Homework #4: Prolog Programming

Some hopefully helpful notes

• Consider the following function definition:

  nrev([], []).
  nrev([H|T0], L) :-
      nrev(T0, T),
      append(T, [H], L).
  

  some things to note:
  a) We still have the base/inductive case split, but notice that no other (unrelated) lines of code appear in between them.
  b) That variables to be matched start with Capital letters (I had problems when I tried lowercase variables).
  c) Code, such as this, goes in the left pane of the Swish Window.
  

      nrev([1,2,3],A).
    

  This is a query--basically it asks: what would A have to be make this statement true. It should be entered into the pane at the bottom right of the Swish window. The correct response is A = [3,2,1] (click the "run" far bottom right to execute the query).

  sumOdd(A,B,C,D) :-
      D is A+C.
  

  This is a valid function; but notice that the interpreter will complain about the variable "B", because it never gets used. The system would prefer:

  sumOdd(A,_,C,D) :-
     D is A+C.
  

• Here is a simple prolog query. [1,2,3,4] = [A,B|T]. It is totally valid. The response will be that A = 1, B = 2 and T = [3,4].
• Next, consider the function "twice".

  twice(A,B) :-
      A is B*2.
  

  Note the results of each of the following queries:

  twice(3,6).
   false
  
  twice(6,3).
   true
  
  twice(C,6).
   C = 12.
  
  twice(6,C).
   ERROR
  

  {Apparently, we know how to compute A given B, but the opposite (albeit, clear to you and me) is not given.}

The Actual Homework Problems:

1. Write a function called "second", which has two params: a list and value. If the list has less than two elements, the value should be "-1". If the list has at least two elements, the value should be the second element of the list. These should be valid (true) queries:

       second([7],-1).
   second([7,3,9],3).
   
   second([1,2,3],A).
   should return that A =2.
   

2. Write _either_ the oddL or evenL function which determines whether the length of a list is odd (or even).

   these should be valid queries:
   oddL([1,2],false).
   evenL([1,2,3,4],true).
   
   oddL([1,2,3],A).
   A = true.
   

3. Write the sumList function which adds up the element values in a list.

   sumList([1,2,3],6).
   sumList([7,3,9],A).
    A = 19.