and 3 .

1. Insertion at the Beginning

Let's add a node with value 6 at the beginning of the doubly linked list we made above.

1. Create a new node

• allocate memory for newNode
• assign the data to newNode .

2. Set prev and next pointers of new node

• point next of newNode to the first node of the doubly linked list
• point prev to null

3. Make new node as head node

• Point prev of the first node to newNode (now the previous head is the second node)
• Point head to newNode

Code for Insertion at the Beginning

2. insertion in between two nodes.

Let's add a node with value 6 after node with value 1 in the doubly linked list.

2. Set the next pointer of new node and previous node

• assign the value of next from previous node to the next of newNode
• assign the address of newNode to the next of previous node

3. Set the prev pointer of new node and the next node

• assign the value of prev of next node to the prev of newNode
• assign the address of newNode to the prev of next node

The final doubly linked list is after this insertion is:

Code for Insertion in between two Nodes

3. insertion at the end.

Let's add a node with value 6 at the end of the doubly linked list.

2. Set prev and next pointers of new node and the previous node

If the linked list is empty, make the newNode as the head node. Otherwise, traverse to the end of the doubly linked list and

The final doubly linked list looks like this.

Code for Insertion at the End

Deletion from a doubly linked list.

Similar to insertion, we can also delete a node from 3 different positions of a doubly linked list.

1. Delete the First Node of Doubly Linked List

If the node to be deleted (i.e. del_node ) is at the beginning

Reset value node after the del_node (i.e. node two)

Finally, free the memory of del_node . And, the linked will look like this

Code for Deletion of the First Node

2. Deletion of the Inner Node

If del_node is an inner node (second node), we must have to reset the value of next and prev of the nodes before and after the del_node .

For the node before the del_node (i.e. first node)

Assign the value of next of del_node to the next of the first node.

For the node after the del_node (i.e. third node)

Assign the value of prev of del_node to the prev of the third node.

Finally, we will free the memory of del_node . And, the final doubly linked list looks like this.

Code for Deletion of the Inner Node

3. Delete the Last Node of Doubly Linked List

In this case, we are deleting the last node with value 3 of the doubly linked list.

Here, we can simply delete the del_node and make the next of node before del_node point to NULL .

Code for Deletion of the Last Node

Here, del_node ->next is NULL so del_node->prev->next = NULL .

Note : We can also solve this using the first condition (for the node before del_node ) of the second case (Delete the inner node).

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1. Complexity of Insertion Operation

• The insertion operations that do not require traversal have the time complexity of O(1) .
• And, insertion that requires traversal has time complexity of O(n) .
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2. Complexity of Deletion Operation

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Table of Contents

• Insertion at the Beginning of doubly linked list
• Insertion in between two nodes
• Delete the First Node of Doubly Linked List
• Deletion of the Inner Node
• Delete the Last Node of Doubly Linked List

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Pointers are one of the core components of the C programming language. A pointer can be used to store the memory address of other variables, functions, or even other pointers. The use of pointers allows low-level memory access, dynamic memory allocation, and many other functionality in C.

In this article, we will discuss C pointers in detail, their types, uses, advantages, and disadvantages with examples.

What is a Pointer in C?

A pointer is defined as a derived data type that can store the address of other C variables or a memory location. We can access and manipulate the data stored in that memory location using pointers.

As the pointers in C store the memory addresses, their size is independent of the type of data they are pointing to. This size of pointers in C only depends on the system architecture.

Syntax of C Pointers

The syntax of pointers is similar to the variable declaration in C, but we use the ( * ) dereferencing operator in the pointer declaration.

• ptr is the name of the pointer.
• datatype is the type of data it is pointing to.

The above syntax is used to define a pointer to a variable. We can also define pointers to functions, structures, etc.

How to Use Pointers?

The use of pointers in C can be divided into three steps:

• Pointer Declaration
• Pointer Initialization
• Pointer Dereferencing

1. Pointer Declaration

In pointer declaration, we only declare the pointer but do not initialize it. To declare a pointer, we use the ( * ) dereference operator before its name.

The pointer declared here will point to some random memory address as it is not initialized. Such pointers are called wild pointers.

