CODEWITHSUNDEEP
prolog
DinoArceuxis
DinoArceuxis

Reputation: 9

List of characters between two parenthesis plus a number

I'm trying to do a predicate parlist(list,num,all).

"list" is basically a list of characters (for example ['a','b','c']), "num" is just a natural number, and "all" is the list that concatenate the list plus the number, except if the length of elements in the original list is 2 or more, then the output list "all" has to be between two parenthesis excluding the number.

Example for a list with length >=2:

parlist(['a','b','c'],3,R).

Output should be:

R=['(','a','b','c',')',3]

Example for a list with length <2:

parlist(['a'],2,R).

Output should be:

R=['a',2]

I'm a little bit lost with prolog, so i dont see how to start, please help. Thanks.

Upvotes: 0

Views: 70

Answers (1)

Nicholas Carey
Nicholas Carey

Reputation: 74325

A lot of Prolog is pattern matching. You have three cases, right?

  • The empty list
    Just make put the number in a list: [ 3 ]

  • Lists of length 1
    Append the number to the list: [ a , 3 ]

  • Lists of length > 1
    Enclose the list in parentheses and add the number at the end: [ '(', a, b, ')', 3 ].

For this you don't even need to iterate over the list. You just need to pattern-match in the head of your predicate clause.

Case 1: the empty list. This is easy:

parlist( [], N, [N] ).

Case 2: Lists of length 1. This is just as easy:

parlist( [X], N, [X,N] ).

Case 3: Lists of length > 1. A little more complex. You could use built-in here, or roll your own.

Builtins: We can use append/3 to effect here:

parlist( [X,Y|Zs], N, R ) :- append( ['(',X,Y|Zs], [')',N], R ).

Roll Your Own: You can roll your own almost as easily. We'll use a little specialized helper predicate here:

parlist( [X,Y|Zs], N, ['(' | R ] ) :- parlist1( ['(',X,Y|Zs], N, R ).

%--------------------------------------------------------------------
% If the list is of length >= 2, add the 1st two elements to the
% result list, and
% - recurse down on the tail.
%
% If the list is of length 0 or 1, we're done: close out the result with
% - the last element of the list (if present),
% - the lead-out parenthesis and
% - the number
%
%--------------------------------------------------------------------
parlist1( [X,Y|Zs] , N , [X,Y|R] ) :- parlist1( Zs , N , R ).
parlist1( []       , N , [  ')',N] ).
parlist1( [X]      , N , [X,')',N] ).

Upvotes: 1

Related Questions