CODEWITHSUNDEEP
prolog
student
student

Reputation: 39

Display list of list prolog

I can't display list of list in prolog please help me for example : I have this list : [[1,2,3][4,5][6,8,7][6,9]] I want to delete this [ [] [] [] [] ] and just display content: 1 2 3 4 5 6 8 7 6 9

I have tried this

WriteListe([]).
write_liste([X|Xs]) :-
 format('-~w~n', [X]),
 write_liste(Xs).

But when i call WriteListe(L) i have this result L=[] -_6782 L = [_6782] .....

? Thanks you

Upvotes: 0

Views: 384

Answers (1)

Will Ness
Will Ness

Reputation: 71119

Every non-empty list [A|B] consists of its head element A and the rest of its elements, a list B. Otherwise it is an empty list, []:

write_liste(X):-                        /* e.g. write_liste([[1,2,3],[4,5]). */
   X = [A|B] -> write_liste( ... ),     /* recurse on "car"                  */
                write_liste( ... ) ;    /* recurse on "cdr"                  */
   X = []    -> true /* skip it */ ; 
   /* else */   format('~w ', X).       /* we've reached the list's fringe   */

Try to fill in the blanks.

In lisp this is informally known as car ⁄ cdr recursion. We process the "car" i.e. the head element, and the cdr i.e. the rest of the elements. And if it can't be taken apart, then it's an atomic element. Since each chain of such cons nodes ([A|B] in Prolog) ends in a sentinel value, an empty list [] which also cannot be taken apart, we skip that, as we're interested only in the actual elements, not in any artificial effects of representation.

Put another way, [] is empty, so there's nothing in it to be printed.

Upvotes: 2

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