Question

Define a function in Scheme (or relation in Prolog) that checks whether a set of elements (represented as a list) is a subset of another set (represented as a list).

Answer 1

Answer:

subset([],[]).

       subset([X|L],[X|S]) :-

           subset(L,S).

       subset(L, [_|S]) :-

           subset(L,S).

Success:

     subset([1,3], [1,2,3]).

     subset(X, [1,3,4]).        % error handling to compare sets in a given order

Fail:

     subset([2,1], [1,2,3]).   % compares in a different order from the first.

Explanation:

The function "Subset" in the source code above accepts two sets, then checks if the first set is a subset of the second. The code returns true if the condition is met.