B-Prolog has logical loops. For example, that's how we can calculate sum of [1,2,3]:
test1 :-
foreach(A in 1..3, [], ac(Sa, 0), (
Sa^1 is Sa^0 + A
)),
writeln(sa(Sa)).
?- test1.
test1.
sa(6)
yes
But when I try two nested loops with accumulators, I get errors:
test2 :-
foreach(_A in 1..3, [Sb], ac(Sa, 0), (
foreach(B in 1..3, [], ac(Sb, 0), (
Sb^1 is Sb^0 + B
)),
writeln(sb(Sb)),
Sa^1 is Sa^0 + Sb
)),
writeln(sa(Sa)).
?- test2.
test2.
*** error(invalid_argument,(+)/2)
Another variant, not including Sb in the list of local variables of the outer loop:
test3 :-
foreach(_A in 1..3, [], ac(Sa, 0), (
foreach(B in 1..3, [], ac(Sb, 0), (
Sb^1 is Sb^0 + B
)),
writeln(sb(Sb)),
Sa^1 is Sa^0 + Sb
)),
writeln(sa(Sa)).
?- test3.
test3.
sb(6)
*** error(invalid_argument,(+)/2)
Is it even possible to have nested loops with accumulators in B-Prolog?
My B-Prolog version is 8.0#1.
Your program runs correctly when compiled.
| ?- cl(sergey)
Compiling::sergey.pl
** Warning: Variable 'Sb' is treated as global in foreach (17-25).
** Warning: Variable 'Sb' is treated as global in list_comprehension (36-38).
compiled in 0 milliseconds
loading...
yes
| ?- test1
sa(6)
yes
| ?- test2
sb(6)
sb(6)
sb(6)
sa(18)
yes
| ?- test3
sb(6)
sb(6)
sb(6)
sa(18)
There must be some problem with the interpreter. This accumulator thing is very ugly and I never use it. In Picat, the successor of B-Prolog, you can use := to "update" variables.
test1 =>
Sa = 0,
foreach(A in 1..3)
Sa := Sa+A
end,
writeln($sa(Sa)).
test2 =>
Sa = 0,
foreach(_A in 1..3)
Sb := 0,
foreach(B in 1..3)
Sb := Sb+B
end,
writeln($sb(Sb)),
Sa := Sa+Sb
end,
writeln($sa(Sa)).
An even better way is to use list comprehension.
test1 =>
Sa = sum([A : A in 1..3]),
writeln($sa(Sa)).
test2 =>
Sa = sum([Sb : _A in 1..3, Sb=sum([B : B in 1..3])]),
writeln($sa(Sa)).
The compiler compiles the summations into programs that use ':='. As lists are not actually constructed, there is no overhead.
http://www.probp.com/manual/node55.html seems to show a nested loop, flattened:
?-foreach(A in [a,b], I in 1..2, ac(L,[]), L^1=[(A,I)|L^0]).
L = [(b,2),(b,1),(a,2),(a,1)]
We just need to recover the end-of-inner loop condition. See if this works:
test2 :-
foreach(A in 1..3,
B in 1..3, [], [ac(Sa, 0), ac(La, []) ac(Sb, 0)], (
( ( La^0 = [] ; La^0 = [A] ) % or just (La^0 = [A])
-> % same A, next B
Sa^1 is Sa^0, Sb^1 is Sb^0 + B
; % new A, B-loop has ended
Sa^1 is Sa^0 + Sb^0, writeln(sb(Sb^0)), Sb^1 is 0 + B
),
La^1 = [A]
)),
Sa_final = Sa + Sb, % process the final iteration
writeln(sa(Sa_final)).
If there can be repetitions in A
s, just index them up as the docs show:
?-foreach((A,I) in ([a,b],1..2), ac(L,[]), L^1=[(A,I)|L^0]).
L = [(b,2),(a,1)]
and break on the changes in I
.
(Not tested).