Let's Play Coding! :D: Implementing Search and Add/Replace Operations on Linked List - Data Structures

Problem Question

To implement Search and Replace, Add Before Value, and Add After Value Operations on a Linked List

Explanation of Problem

In this program we would be implementing a Linked List. Make sure you have a strong understanding of pointers to understand Linked Lists. A linked list is a basic data structure that is used in dynamic memory allocation applications. It comprises of 'nodes' which are linked together to form a sequence of nodes called Linked List. The linkage is done using memory addresses of adjacent nodes (next node in singly linked list, and both next & previous node in doubly linked list).

In this program we use a struct to implement the node of our linked list. We will implement addition function to get some data in linked list before we can perform desired operations on it. Adding a new node to the list means, creating a new node structure, allocating memory to it and linking it to the list.

Code

#include<iostream>

/**@Title: LinkedList v1.5.cpp*
*@Programming Paradigm: Procedural*
*@Language: C++*
*@Compiler: GNU GCC*
*@IDE: Code::Blocks 13.12*
*@Author: Rogue Coder*
*@URL: http://letsplaycoding.blogspot.com/*
*@Date: 27-11-2014*
*/

struct node
{
  int data;
  node* next;
};

void addAfter(node* rootNode);
void addBefore(node* rootNode);
void addAtLast(node** rootNode);
void replaceValue(node* rootNode);
node* findNode(int SearchKey, node* rootNode);
void displayList(node* rootNode);

int main()
{
  int choice;
  node *startList = NULL;
  std::cout << "Welcome to LinkedList v1.5" << std::endl << "Made by Rogue Coder" << std::endl;
  do
  {
    std::cout << std::endl << "1 : Add a Node to the list" <<
         std::endl << "2 : Add Value After" <<
         std::endl << "3 : Add Value Before" <<
         std::endl << "4 : Find And Replace Value" <<
         std::endl << "5 : Display List" <<
         std::endl << "6 : Exit" <<
         std::endl << "Enter your choice : ";
    std::cin>>choice;
    switch(choice)
    {
    case 1:
        addAtLast(&startList);
        break;
    case 2:
        addAfter(startList);
        break;
    case 3:
        addBefore(startList);
        break;
    case 4:
        replaceValue(startList);
        break;
    case 5:
        displayList(startList);
        break;
    case 6:
      std::cout<<std::endl<<"Thank you for using LinkedList v1.4"<<std::endl<<"Made by Rogue Coder"
           <<std::endl<<"Press any key to exit"<<std::endl;
      break;
    default:
      std::cout<<"\a\aWrong Choice\a\a"<<std::endl;
      break;
    }
  }
  while(choice != 6);
  std::cin.get();
  return 0;
}

void addAtLast(node** rootNode)
{
  node* newNode = new node;
  std::cout<<std::endl<<"Enter data : ";
  std::cin>>newNode->data;
  if(*rootNode == NULL)
  {
    *rootNode = newNode;
  }
  else
  {

    node* currentNode = *rootNode;
    while(currentNode->next != NULL)
    {
      currentNode = currentNode->next;
    }
    currentNode->next=newNode;
  }
  newNode->next=NULL;
}

node* findNode(int SearchKey, node* rootNode)
{
    if (rootNode == NULL)
    {
        std::cout<<std::endl<<"\aList Empty\a";
    }
    else
    {
        node* currentNode = rootNode;
        while (currentNode)
        {
            if(SearchKey == currentNode -> data)
            {
                return currentNode;
            }
            currentNode = currentNode -> next;
        }
    }
    return NULL;
}

void addAfter(node* rootNode)
{
    int searchKey, newData;
    node *searchNode = NULL, *tempNode = NULL, *newNode = new node;
    std::cout<<"Enter the Value to find: ";
    std::cin>>searchKey;
    std::cout<<"Enter the Value to add: ";
    std::cin>>newData;
    searchNode = findNode(searchKey, rootNode);
    if (searchNode != NULL)
    {
        tempNode = searchNode -> next;
        searchNode -> next = newNode;
        newNode -> next = tempNode;
        newNode -> data = newData;
    }
    else
    {
        std::cout<<"Value Not Found!";
    }
}

void addBefore(node* rootNode)
{
    int searchKey, newData;
    node *searchNode = NULL, *newNode = new node;
    std::cout<<"Enter the Value to find: ";
    std::cin>>searchKey;
    std::cout<<"Enter the Value to add: ";
    std::cin>>newData;
    searchNode = findNode(searchKey, rootNode);
    if (searchNode != NULL)
    {
        newNode -> data = searchNode -> data;
        newNode -> next = searchNode -> next;
        searchNode -> next = newNode;
        searchNode -> data = newData;
    }
    else
    {
        std::cout<<"Value Not Found!";
    }
}

void replaceValue(node* rootNode)
{
    int searchKey, newData;
    node *searchNode = NULL;
    std::cout<<"Enter the Value to find: ";
    std::cin>>searchKey;
    std::cout<<"Enter new Value: ";
    std::cin>>newData;
    searchNode = findNode(searchKey, rootNode);
    if (searchNode != NULL)
    {
        searchNode -> data = newData;
    }
    else
    {
        std::cout<<"Value Not Found!";
    }
}

void displayList(node* rootNode)
{
  node *currentNode = rootNode;
  if(currentNode == NULL)
  {
    std::cout<<std::endl<<"\aList Empty\a"<<std::endl;
  }
  else
  {
    std::cout<<std::endl;
    while(currentNode != NULL)
    {
      std::cout<<currentNode->data<<"->";
      currentNode=currentNode->next;
    }
    std::cout<<"End of List"<<std::endl;
  }
}

