How to add item to list in prolog

Question

I have a list in prolog that contains several items. I need to 'normalized' the content of this list and write the result to a new list. But I still have problem in doing it. The following code shows how I did it:

 normalizeLists(SourceList, DestList) :-

 % get all the member of the source list, one by one
 member(Item, SourceList),

 % normalize the item
 normalizeItem(Item, NormItem),

 % add the normalize Item to the Destination List (it was set [] at beginning)
 append(NormItem, DestList, DestList).

The problem is in the append predicate. I guess it is because in prolog, I cannot do something like in imperative programming, such as:

DestList = DestList + NormItem,

But how can I do something like that in Prolog? Or if my approach is incorrect, how can I write prolog code to solve this kind of problem.

Any help is really appreciated.

Cheers

CapelliC CapelliC · Accepted Answer · 2012-12-15T06:33:05

Variables in Prolog cannot be modified, once bound by unification. That is a variable is either free or has a definite value (a term, could be another variable). Then append(NormItem, DestList, DestList) will fail for any NormItem that it's not an empty list.

Another problem it's that NormItem it's not a list at all. You can try

normalizeLists([], []).
normalizeLists([Item|Rest], [NormItem|NormRest]) :-

 % normalize the item
 normalizeItem(Item, NormItem),

 normalizeLists(Rest, NormRest).

or (if your Prolog support it) skip altogether such definition, and use an higher order predicate, like maplist

...
maplist(normalizeItem, Items, Normalized),
...

How to add item to list in prolog

1 Answers