Prolog：采用两个元素的列表，并且仅当第一个元素与第二个元素相同时才返回true

Question

I'm a newbie prolog programmer, and for an assignment, I have to have a basic program that succeeds if and only if list X is a list of two elements, with the first as the same as the second.

From my view of prolog, programs seem to be pretty small, so I typed this in:

firstPair(x,x).

When I run it under swipl, I get this as output:

Syntax error: Operator expected

Is there something more that needs to be done? I thought that if I executed this with say, firstPair(1,2). this would be all it would need to know that it is false.

Answer 1

First, lowercase x is not a variable, it's an atom . Make x uppercase to fix the problem:

firstPair(X,X).

Second, you do not type this into the interpreter. Rather, you write it into a file firstPair.pl , and then read that file into Prolog.

At the command prompt, type this:

['firstPair.pl'].

Press enter. Now you can use your firstPair/2 rule.

Finally, since the assignment talks about lists, I think the instructor wanted you to write firstPair/1 , not firstPair/2 :

firstPair([X,X]).

Answer 2

Your program/fact

firstPair(X,X).

will succeed if the two arguments given it can be unified , whether they are lists, atoms, variables, etc. To meet your specification, a

program that succeeds if and only if list X is a list of two elements, with the first as the same as the second.

You need something like this:

list_of_two_elements( [X,X] ).

This will succeed if passed a single term that is (or can be unified with) a list of two elements that are, or can be made through unification, identical. For instance, all of the following will succeed:

• list_of_two_elements( X ).

  on success, the variable X will be unified with a list of two elements containing the same unbound variable, something like [V1,V1] .

• list_of_two_elements( [1,1] ).

• list_of-two_elements( [1,X] ). (on success, X here will have been unified with the integer 1 .)