Split a number into a list of digits in Prolog

Question

I've been having trouble trying to split numbers into lists using Prolog, e.g. 123456 becomes [1,2,3,4,5,6].

Can you please help me work out how to do this?

Answer 1

You could first create a reverse list:

//Base step
splitRev(0,[]).
//Recursive step
splitRev(N,[A|As]) :- N1 is floor(N/10), A is N mod 10, splitRev(N1,As).

The recursive step works like this:

N1 is floor(N/10)

divides N by 10 and rounds it down. So 538 becomes 53.8 becomes 53. It cuts off the last digit.

A is N mod 10

takes the remainder of N divided by 10. 538 mod 10 equals 8. So you get only the last digit.

Now for splitting the list you only need to reverse the list created by splitRev/2. So predicate split/2 is defined as:

split(N,L1) :- splitRev(N,L2), reverse(L1,L2).

Note that reverse/2 is a built-in predicate.

I hope this helps!

Answer 2

the builtins available are ISO standard:

?- number_codes(123456,X),format('~s',[X]).
123456
X = [49, 50, 51, 52, 53, 54].

?- number_chars(123456,X),format('~s',[X]).
123456
X = ['1', '2', '3', '4', '5', '6'].

I also have some very old code I developed for my interpreter. := must be renamed is to run with standard Prologs. But then you are best served from above builtins...

itoa(N, S) :-
    N < 0, !,
    NN := 0 - N,
    iptoa(NN, SR, _),
    reverse(SR, SN),
    append("-", SN, S).
itoa(N, S) :-
    iptoa(N, SR, _),
    reverse(SR, S).

iptoa(V, [C], 1) :-
    V < 10, !,
    C := V + 48.
iptoa(V, [C|S], Y) :-
    M := V / 10,
    iptoa(M, S, X),
    Y := X * 10,
    C := V - M * 10 + 48.

edit here the additional call required to get numbers:

?- number_codes(123456,X), maplist(plus(48),Y,X).
X = [49, 50, 51, 52, 53, 54],
Y = [1, 2, 3, 4, 5, 6].