CODEWITHSUNDEEP
prolog
wholerabbit
wholerabbit

Reputation: 11536

Variable assignment in recursion

Given this:

useAdd(List, What) :-                                         
    addToList(List, What, New),                               
    reverseList(New, T),                                      
    write("done "),                                           
    write(T), nl, !.                                          

addToList(List, What, [ What | List]).                        

reverseList([], List) :- write(List), nl.                     
reverseList([H | Old], []) :- reverseList(Old, [ H ]).        
reverseList([ H | Old], New) :- reverseList(Old, [ H | New]).

Why does this happen (in swipl)?

?- useAdd([42, 100, ok], hello).
[ok,100,42,hello]
done []
true.

Obviously assigning to New in useAdd amounted to something, because the correct list is passed to reverseList. But then, unlike New, T in useAdd is empty.

Upvotes: 1

Views: 43

Answers (1)

Will Ness
Will Ness

Reputation: 71065

Your definition of reverseList/2 uses T as the reversed list's tail. The reversed list is built and printed at the deepest level of recursion, and then nothing is done with that created value:

2 ?- reverseList( [1,2,3,4], [0]).
[4,3,2,1,0]
true.

Instead, pass it in as a fresh logical variable, as a third parameter, pass it along unchanged between the invocations, and finally set it at the deepest level:

reverseList([], List, T) :- List = T, write(List), nl. 
reverseList([H | Old], [], T) :- reverseList(Old, [ H ], T).
reverseList([ H | Old], New, T) :- reverseList(Old, [ H | New], T).

Then call it as reverseList( [1,2,3,4], Newtail, T) to get your T set at the deepest level of recursion:

6 ?- reverseList( [1,2,3,4], [0], T).
[4,3,2,1,0]
T = [4, 3, 2, 1, 0].

The definition is all over the place, the naming is terrible, and it used T probably for "tail", so you should name the new argument something like R instead, to at least hint at the fact that it will hold the reversed list as the result of an invocation.

Upvotes: 1

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