CODEWITHSUNDEEP
prolog
user550738
user550738

Reputation:

how to set value of an arg in Prolog?

I have an assignment question like so:

Write a program to find the last element of a list. e.g.
?- last(X, [how, are, you]).
X = you
Yes

I'm currently finding the last element like this:

last([Y]) :-
    write('Last element ==> '),write(Y).
last([Y|Tail]):-
    last(Tail).

And it works. My question is, how do I change it to accept and set the addition X parameter and set it correctly?

I tried this, but it's not working ...

last(X, [Y]) :-
    X is Y.

last(X, [Y|Tail]):-
    last(X, Tail).

Upvotes: 0

Views: 1891

Answers (2)

Peter Geiger
Peter Geiger

Reputation: 103

Using the unification operator is not the preferred way to unify in a case like this. You can use unification in a much more powerful way. See the following code:

last(Y, [Y]).  %this uses pattern matching to Unify the last part of a list with the "place holder"
               %writing this way is far more concise.
               %the underscore represents the "anonymous" element, but basically means "Don't care"

last(X, [_|Tail]):-
last(X, Tail).

Upvotes: 1

Haile
Haile

Reputation: 3180

Most obvious problem: (is)/2 works with numbers only. (link)

-Number is +Expr True when Number is the value to which Expr evaluates

You want to use the unification operator (=)/2 (link):

last(X, [Y]) :-
    X = Y,
    !.

last(X, [_|Tail]):-
    last(X, Tail).

Let's try:

?- last(X, [1, 2, 3]).
X = 3.

?- last(X, [a, b, c]).
X = c.

Upvotes: 1

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