Explanation of Code

#include <iostream> -> The compiler calls the Preprocessor to include the IOSTREAM(Standard Input / Output Streams Library) header file into the program, thus letting the use of the Standard Input / Output Streams functions like std::cin and std::cout. As per C++11 specification, including <iostream> automatically includes also <ios>, <streambuf>, <istream>, <ostream> and <iosfwd>.

int main() -> The entry point of the program where the execution starts. This function has to be named main. As per the ANSI specification, the return type has to be int. Since the return type is specified as int in my program, I have to use a return statement at the end of my code. So I use return 0 since zero returned from a function, by convention, implies a correct execution of the program. The return values are used to debug the program.

std::cin (extern istream cin) -> Standard Input Stream, and object of class istream. It is generally used with the extraction operator (>>), though we can use member functions like get (cin.get()), read (cin.read()), etc. for the input. The use of extraction operator is much more popular due to the fact that it aids in getting formatted input.

std::cout (extern ostream cout) -> Standard Output Stream, and object of class ostream. It is generally used with the insertion operator (<<), though we can use member functions like write (cout.write()) for the output. The use of insertions operator is much more popular due to the fact that it aids in giving formatted output.

using namespace std; -> In modern IDEs, we have to explicitly write std::cout instead of cout to use the ostream cout object. Namespace std helps in easing off the pain of writing std:: again and again. Though make sure you are not trapped! The classes defined in std should not be redefined by you. So in case you want to define a class 'distance', you can't do so if you have used std namespace. Though you can define 'Distance' (capital D).

std::endl (ostream& endl (ostream& os)) -> This is a function which is used to insert a newline character and flush the stream. Because this function is a manipulator, it is designed to be used alone with no arguments in conjunction with the insertion (<<) operations on output streams.

struct node
{
int data;
node* next;
}; ->

This is where we create the struct node that is going to be the building block of our linked list. It comprises of an int 'data' where the user shall store the data they wish. Please note, you can use as many variables and of as many types in the struct, but be sure you handle them correctly. For simplicity we have used int type in our case. The other variable inside our node is 'next', which is a pointer to node. This would be used to build the linkage between the nodes of our linked list. 'next' is going to hold the address of the next node in the sequence.

node* startList; -> The global pointer ‘startList’ which we are going to use to point to the first node/root node/ start node of the linked list, we are going to implement in this program.

int choice; -> This variable 'choice' will be used for the user’s choice in the menu driven program.

void addAtLast(node** rootNode)
{
  node* newNode = new node;
  std::cout<<std::endl<<"Enter data : ";
  std::cin>>newNode->data;
  if(*rootNode == NULL)
  {
    *rootNode = newNode;
  }
  else
  {

    node* currentNode = *rootNode;
    while(currentNode->next != NULL)
    {
      currentNode = currentNode->next;
    }
    currentNode->next=newNode;
  }
  newNode->next=NULL;
} ->

This function is used to add a new node at the end of our linked list. It asks for user input and the data is stored in the ‘newNode’ which is the pointer to the new node that we are going to add to our list. The first statement of this function means, memory is allocated for a node type variable and a pointer is returned which we capture in the variable ‘newNode’. After the user input the program checks if the list is empty which we come to know if the ‘rootNode’, i.e., root node of the linked list is initialised as yet or not. If the root node is not yet initialised for our list, we make the ‘rootNode’ pointer to point to the ‘newNode’ we just created. Else, we traverse through the list. Starting from the root node, we hold the position in a new pointer, ‘currentNode’. Since the last node’s ‘next’ won’t point to any node, thus we use currentNode->next != NULL for that. Till so is the case, we keep on assigning the address of the next node in sequence to the pointer ‘currentNode’. So as and when we encounter the last node in the list, we come out of the loop. The pointer ‘currentNode’ now holds the address to the last node in the list. So we define the ‘next’ for this last node of ours as the newNode we just created. Now the next of newNode is set to NULL, hence the newNode is added at the end of the list.

