II. Notes on Data Structuring * - Cornell University

NOTES ON DATA STRUCTURING 139
in the chain. The empty sequence is represented by a value which could not
possibly be an address (say zero or minus one); and the link location of the
last item in the sequence contains this value. The first item in the chain is
pointed to by the base location of the sequence.
A single linked chain is useful when the direction in which the sequence
will be read is known; for the links have to point in this direction. In the
case of a stack they will point backwards, and in the case of input and output
sequences and queues they will point forwards. In the case of an input or
output sequence, the base location of the external variable which is to hold
the initial and/or final value of the sequence points permanently at the
beginning of the chain, while the base location of the sequence itself steps
through the sequence. In the case of a queue, two base locations are used,
to point to each end of the sequence.
1
Stack
(a)
end
FIG. 7.
[
Queue
:L...--."-"-
I
Deque
write pointer [., write pointer
read pointer ,[ , read pointer
,, ,
!, "I
ll
(b) (c)
Sequences (Chained Representation)
-
difference
of two
pointers
One possible advantage of the single-chained representation in the case of
stacks is that several stacks can share the same initial segments, which may
save space and time in some applications. However, when an item is popped
up from such a stack, the storage space which it occupies cannot be immedi-
ately returned to the free chain, since it may be in use as part of another
stack. One solution to this problem is never to return storage explicitly,
but to wait until the free chain is exhausted. Then all currently allocated
sequences are scanned, and all blocks currently in use are marked. Then all
unmarked blocks are collected onto the free chain. This is known as a scan-