Categorise List in Prolog

Question

Alright so I am coding a parser for arithmetic equations. I get the input in a list, e.g. "10+20" = [49,48,43,50,48] and then I convert all the digits to there corresponding numbers e.g. [49,48,43,50,48] = [1,0,43,2,0] and from there I want to put integers > 10 back together.

Converting from ascii -> digits I use a maplist and number_codes to convert.

One approach I had was to just traverse the list and if it's 0-9 store it in a variable and then check the next number, 0-9 append it to the other variable and so on until I hit an operator. I can't seem to simply append digits as it were. Here's my current code.

expression(L) :-
    maplist(chars, L, Ls).

chars(C, N) :-
    (
        C >= "0", "9" >= C -> number_codes(N, [C]);
        N is C
    ).

Not sure if there's a simple way to add to my code (as far as I know, maplist only gives back a list of equal length to the list passed in but I could be mistaken).

Any help is appreciated :)

score 1 · Answer 1 · answered Sep 11 '12 at 10:01

Yes, maplist only 'gives back' a list of equal length. Moreover, maplist applies a predicate only to one element (basically it's context-free). Therefore, it is not possible to do what you want (combine digits between operators to a single number) with maplist and you would have to write the recursion yourself.

However, you can do something way easier than all this converting back and forth:

expression(L, E):-
    string_to_atom(L,A),
    atom_to_term(A,E,[]).

Which works like this:

2 ?- expression("1+2",E).
E = 1+2.

3 ?- expression("1+2",E), X is E.
E = 1+2, X = 3.

4 ?- expression("1+2",E), X+Y =  E.
E = 1+2, X = 1, Y = 2.

5 ?- expression("1+2+3",E), X+Y =  E.
E = 1+2+3, X = 1+2, Y = 3.

Naturally, if you want a list with all the numbers involved you will have to do something recursive but this is kinda trivial imho.

If however you still want to do the converting, I suggest checking Definite Clause Grammars; it will simplify the task a lot.

@WhaleFanny I havent worked with sicstus; maybe you need to include a library or check for a similar predicate — Thanos Tintinidis, Sep 11 '12 at 11:14
[`atom_codes/2`](http://www.swi-prolog.org/pldoc/doc_for?object=atom_codes/2) is ISO Prolog and seems to do what you expect from `string_to_atom/2` — m09, Sep 11 '12 at 11:57

score 1 · Answer 2 · edited May 23 '17 at 11:45

1

I answered some time ago with an expression parser.

It will show you how to use DCG for practical tasks, and I hope you will appreciate the generality and simplicity of such approach.

Just a library predicate is required from SWI-Prolog, number//1, easily implemented in Sicstus. Let me know if you need more help on that.

edited May 23 '17 at 11:45

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answered Sep 11 '12 at 14:01

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Categorise List in Prolog

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