node* findNode(int SearchKey, node* rootNode)
{
    if (rootNode == NULL)
    {
        std::cout<<std::endl<<"\aList Empty\a";
    }
    else
    {
        node* currentNode = rootNode;
        while (currentNode)
        {
            if(SearchKey == currentNode -> data)
            {
                return currentNode;
            }
            currentNode = currentNode -> next;
        }
    }
    return NULL;
} ->
This function is usd to find the address of the the node which contains the value that we want to search. This function is not available for the user as a direct functionality, but instead, is used internally by other functions which I would explain shortly. Observe the function's signature. You would see we have 2 arguments passed to this function. The first one, 'searchKey', holds the value that the user is looking for. The second argument, 'rootNode', is used to hold the address of the starting node of the list, just like the other functions.
We first check if the list is empty. To achieve that, we look at the address held by the 'rootNode' variable. If it is 'NULL', then our list is empty as yet. Otherwise, we traverse through the list starting from the root node until we find a node where the pointer 'next' holds NULL. While we are traversing through the list, if we encounter the data that we're looking for, we end the search and return the address of the node where hit the sucessful search.
This means, our function returns the first successful search back. If the element is not found on the list, the function returns NULL.

void addAfter(node* rootNode)
{
    int searchKey, newData;
    node *searchNode = NULL, *tempNode = NULL, *newNode = new node;
    std::cout<<"Enter the Value to find: ";
    std::cin>>searchKey;
    std::cout<<"Enter the Value to add: ";
    std::cin>>newData;
    searchNode = findNode(searchKey, rootNode);
    if (searchNode != NULL)
    {
        tempNode = searchNode -> next;
        searchNode -> next = newNode;
        newNode -> next = tempNode;
        newNode -> data = newData;
    }
    else
    {
        std::cout<<"Value Not Found!";
    }
} ->

This is the function that is used to add a node after the value that the user inputs. The function expects root node's address as the agument, which gets stored in the 'rootNode' variable.
The function prompts the user to enter the value he's looking for and the data to be added after that value. Once the program gets the user inputs, it calls the findNode function to lookup the value in the Linked List. Once we get the address of the node where the value matches, the current next node of this node is stored in a temp node ('tempNode' variable). A new node ('newNode' variable) is brought up, and the node which we found from our call to 'findNode' function, starts pointing to this new node as it's next node. 'newNode' starts to point to tempNode in it's next pointer, and stores the value user wanted to insert in data variable.
If the value was not found in the list, then the function displays Value not found and returns.

void addBefore(node* rootNode)
{
    int searchKey, newData;
    node *searchNode = NULL, *newNode = new node;
    std::cout<<"Enter the Value to find: ";
    std::cin>>searchKey;
    std::cout<<"Enter the Value to add: ";
    std::cin>>newData;
    searchNode = findNode(searchKey, rootNode);
    if (searchNode != NULL)
    {
        newNode -> data = searchNode -> data;
        newNode -> next = searchNode -> next;
        searchNode -> next = newNode;
        searchNode -> data = newData;
    }
    else
    {
        std::cout<<"Value Not Found!";
    }
} ->

This function is used to insert the new value before the element searched by the user. There are two ways in which this part can be implemented. One is to find the element, and the insert a new node before that. The other way is to add a new node after the searched node, copy the contents of searched node to this new node, and replacevalue of searched node with new value. Although, former is a bteer approach, our implementation uses the latter. I have gone with the second approach so that I could reuse search operation from findNode function.
So what are we doing here? We find the node which has the data the user is looking for. Then we push a new node after that one, and copy the contents of searched node to the new node(newNode -> data = searchNode -> data and newNode -> next = searchNode -> next). Then we replace the contents of original node with new values. In essence, we get the new value inserted before the searched value.
Again, if the value is not found, the user is prompted with 'Value not found' message.
void replaceValue(node* rootNode)
{
    int searchKey, newData;
    node *searchNode = NULL;
    std::cout<<"Enter the Value to find: ";
    std::cin>>searchKey;
    std::cout<<"Enter new Value: ";
    std::cin>>newData;
    searchNode = findNode(searchKey, rootNode);
    if (searchNode != NULL)
    {
        searchNode -> data = newData;
    }
    else
    {
        std::cout<<"Value Not Found!";
    }
} ->
This function is used to replace the first occurence of the value that the user searches in the list, with a value that the user inputs. Both search and new values are taken from the user as inputs. The function then calls findNode function to get the address of the nodw where the search value resides. Then the new value replaces the data already sitting there. If the value is not present in the list, a message is shown to the user to indicate the same.

void displayList(node* rootNode)
{
  node *currentNode = rootNode;
  if(currentNode == NULL)
  {
    std::cout<<std::endl<<"\aList Empty\a"<<std::endl;
  }
  else
  {
    std::cout<<std::endl;
    while(currentNode != NULL)
    {
      std::cout<<currentNode->data<<"->";
      currentNode=currentNode->next;
    }
    std::cout<<"End of List"<<std::endl;
  }
} ->

This function is used to display the list. The functions accepts the address of the root node as the argument. It then starts traversing through the list from root node, until it finds a node which points to NULL as the next element, indicating the end of list. While traversing through, the function displays the values held in each node on the screen separated by '->'

do{..}while() -> The program loop which encapsulates the whole program. Until the user chooses to exit the program, the control loops within this.

std::cin.get() -> This statement is used to pause our program, until user presses a key. This function is not necessary in your program, I use it to see my outputs at a paused screen. If you use cmd to run your programs, you might not need this. If you use linux/unix you might not need this. Moreover, removing this line of code from this program, doesn't affect the functionality of the program.