- Recall that memory space used to stored elements in
a linked list were previous obtained (reserved)
by using the memory allocation operator
new
- When we delete elements from any
dynamic data structure, it is
of utmost importance
that we unreserve
the memory space that was used to store the variable.
Because if you do not un-reserve the memory space, your program may exhause all available memory space and a subsequent new will fail; causing your program to crash
- The delete
operator is used to
un-reserve (return)
the reserved memory space that was previously reserved by
new
- The delete operator
needs to know the (first) address of the memory space -
which is also the value returned by the
new
operator
- Syntax and Example usage:
Syntax: delete AddressValue;
Example of usage: ListElem *p; p = new ListElem; ... delete p;
- How to remove the
FIRST element from a list:
Code used to delete first element from a list: ListElem *p; if (head != NULL ) // Must make sure list is not empty { p = head; // Save head head = head->next; delete p; // Use saved value to return memory !! } - Example Program:
(Demo deletion at HEAD of list) ----
click here
- Just like inserting at tail, before you can delete an element
from other than the head, you must find
that element
- Not only must you find the right element, you must also
maintain sufficient information
in order to perform the assigned task.
- Consider the steps to
remove the
LAST element from a list:
(general case)
- Find the last element
in the list --- this element is identified by
last->next == NULL
- Set last_but_one->next = NULL ---
this will effectively make the last_but_one
into the new last element of the list
(it "unlinks" the last element)
- Finally, return the memory space used to store the last element: delete last;
- Find the last element
in the list --- this element is identified by
last->next == NULL
- Notice
that we must we
cannot
obtain last_but_one from
last !!!
So we must be careful to maintain information on the last_but_one element while we are searching for the last element !!!
- As in insertion, we must also determine
and handle exception cases in deletion...
- There is ONE special case
in deleting from the tail: when the list has
only one element left...
I.e., AFTER deletion of the (only) element, the list becomes empty (kinda like the empty list case....)
- How to handle deletion that results in an empty list:
- Example Program:
(Demo deletion at TAIL of list) ----
click here
Code used to delete the last element from a list:
ListElem *last;
ListElem *last_but_one;
if ( head != NULL ) // Delete only from non-empty list
{
if ( head->next == NULL )
{ // Handle special case: List has 1 element
last = head; // Save for delete operator
head = NULL;
delete last; // Use saved value to delete element
}
else
{ // The general case
// Find the "last_but_one" element
last_but_one = head;
while ( last_but_one->next->next != NULL )
{
last_but_one = last_but_one->next;
}
// Delete element following the "last_but_one" element
last_but_one->next = NULL;
delete last;
}
}
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