Inserting a new element at the TAIL of a Linked List

Inserting new elements at tail

Because a linked list is a sequential data structure, when you want to insert a new element anywhere other than the HEAD of the list, you must first FIND the location
Effect of inserting a new element at the tail of a linked list:

Recipe for inserting a new element at the tail of a linked list:

Find the last element in the list
- How can you tell which is the last element ??? (next value is NULL)
Make the last element point to the new element (the new element will thus "follow" the old last element)
Set the "next" linkage variable in the new element to NULL to indicate the end of the list

Pictorially:

Computer instructions

Code for inserting at tail of non-empty list
elem = new ListElem; // Make new object elem->value = ...; // Fill elem with information // Find the last element in the list lastElem = head; while (lastElem->next != NULL) lastElem = lastElem->next; // Link "elem" to lastElem: lastElem->next = elem; // Mark new elem as the last element: elem->next = NULL;

CAVEAT when inserting in linked list other than the start....

The insertion (and later deletion) of new elements into a linked list at a position other than the START of the list, always involves considering some special cases -- and each special case requires a different way to handle the insertion.
The most common exception case is the empty list

Consider the exception case for insert at tail when we try to insert a new element into an empty list:

Code for inserting at tail of non-empty list
elem = new ListElem; elem->value = ...; // Find the last element in the list lastElem = head; // head == NULL while (lastElem->next != NULL) lastElem = lastElem->next; lastElem->next = elem; elem->next = NULL;

The expression lastElem->next is illegal when lastElem is equal NULL !!!
Thus, the program segment for insert a new linked list element at the tail does not work when the list is empty

We must (first identify and then) handle exception cases separately...
In the insertion at the TAIL of the list, there is only ONE exception case (empty list).
(Other type of inserts may have more exception cases...)

Complete program segment for inserting a new list element at the end of a list:

Complete algorithm to insert at tail of list
elem = new ListElem; elem->value = ...; if ( head == NULL ) // Detect and handle exception case.... { head = elem; elem->next = NULL; } else { // Find the last element in the list lastElem = head; while (lastElem->next != NULL) lastElem = lastElem->next; // Link "elem" to lastElem: lastElem->next = elem; // Mark new elem as the last element: elem->next = NULL; }

Example Program: (Demo insertion at TAIL of list) ---- click here