Prolog Lists, Add 1 to tail of the list

1
votes

I'm trying to add either 1 or 2 to numbers in a list in SWI-Prolog.

I've managed to add 1 or 2 to the head of the list however I'm having difficulties adding onto each element on the tail of the list. I don't want to add 1 or 2 to EVERY element at the same time just at separate times. I.e. If my input is add([2,3,4], X).

I would like the possibilities of X to be the following: X = [3,3,4] X = [4,3,4] X = [2,4,4] X = [2,5,4] X = [2,3,5] X = [2,3,6]

My code at present is:

add([],[]).
add([H1|T1],[H2|T2]) :-
                    is(H2,+(H1,1)), T1=T2;
                    is(H2,+(H1,2)), T1=T2.

Obviously this only adds 1 or 2 onto the head of the list and not the tail. Therefore does anyone know how I may go about adding 1 or 2 onto the elements in the tail of my list?

2
listmathprolog
Just curious, why are you using prefix notation for infix operations? – Eugene Sh.
To be honest I'm new to prolog and that way just sort of made sense. I'm sure my way of coding isn't the optimal way to do so haha – Michelle1243

2 Answers

0
votes

sometime more verbose can be clearer:

add([],[]).
add([H1|T],[H2|T]) :-
 H2 is H1+1.
add([H1|T],[H2|T]) :-
 H2 is H1+2.
add([H|T1],[H|T2]) :-
 add(T1,T2).

now alternatives are listed out, the last one just handles - recursively - the remaining elements

Anyway, your code is just missing a line:

add([],[]).
add([H1|T1],[H2|T2]) :-
 is(H2,+(H1,1)), T1=T2;
 is(H2,+(H1,2)), T1=T2;
 H1=H2, add(T1,T2).

After comment, here is how to subtract and keep only positive values:

add([H1|T1],[H2|T2]) :-
 H2 is H1-1, H2 > 0, T1=T2;
 ...
1
votes

First define a predicate addX/3 that will add X to one of the members of the first list:

addX([], [],_).     % base case
addX([H|T], [H1 | T], X) :- H1 is H + X.   % add to first element
addX([H|T], [H | T1], X) :- addX(T, T1, X). % or retain the first element and add to some element in the tail

Than using it define your add predicate as addX with X=1 or X=2:

add(L, R) :- addX(L, R, 1).
add(L, R) :- addX(L, R, 2).

Testing:

?- add([2,3,4], X).
X = [3, 3, 4] ;
X = [2, 4, 4] ;
X = [2, 3, 5] ;
X = [2, 3, 4] ;
X = [4, 3, 4] ;
X = [2, 5, 4] ;
X = [2, 3, 6] ;
X = [2, 3, 4].