More examples

1
More examples

• Todays examples are related to coursework

2
Once upon a time, an insect is crawling along a
wire cube,

• In the original program, the relation arrow(X,Y)
  represents a directed link. How do we extend it
  to a bi-directional link?
• Easy using disjunction
• connected(X, Y)- arrow(X, Y).
• connected(X, Y)- arrow(Y, X).

3
Re-look at cango program

• Can we simply replace arrow by connected?
• cango(X,Y)- connected(X,Y).
• cango(X,Y)- connected(X,Z), cango(Z,Y).
• No. There are circles in the map there is a
  danger of being trapped in a circle. Thus, we
  must add something to avoid this.

4
The new cango program with circle detection

• Need an extra list to store
• the path generated so far
• cango(X,Y, CurrentPath)-
• connected(X,Y),
• \ member(Y,CurrentPath).
• cango(X,Y, CurrentPath)-
• connected(X,Z),
• \ member(Z,CurrentPath),
• cango(Z,Y, ZCurrentPath).
• (\ means not)

• If we want to return a path,
• then add the 4th argument
• cango(X,Y, P, FinalPath)-
• connected(X,Y),
• \ member(Y,P),
• FinalPath YP.
• cango(X,Y, CP, FP)-
• connected(X,Z),
• \ member(Z,CP),
• cango(Z,Y, ZCP, FP).

5
When call cango, we must initialize the 3rd
argument Question What is the CurrentPath
initially?

• Answer it is the start point.
• ?- cango(a,h, a, Path).
• Path h,d,b,a,
• Why the returned Path is reversed?
• In each recursion, the 3rd argument is added an
  element
• at its front a, b,a, d,b,a, and finally
  h,d,b,a
• This kind of list is called an accumulator

6
How to reverse a list another example of using
accumulator

• Naïve reverse
• rev1(HT, R)-
• rev1(T, RT),
• append(RT,H, R).
• append(, L, L).
• append(HT,X,HL)-
• append(T,X,L).

• More efficient reverse the
• 3rd arg. is an accumulator
• rev2(L, RL)- init accumulator
• rev2(L, RL, ).
• rev2(HT, RL, CL)-
• rev2(T, RL, HCL).
• rev2(, RL, CL)- base
• RLCL.

7
Formatting output

• Assume a route is represented by a list of
  stations and a list of tube lines,
• How to print it out? E.g. tube line is bakerloo,
  central, and stations are
• baker_street, regents_park, oxford_circus,
  tottenhan_court_road, holborn
• a simple solution (you can improve it)
• myprint(_, _) - nl.
• myprint(X,YT1,LT2)-
• connected(X,Y,L),
• write(Take ), write(L), write( line),
• write( from ), write(X), write( to ),
  write(X), nl,
• myprint(YT1, LT2).
• myprint(X,YT1,LT2)-
• \ connected(X,Y,L), need to change to
  another line
• myprint(X,YT1, T2).

8
Implementing a user interface

• An example from last lecture - repeat
  promptreaddo
• sq- write(Give a number ), read(X),
  sq_loop(X).
• sq_loop(X)- Xexit, write(bye), nl. stop
• sq_loop(X)- \ number(X),
• write(input error), nl, sq. restart, ask
  for another
• sq_loop(X)- number(X),
• C is XX, write(sqr(X)), write( ), write(C),
  nl,
• sq. carry on getting input