2. Pointer Initialization

Pointer initialization is the process where we assign some initial value to the pointer variable. We generally use the ( & ) addressof operator to get the memory address of a variable and then store it in the pointer variable.

We can also declare and initialize the pointer in a single step. This method is called pointer definition as the pointer is declared and initialized at the same time.

Note: It is recommended that the pointers should always be initialized to some value before starting using it. Otherwise, it may lead to number of errors.

3. Pointer Dereferencing

Dereferencing a pointer is the process of accessing the value stored in the memory address specified in the pointer. We use the same ( * ) dereferencing operator that we used in the pointer declaration.

Dereferencing a Pointer in C

C Pointer Example

Types of pointers in c.

Pointers in C can be classified into many different types based on the parameter on which we are defining their types. If we consider the type of variable stored in the memory location pointed by the pointer, then the pointers can be classified into the following types:

1. Integer Pointers

As the name suggests, these are the pointers that point to the integer values.

These pointers are pronounced as Pointer to Integer.

Similarly, a pointer can point to any primitive data type. It can point also point to derived data types such as arrays and user-defined data types such as structures.

2. Array Pointer

Pointers and Array are closely related to each other. Even the array name is the pointer to its first element. They are also known as Pointer to Arrays . We can create a pointer to an array using the given syntax.

Pointer to Arrays exhibits some interesting properties which we discussed later in this article.

3. Structure Pointer

The pointer pointing to the structure type is called Structure Pointer or Pointer to Structure. It can be declared in the same way as we declare the other primitive data types.

In C, structure pointers are used in data structures such as linked lists, trees, etc.

4. Function Pointers

Function pointers point to the functions. They are different from the rest of the pointers in the sense that instead of pointing to the data, they point to the code. Let’s consider a function prototype – int func (int, char) , the function pointer for this function will be

Note: The syntax of the function pointers changes according to the function prototype.

5. Double Pointers

In C language, we can define a pointer that stores the memory address of another pointer. Such pointers are called double-pointers or pointers-to-pointer . Instead of pointing to a data value, they point to another pointer.

Dereferencing Double Pointer

Note: In C, we can create multi-level pointers with any number of levels such as – ***ptr3, ****ptr4, ******ptr5 and so on.

6. NULL Pointer

The Null Pointers are those pointers that do not point to any memory location. They can be created by assigning a NULL value to the pointer. A pointer of any type can be assigned the NULL value.

It is said to be good practice to assign NULL to the pointers currently not in use.

7. Void Pointer

The Void pointers in C are the pointers of type void. It means that they do not have any associated data type. They are also called generic pointers as they can point to any type and can be typecasted to any type.

One of the main properties of void pointers is that they cannot be dereferenced.

8. Wild Pointers

The Wild Pointers are pointers that have not been initialized with something yet. These types of C-pointers can cause problems in our programs and can eventually cause them to crash. If values is updated using wild pointers, they could cause data abort or data corruption.

9. Constant Pointers

In constant pointers, the memory address stored inside the pointer is constant and cannot be modified once it is defined. It will always point to the same memory address.

10. Pointer to Constant

The pointers pointing to a constant value that cannot be modified are called pointers to a constant. Here we can only access the data pointed by the pointer, but cannot modify it. Although, we can change the address stored in the pointer to constant.

Other Types of Pointers in C:

There are also the following types of pointers available to use in C apart from those specified above:

• Far pointer : A far pointer is typically 32-bit that can access memory outside the current segment.
• Dangling pointer : A pointer pointing to a memory location that has been deleted (or freed) is called a dangling pointer.
• Huge pointer : A huge pointer is 32-bit long containing segment address and offset address.
• Complex pointer: Pointers with multiple levels of indirection.
• Near pointer : Near pointer is used to store 16-bit addresses means within the current segment on a 16-bit machine.
• Normalized pointer: It is a 32-bit pointer, which has as much of its value in the segment register as possible.
• File Pointer: The pointer to a FILE data type is called a stream pointer or a file pointer.

Size of Pointers in C

The size of the pointers in C is equal for every pointer type. The size of the pointer does not depend on the type it is pointing to. It only depends on the operating system and CPU architecture. The size of pointers in C is 

• 8 bytes for a 64-bit System
• 4 bytes for a 32-bit System

The reason for the same size is that the pointers store the memory addresses, no matter what type they are. As the space required to store the addresses of the different memory locations is the same, the memory required by one pointer type will be equal to the memory required by other pointer types.

