- Recall that:
You must first FIND the location of insertion before you can insert an element in a list.
- So: to
insert an element at the tail
of a list,
you must first
find the last element
of the list
The last element of the list is identify by the value of the next field is NULL
- Algorithm to
find
the last element in list
ListElem *head; // Assume head is initialized.... ListElem *lastElem; // lastElem will point to the last element
lastElem = head; while (lastElem->next != NULL) lastElem = lastElem->next; - Result:
- The following diagram
show the Before and After state
of inserting a new element at tail:
New links that you need to make:
- Computer instructions to make the links
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;
lastElem->next = elem; // Link "elem" to lastElem: elem->next = NULL; // Mark new elem as the last element:
- The
insert algorithm
above
does NOT change the head
of the list
- It is
generally true that:
-
Inserting a new element
at the tail
does NOT change the head
- However, this is
NOT true
when
the list is EMPTY:
Before insertion
head = NULL;
After insertion
head = address of the new list element;
- General fact of linked lists:
- Insertion
and
deletion
algorithm
at a
position other than the start
of the list,
always
consists of:
- A general case
- One or more special cases
- The
special cases
requires a
different way to handle the insertion
- Each special cases must be detected (with an if-statement)
- Insertion
and
deletion
algorithm
at a
position other than the start
of the list,
always
consists of:
- The
most common exception case
is the
empty list
(This is the case here with inserting at tail)
- Inserting at tail in an
empty list:
Fact: head = NULL; else points to a new List element
elem->next = NULL; // Mark element as last element head = elem; // It is the first element in list
- Complete algorithm to insert new element at the end of a 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; }
- Input parameters: What information
does the function need to do the job
- The start (head) of the list
- The new list element
NOTE: we assume that the value in the list element has already been initialized (that is NOT part of the insert algorithm !)
- Output parameters: What information
does the function return
- The start (head) the new list
- Again,
there are 2 ways to return
the head
value:
- Return the
new head
as the
function value
- Return the new head using a variable (that must be passed by-reference
- Return the
new head
as the
function value
- Algorithm:
ListElem * InsertAtTail(ListElem * head, ListElem * elem) { ListElem *lastElem; if ( head == NULL ) // Detect and handle exception case.... { elem->next = NULL; return(elem); // new head } 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; return(head); // new head } }
- How to use this function:
int main(int argc, char *argv[]) { ListElem *head, *p; head = NULL; // Empty list p = new ListElem; p->value = 1500; head = InsertAtTail(head, p); // <--- record the new head in "head" }
- Example Program:
(Demo above code)
- Prog file: click here
- Algorithm:
void InsertAtTail(ListElem * & head, ListElem * elem) { ListElem *lastElem; if ( head == NULL ) // Detect and handle exception case.... { elem->next = NULL; head = elem; // Return new head } else { // General case // 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; // Technically, we should return new head with the statement: // head = head; } }
- How to use this function:
int main(int argc, char *argv[]) { ListElem *head, *p; head = NULL; // Empty list p = new ListElem; p->value = 1500; InsertAtTail(head, p); // <--- new head returned in "head" }
- Example Program:
(Demo above code)
- Prog file: click here