prolog how to use math operation

Question

I am a novice in prolog programming, i use swi-prolog. Now I'm stucked by some math problems

as we know the predicate :A is 3+3.works well,the answer is A=6.

but if I want to find two digits (A and B) from 0~9 that a+b=6 6 is A+B does't work. so I want to know if there is a easy way to do this? And what if I want to find 3 digits (A,B and C)from 0~9 that A+B+C=13 how to do this ?

score 2 · Accepted Answer · answered Oct 09 '15 at 12:50

the simpler way, working in every Prolog implementation: declare a predicate digit/1 (the notation predicate/N means that predicate has N arguments)

digit(D) :- member(D, [0,1,2,3,4,5,6,7,8,9]).

then you can ask

?- digit(A),digit(B),6 is A+B.
A = 0,
B = 6 ;
A = 1,
B = 5 ;
...

since sum is symmetric, maybe you want to reduce the duplicate solution with

?- digit(A),digit(B),A=<B,6 is A+B.

Using library(clpfd) you can avoid defining your digit/1 predicate, and gain a lot of functionality:

?- [library(clpfd)].
true.

?- [A,B,C] ins 0..9, A+B+C #= 13, label([A,B,C]).
A = 0,
B = 4,
C = 9 ;
A = 0,
B = 5,
C = 8 
...

note that now the incognite can stay to the left of the 'assignment'...

score 2 · Answer 2 · edited Oct 11 '15 at 22:00

My advice to you as a Prolog novice is this:

First things first!
Use clpfd! You can start right now by reading the tag information section on tag clpfd.
Pick up the habit of starting new file with extension .pl with the following line:
```
:- use_module(library(clpfd)).
```
Use clpfd everytime you want to express Prolog relations over integers: Every. Single. Time.
Witness that clpfd is already widely available and that it is still gaining ground.
Realize that you will profit the most if you learn clpfd first—not 1970's style (is)/2!

Why? (is)/2 is a low-level feature best used by constraint solver implementors.

score 1 · Answer 3 · edited May 23 '17 at 11:58

Doing math with prolog is interesting. This looks like an assignment and I would not like to solve it, but I will try to help you find the answer yourself. Given the restricted range of your problem you could probably define every integer from 0 to 9 by creating a simple prolog programm. Keep in mind that you can define also functions like:

add3(A, B, C,SUM) :- SUM is A + B + C.

You could try to solve the problem by an Equation Solver. See this answer: Equation solver in SWI-Prolog

Or using Constraint Logic Programming. http://www.swi-prolog.org/man/clpqr.html

prolog how to use math operation

3 Answers3