prolog problem

vbx_wx · 12-10-2010, 11:42 AM

Hello i have a problem in prolog,i hope someone can help me,Im trying to swap the first 2 elements of a list:

Code:

swap([X,Y | Rest], [Y,X | Rest1]).

And when i do :

Code:

swap([1,2,3,4], What).

I get: What = [2,1|_3ac]
yes

paulsm4 · 12-10-2010, 05:53 PM

Hi, vbx_wx -

I turned this up from Google:
http://www.tek-tips.com/viewthread.c...1629112&page=1

Or this:

Code:

http://forums.devarticles.com/programming-tools-11/prolog-lists-1650.html

% swap first two elements of list
swap([X,Y|Rest], [Y,X|Rest1]).

The same link continues:

Code:

http://forums.devarticles.com/programming-tools-11/prolog-lists-1650.html

when tested it shows:
swap([1,2,3],SL).
SL = [2,1,3] ? ; % swaps first two
SL = [1,3,2] ? ; % swaps last two (first two of tail)
SL = [1,2,3]
yes

swap([1,2,3,4],SL).
SL = [2,1,3,4] ? ; % swaps first two
SL = [1,3,2,4] ? ; % swaps middle two (first two of tail)
SL = [1,2,4,3] ? ; % swaps last two (first two of new tail)
SL = [1,2,3,4]
yes

swap([1,2,3,4,5],SL).
SL = [2,1,3,4,5] ? ; % swaps first two
SL = [1,3,2,4,5] ? ; % swaps next two
SL = [1,2,4,3,5] ? ; % swaps next two
SL = [1,2,3,5,4] ? ; % swaps last two
SL = [1,2,3,4,5]
yes

Clocksin and Mellish is the best book available for learning Prolog:

This link also looks interesting:
http://ktiml.mff.cuni.cz/~bartak/prolog/index.html

'Hope that helps .. at least a little bit