Question

hi everyone I’m here again, this week I got this Homework:

I should find all the numbers between 10 and 10000 which has property like the following example:

89 = 8^1 + 9^2
2427 = 2^1 + 4^2 + 2^3 + 7^4 = 2 + 16 + 8 + 2401

I have this in Haskell implemented, and it works just fine(I think) and return a List like this:

[89,135,175,518,598,1306,1676,2427]

and then I tried to write it in Prolog(as required too) like the following:

num(0).
num(1).
num(2).
num(3).
num(4).
num(5).
num(6).
num(7).
num(8).
num(9).

allNarc(X):- num(A),num(B),num(C),num(D),
             X = A*1000+B*100+C*10+D,Y = A**1+B**2+C**3+D**4,
             X =:= Y,X>10.
allNarc(X):- num(B),num(C),num(D),
             X = B*100+C*10+D,Y = B**1+C**2+D**3,
             X =:= Y,X>10.
allNarc(X):- num(C),num(D),
             X = C*10+D,Y = C**1+D**2,
             X =:= Y,X>10.

the result is something like this:

?- allNarc(X).
X = 1*1000+3*100+0*10+6 ;
X = 1*1000+6*100+7*10+6 ;
X = 2*1000+4*100+2*10+7 ;
X = 0*100+4*10+3 ;    <- 43
X = 0*100+6*10+3 ;    <- 63
X = 1*100+3*10+5 ;
X = 1*100+7*10+5 ;
X = 5*100+1*10+8 ;
X = 5*100+9*10+8 ;
X = 8*10+9 ;
false.

clearly 43 and 64 should not belong to this group and the result is just ugly, can anyone help me to get output like the result in my Haskell implementation?

Answer 1

Two points.

1. when you say X = you are unifying the left and right sides of =. If you want X to be a number equal to evaluating the expression, use X is <expression here> .
2. you want to prevent a zero in the first position: num(A), A \= 0, num(B), ...

After making those changes I get:

?- allNarc(X).
X = 1306 ? ;
X = 1676 ? ;
X = 2427 ? ;
X = 135 ? ;
X = 175 ? ;
X = 518 ? ;
X = 598 ? ;
X = 89 ? ;
no

You could also use bagof to collect the values. e.g. bagof(X,allNarc(X),Narcs). Narcs is then a list of your values.