You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
72 lines
1.3 KiB
72 lines
1.3 KiB
3 years ago
|
|
||
|
|
Exercise 4.76: Our implementation of and
|
||
|
|
as a series combination of queries (Figure 4.5) is elegant, but it is
|
||
|
|
inefficient because in processing the second query of the and we must
|
||
|
|
scan the data base for each frame produced by the first query. If the data
|
||
|
|
base has
|
||
|
|
n
|
||
|
|
elements, and a typical query produces a number of output frames
|
||
|
|
proportional to
|
||
|
|
n
|
||
|
|
(say
|
||
|
|
|
||
|
|
n
|
||
|
|
|
||
|
|
|
||
|
|
/
|
||
|
|
|
||
|
|
|
||
|
|
k
|
||
|
|
|
||
|
|
), then scanning the data base for each
|
||
|
|
frame produced by the first query will require
|
||
|
|
|
||
|
|
|
||
|
|
n
|
||
|
|
2
|
||
|
|
|
||
|
|
|
||
|
|
/
|
||
|
|
|
||
|
|
|
||
|
|
k
|
||
|
|
|
||
|
|
calls to the
|
||
|
|
pattern matcher. Another approach would be to process the two clauses of the
|
||
|
|
and separately, then look for all pairs of output frames that are
|
||
|
|
compatible. If each query produces
|
||
|
|
|
||
|
|
n
|
||
|
|
|
||
|
|
|
||
|
|
/
|
||
|
|
|
||
|
|
|
||
|
|
k
|
||
|
|
|
||
|
|
output frames, then this means
|
||
|
|
that we must perform
|
||
|
|
|
||
|
|
|
||
|
|
n
|
||
|
|
2
|
||
|
|
|
||
|
|
|
||
|
|
/
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
k
|
||
|
|
2
|
||
|
|
|
||
|
|
|
||
|
|
compatibility checks—a factor of
|
||
|
|
k
|
||
|
|
|
||
|
|
fewer than the number of matches required in our current method.
|
||
|
|
|
||
|
|
Devise an implementation of and that uses this strategy. You must
|
||
|
|
implement a procedure that takes two frames as inputs, checks whether the
|
||
|
|
bindings in the frames are compatible, and, if so, produces a frame that merges
|
||
|
|
the two sets of bindings. This operation is similar to unification.
|