Mathematical operations with atoms in prolog

Question

If I have three atoms X, Y and Z where

X = 1
Y = 2
Z = +

How do I put them together so that X Z Y = 3?

Edit: Using ThanosQR's univ solution, I have modified my code as follows:

% Parse list
parse_list([stop|_], _) :- !.   % stop predicate if element is "stop"
parse_list([X, Y|Z]) :- % go through the list line by line
    number(X, Number_1),
    number(Z, Number_2),
    operation(Y, Operation),
    Line =.. [Operation, Number_1, Number_2], 
    Result is Line,
    write(Result).

number(one, 1). 
operation(plus, +). % etc... etc...

I get false... Not sure why..

Please state your Prolog system (probably SWI) and the actual failing query. — false, Nov 29 '11 at 22:39
You get false since the head of the second clause is incorrect. It surely should be `parse_list([X,Y,Z|Rest]) :-`. Also, the second clause has no recursion after the last write: `parse_list([Rest]).` That seems to be missing. Finally, the head of the first clause has two arguments, the head of the second has one. This is inconsistent! — twinterer, Nov 30 '11 at 12:35
Thanks, I'll try your suggestions. Thats a typo in the head of the first clause, it should be one argument. — chutsu, Nov 30 '11 at 13:08

false · Answer 1 · 2011-11-29T23:01:44.937

3

First, a general remark: If you mix impure predicates like write/1 into otherwise pure code, you end up with a very hard to debug program. So first try to identify the relation you are after. Maybe it is something like tokens_meaning(Tokens,Meaning) where Tokens it the list of tokens, as above, and Meaning is some expression to evaluate.

For identifying one problem, it suffices to look at the following fragment:

...
parse_list([X, Y|Z]) :- % go through the list line by line
    number(X, Number_1),
    number(Z, Number_2),
    ...

number(one, 1).

So what you expect here is that you have a term of the following form:

[one, Y|one]

That is, the Z should be the rest of the list. So either [] or something longer like [_] but not one.

But in your case you expect a name like one. That certainly does not make sense. Probably you meant:

parse_list([X, Y, Z| Whatever]) :-

edited Nov 29 '11 at 23:01

answered Nov 29 '11 at 22:51

false

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The reason I'm doing: parse_list([X, Y|Z]) is because I'm assuming the list will always have three elements [X, Y, Z]. And can you elaborate more on the pure code and unpure, am not too sure I understand what you mean by that. – chutsu Nov 30 '11 at 10:47
1

@chutsu: It is still difficult for me to figure out what you actually want. `parse_list([X, Y, Z]) :- ...` might be what you want. But there are still too many things open. Otherwise, please [refer to this thread](http://stackoverflow.com/q/8305811/772868) for how a program with I/O and pure Prolog intertwined might become a cleaner program. – false Dec 01 '11 at 13:32
Yes, indeed. Turns out I failed to define one of the numbers in number/2 that is what caused the fail. Sorry for the headache I have caused you. And thanks very much for helping me on this matter. – chutsu Dec 01 '11 at 18:20

score 1 · Accepted Answer · answered Nov 29 '11 at 21:17

1

try the univ operator:

R =.. [Z,X,Y], A is R.

answered Nov 29 '11 at 21:17

Thanos Tintinidis

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Wow that's great, but when I use it in my code it returns false. – chutsu Nov 29 '11 at 21:36
@chutsu does it work outside your code? what exactly are you doing in your code? – Thanos Tintinidis Nov 29 '11 at 22:02
@chutsu just saw your edit, i think that twinterer spotted the problems; if you write [X,Y|Z] = L when L=[a,b,c] you will get X=a,Y=b, Z=[c], not Z=c – Thanos Tintinidis Nov 30 '11 at 18:48

Mathematical operations with atoms in prolog

2 Answers2