How to find the size of pointers in C?

We can find the size of pointers using the sizeof operator as shown in the following program:

Example: C Program to find the size of different pointer types.

As we can see, no matter what the type of pointer it is, the size of each and every pointer is the same.

Now, one may wonder that if the size of all the pointers is the same, then why do we need to declare the pointer type in the declaration? The type declaration is needed in the pointer for dereferencing and pointer arithmetic purposes.

C Pointer Arithmetic

The Pointer Arithmetic refers to the legal or valid 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. These operations include:

• Increment in a Pointer
• Decrement in a Pointer
• Addition of integer to a pointer
• Subtraction of integer to a pointer
• Subtracting two pointers of the same type
• Comparison of pointers of the same type.
• Assignment of pointers of the same type.

C Pointers and Arrays

In C programming language, pointers and arrays are closely related. An array name acts like a pointer constant. The value of this pointer constant is the address of the first element. For example, if we have an array named val then val and &val[0] can be used interchangeably.

If we assign this value to a non-constant pointer of the same type, then we can access the elements of the array using this pointer.

Example 1: Accessing Array Elements using Pointer with Array Subscript

Not only that, as the array elements are stored continuously, we can pointer arithmetic operations such as increment, decrement, addition, and subtraction of integers on pointer to move between array elements.

Example 2: Accessing Array Elements using Pointer Arithmetic

This concept is not limited to the one-dimensional array, we can refer to a multidimensional array element as well using pointers.

To know more about pointers to an array, refer to this article – Pointer to an Array

Uses of Pointers in C

The C pointer is a very powerful tool that is widely used in C programming to perform various useful operations. It is used to achieve the following functionalities in C:

• Pass Arguments by Reference
• Accessing Array Elements
• Return Multiple Values from Function
• Dynamic Memory Allocation
• Implementing Data Structures
• In System-Level Programming where memory addresses are useful.
• In locating the exact value at some memory location.
• To avoid compiler confusion for the same variable name.
• To use in Control Tables.

Advantages of Pointers

Following are the major advantages of pointers in C:

• Pointers are used for dynamic memory allocation and deallocation.
• An Array or a structure can be accessed efficiently with pointers
• Pointers are useful for accessing memory locations.
• Pointers are used to form complex data structures such as linked lists, graphs, trees, etc.
• Pointers reduce the length of the program and its execution time as well.

Disadvantages of Pointers

Pointers are vulnerable to errors and have following disadvantages:

• Memory corruption can occur if an incorrect value is provided to pointers.
• Pointers are a little bit complex to understand.
• Pointers are majorly responsible for memory leaks in C .
• Pointers are comparatively slower than variables in C.
• Uninitialized pointers might cause a segmentation fault.

In conclusion, pointers in C are very capable tools and provide C language with its distinguishing features, such as low-level memory access, referencing, etc. But as powerful as they are, they should be used with responsibility as they are one of the most vulnerable parts of the language.

FAQs on Pointers in C

Q1. define pointers..

Pointers are the variables that can store the memory address of another variable.

Q2. What is the difference between a constant pointer and a pointer to a constant?

A constant pointer points to the fixed memory location, i.e. we cannot change the memory address stored inside the constant pointer. On the other hand, the pointer to a constant point to the memory with a constant value.

Q3. What is pointer to pointer?

A pointer to a pointer (also known as a double pointer) stores the address of another pointer.

Q4. Does pointer size depends on its type?

No, the pointer size does not depend upon its type. It only depends on the operating system and CPU architecture.

Q5. What are the differences between an array and a pointer?

The following table list the differences between an array and a pointer : Pointer Array A pointer is a derived data type that can store the address of other variables. An array is a homogeneous collection of items of any type such as int, char, etc. Pointers are allocated at run time. Arrays are allocated at runtime. The pointer is a single variable. An array is a collection of variables of the same type. Dynamic in Nature Static in Nature.

Q6. Why do we need to specify the type in the pointer declaration?

Type specification in pointer declaration helps the compiler in dereferencing and pointer arithmetic operations.

• Quiz on Pointer Basics
• Quiz on Advanced Pointer

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