# Explain what this prolog program does?

I am working on Prolog and I came across the below program. When I execute this program with a query for example: mysterious([2,3,4,5,6,7,8],L). It has given me the answer 20. I did not understand the flow of getting this result. can anyone help me to understand this code?...How does it produce the answer 20 for the above mentioned query?

```mysterious([],0).
mysterious([X],X).
mysterious([X,Y|Xs], Res) :-
mysterious(Xs, Res1),
Res is X + Res1.
```

Because this is such a basic program, I will give you a hint. First off, this:

```foo([]).
foo([X]).
foo([X,Y|Rest]) :-
foo(Rest).
```

could be also written as:

```foo([]).
foo([First|Rest]) :-
bar(Rest, First).

bar([], Last).
bar([This|Rest], Prev) :-
foo(Rest).
```

It is unnecessarily verbose, but more explicit. In your case, however, it makes something obvious:

```mysterious([], 0).
mysterious([X|Rest], Result) :-
myst_1(Rest, X, Result).

myst_1([], Result, Result).
myst_1([_Y|Rest], X, Result) :-
mysterious(Rest, Result1),
Result is X + Result1.
```

Something which you did not mention in your question is that the compiler must have given you a "singleton variable" warning on the Y in the last clause of mysterious/2. This should have been a strong indication of what is going on already.

I can help you by commenting lines, maybe you will understand the prolog a little more, let's start:

Prolog programs describe relations, defined by means of clauses. Pure Prolog is restricted to Horn clauses. There are two types of clauses: facts and rules. (by wikipedia)

```% comment in prolog starts with '%'
mysterious([],0).  % its called 'predicate', something like functions in imperative
% languages. it has two arguments: [], and 0. and it is ended
% by dot (.). Clauses with empty bodies are called facts.
% (this is fact).

mysterious([X],X). % is actually a "rule" that says, "if the list has one element,
% then the result is just that element". (by @mbratch)

mysterious([X,Y|Xs], Res) :- % now, when argument 1 is list which looks like
% [SomeElement, NextElement|LastElements],
mysterious(Xs, Res1),      % call 'mysterious' with only LastElements
% and variable Res1 (skip first two elements of list)
Res is X + Res1.           % then add first element to Res1.

% it calculates the sum of every second element of list, beginning on first.
```

### Selecting multiple tags with condition

I want to select all editable form elements (input[type=text], input[type="radio"], input[type="checkbox"], select and textarea) that are not readonly or disabled and have a name. Is it possible to...

### Windows 8 App Cert Kit no longer runs?

I am trying to do the App Cert check on my Metro app, but it won't run anymore! Anybody else having this same issue? I've searches all over the Internet, with not even a single related result.

###### About UNIX Resources Network

Original, collect and organize Developers related documents, information and materials, contains jQuery, Html, CSS, MySQL, .NET, ASP.NET, SQL, objective-c, iPhone, Ruby on Rails, C, SQL Server, Ruby, Arrays, Regex, ASP.NET MVC, WPF, XML, Ajax, DataBase, and so on.