10.3 Implementing pointers and objects
10.31¶
Draw a picture of the sequence $\langle 13, 4, 8, 19, 5, 11 \rangle$ stored as a doubly linked list using the multiplearray representation. Do the same for the singlearray representation.

A multiplearray representation with $L = 2$,
$$ \begin{array}{rccccccc} \hline index & 1 & 2 & 3 & 4 & 5 & 6 & 7 \\ \hline next & & 3 & 4 & 5 & 6 & 7 & \diagup \\ \hline key & & 13 & 4 & 8 & 19 & 5 & 11 \\ \hline prev & & \diagup & 2 & 3 & 4 & 5 & 6 \\ \hline \end{array} $$

A singlearray version with $L = 1$,
$$ \begin{array}{rcccccccccccccccccc} \hline index & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9 & 10 & 11 & 12 & 13 & 14 & 15 & 16 & 17 & 18 \\ \hline key & 13 & 4 & \diagup & 4 & 7 & 1 & 8 & 10 & 4 & 19 & 13 & 7 & 5 & 16 & 10 & 11 & \diagup & 13 \\ \hline \end{array} $$
10.32¶
Write the procedures $\text{ALLOCATEOBJECT}$ and $\text{FREEOBJECT}$ for a homogeneous collection of objects implemented by the singlearray representation.
10.33¶
Why don't we need to set or reset the $prev$ attributes of objects in the implementation of the $\text{ALLOCATEOBJECT}$ and $\text{FREEOBJECT}$ procedures?
We implement $\text{ALLOCATEOBJECT}$ and $\text{FREEOBJECT}$ in the hope of managing the storage of currently nonused object in the free list so that one can be allocated for reusing. As the free list acts like a stack, to maintain this stacklike collection, we merely remember its first pointer and set the $next$ attribute of objects. There is no need to worry the $prev$ attribute, for it hardly has any impact on the resulting free list.
10.34¶
It is often desirable to keep all elements of a doubly linked list compact in storage, using, for example, the first $m$ index locations in the multiplearray representation. (This is the case in a paged, virtualmemory computing environment.) Explain how to implement the procedures $\text{ALLOCATEOBJECT}$ and $\text{FREEOBJECT}$ so that the representation is compact. Assume that there are no pointers to elements of the linked list outside the list itself. ($\textit{Hint:}$ Use the array implementation of a stack.)
FREEOBJECT(x)
p = F.top  1
p.prev.next = x
p.next.prev = x
x.key = p.key
x.prev = p.prev
x.next = p.next
PUSH(F, p)
10.35¶
Let $L$ be a doubly linked list of length $n$ stored in arrays $key$, $prev$, and $next$ of length $m$. Suppose that these arrays are managed by $\text{ALLOCATEOBJECT}$ and $\text{FREEOBJECT}$ procedures that keep a doubly linked free list $F$. Suppose further that of the $m$ items, exactly $n$ are on list $L$ and $m  n$ are on the free list. Write a procedure $\text{COMPACTIFYLIST}(L, F)$ that, given the list $L$ and the free list $F$, moves the items in $L$ so that they occupy array positions $1, 2, \ldots, n$ and adjusts the free list $F$ so that it remains correct, occupying array positions $n + 1, n + 2, \ldots, m$. The running time of your procedure should be $\Theta(n)$, and it should use only a constant amount of extra space. Argue that your procedure is correct.
We represent the combination of arrays $key$, $prev$, and $next$ by a multiblearray $A$. Each object of $A$'s is either in list $L$ or in the free list $F$, but not in both. The procedure $\text{COMPACTIFYLIST}$ transposes the first object in $L$ with the first object in $A$, the second objects until the list $L$ is exhausted.