II. Notes on Data Structuring * - Cornell University

138 c. A, R, HOARE
redundant information (e.g. length of previous
the sequence.
f. Stack
~////.~
"//////A
"//////A
]top of stack
I J
. . . . . . . .
Queue
NNt~
(a) (b)
I~ tog or length indication
first free
read pointer
Fie. 6. Sequences (variable length items)
item) between each item in
z////~2A
z/2"//,/A
Deque
(c)
2 length
1 length
5 length
When a sequence is itself a part of an item of some other sequence, the
contiguous representation of the item-sequence may be used. This will
normally be accompanied by a count giving the length of the sequence, so
that the actual size of each item can be computed when the item is read.
8.2.2. Chained Representation
In order to avoid reshuffling problems mentioned in the previous section, it
is usual to introduce indirect or chained methods of storage allocation, using
either fixed length or variable length units of allocation. The available store
is split into areas, some of which will be in use for storing items of some
sequence, and others will be free. The free areas are also linked together as a
chained sequence. Whenever a programmer's sequence requires extension,
an area (or part of an area) is acquired from the free chain; and whenever a
sequence is shortened by reading, an area can be returned to the free chain.
In the case of fixed-length items, the administration of dynamic storage
allocation with explicit deallocation presents no problems. The problems of
variable length allocation will not be treated here; they are best avoided by
the use of blocking (see next section).
The simplest form of chain is the single linked chain. Each item of the
sequence has adjoined to it, in a link location, the address of the next item