Re: if

[Fergus Henderson]

On 29-Jan-2001, Calum Grant <address@hidden> wrote:
> That's not really specific to GNU, but I'll try to answer.
> 
> There actually are "if" predicates in Prolog, and alternative branches of
> execution can be taken conditionally.  The syntax is just a little different
> and problems tend to be formulated differently.  e.g. in an imperative
> language,
> 
>       printnum(n):
>               if(n=1) return "one"
>               else if(n=2) return "two"
> 
> while in Prolog,
> 
>       printnum(1, one).
>       printnum(2, two).
> 
> By default, every "branch" of the printnum predicate is taken, so
> printnum(N, M) would yield two answers: N=1, M=one, N=2, M=two.  However if
> you call printnum(2, M), you would just get M=two.  The first branch is not
> taken, since the condition N=1 fails.
> 
> printnum can also be rewritten as
> 
>       printnum(N, M) :- N=1, M=one; N=2, M=two.
> 
> which makes it even more clear what the conditions are (N=1 or N=2).

That's all true, but it's important to mention Prolog's if-then-else
construct.  What you've shown with multiple clauses is more like
Prolog's analog of the `switch' statement in C.  But Prolog does
have a very direct analog of if-then-else, in fact it's even called
"if-then-else", although it is spelt `... -> ... ; ...'.
For example,

        printnum(n):
                if(n=1) return "one"
                else return "many"

would be written in Prolog as

        printnum(N, M) :-
                ( N = 1 -> M = "one"
                ; M = "many"
                ).

(the parentheses here are optional but highly advisable).

Likewise 

        printnum(n):
                if(n=1) return "one"
                if(n=2) return "two"
                else return "many"

would be

        printnum(N, M) :-
                ( N = 1 -> M = "one"
                ; N = 2 -> M = "two"
                ; M = "many"
                ).

You can also achieve similar effects using multiple clauses and cut,
but using Prolog's if-then-else is much better style, IMNSHO.

-- 
Fergus Henderson <address@hidden>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.