Re: [eclipse-clp-users] List of Structures

From: Joachim Schimpf <jschimpf_at_...311...>
Date: Mon, 02 Feb 2015 18:34:49 +0000
On 02/02/2015 06:55, Edgaonkar, Shrirang wrote:
> I have been working on the following problem for some time. I wish to filter the following List.
> :- local struct( goodsinfo( goodsid, goodsname, price, saledate, term, zaikoamount, registerdate,
> categoryid, searchViews ) ).
> Result = [goodsinfo(101, "laptop", "tablet", "iPad"), goodsinfo(102, "notebook", "pendrive", "tab")]

Your structure syntax is wrong.  You have declared a structure
goodsinfo/9 (i.e. with 9 arguments), but the example list contains
goodsinfo/4 structures.

Note that there are two ways to write structures in ECLiPSe:

1. Canonical (Prolog-style) notation:  This style can be used without
any preceding struct-declaration, and has the simple form

    functor_name(argument1, ..., argumentN)

i.e. you use *round* brackets and you must write *all* N arguments
in the correct order from 1 to N.  The arity of the structure is N.

2. ECLiPSe's struct-syntax:  This requires a preceding struct-declaration,
and has the form

    functor_name{arg_name:arg_value, ...}

i.e. you use *curly* brackets, and you label every argument with its declared
field name.  You do *not* have to write all N arguments, and you can write the
arguments in any order.  The system knows the arity and argument position from
the struct-declaration.

ECLiPSe basically macro-expands the f{...} syntax into f(...) syntax.  See how
it works by issuing your struct-declaration above, and trying a query like

?- X = goodsinfo{price:9.99, goodsname:"mouse"}.

X = goodsinfo(_275, "mouse", 9.99, _278, _279, _280, _281, _282, _283)
Yes (0.00s cpu)

You see that the convenient {}-syntax has been expanded into the internal form
with all 9 arguments, the price and goodsname arguments have been put into the
correct argument slots, and the remaining arguments filled with free variables.

> I am trying something like this
> (foreach(goodsinfo( goodsid:GoodsId, goodsname, term, searchViews ),Result), fromto(List,Out,In,[]) do
>     X[goodsid of goodsinfo] == 101 -> Out=[X|In] ; Out=In, writeln( X[goodsid of goodsinfo] )),

The loop structure is essentially correct.  The following would work:

filter101(Xs, Ys) :-
    ( foreach(X,Xs), fromto(Ys,Ys2,Ys1,[]) do
        ( goodsinfo{goodsid:101}=X -> Ys2=[X|Ys1] ; Ys2=Ys1 )

        %%% Alternatively:
        %%% ( X[goodsid of goodsinfo]=:=101 -> Ys2=[X|Ys1] ; Ys2=Ys1 )
        %%% Alternatively:
        %%% arg(goodsid of goodsinfo, X, I), (I==101 -> Ys2=[X|Ys1] ; Ys2=Ys1 )

-- Joachim
Received on Mon Feb 02 2015 - 18:34:59 CET

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