Question

Code in Prolog a predicate swapFirstTwo/2 that swaps the rst two elements of the given list.
If the list is too short, it must fail.
?- swapFirstTwo([a,b,c,d,e], Ys).
Ys = [b,a,c,d,e].
?- swapFirstTwo([a], Ys).
false.

Answer 1

The following is the required Prolog Predicate.

swapFirstTwo([A,B|T],[B,A|T]).

The answer seems really simple as the answer is actually very simple.

As you can see the functor here is swapFirstTwo/2 as instructed in the question, that is the predicate has an arity of 2.

The first argument here is [A,B|T] and the second argument is simply [B,A|T] , which simply means that it is the same list but with the first two elements swapped.

A and B represents the first and the second elements of the list respectively, and T represents the remaining elements of the list.

Also, the predicate will return false if the number of elements in the list is less than 2.

Note:- A, B and T needs to be in capital. This denotes that A,B and T are variables. if these characters are not written in capital the predicate would not work correctly.

Below is a list of some sample outputs:-

?- swapFirstTwo([],Ys).
false.

?- swapFirstTwo([a],Ys).
false.

?- swapFirstTwo([z],Ys).
false.

?- swapFirstTwo([a,b],Ys).
Ys = [b, a].

?- swapFirstTwo([a,1],Ys).
Ys = [1, a].

?- swapFirstTwo([a,1,b],Ys).
Ys = [1, a, b].

?- swapFirstTwo([2,1,b],Ys).
Ys = [1, 2, b].

?- swapFirstTwo([a,b,c,d,e],Ys).
Ys = [b, a, c, d, e].

?- swapFirstTwo([x,y,z,a,b],Ys).
Ys = [y, x, z, a, b].

?- swapFirstTwo([&,1,z,a,b],Ys).
Ys = [1, &, z, a, b].

If you have any query regarding the answer let me know in the comment section, I would love to help.