SWI-Prolog -- scanl/7

Availability::- use_module(library(apply)).(can be autoloaded)

scanl(:Goal, +List, +V0, -Values)

scanl(:Goal, +List1, +List2, +V0, -Values)

scanl(:Goal, +List1, +List2, +List3, +V0, -Values)

scanl(:Goal, +List1, +List2, +List3, +List4, +V0, -Values)

Scan an ensemble of m (0 <= m <= 4) lists of length n head-to-tail ("scan-left"), using columns of m list elements as arguments for Goal. The scanl family of predicates is defined as follows, with V0 an initial value and V the final value of the scanning operation:

scanl(G, [X_11, ..., X_1n],
         [X_21, ..., X_2n],
         ...,
         [X_m1, ..., X_mn], V0, [V0, V1, ..., Vn] ) :-
   call(G, X_11, ..., X_m1, V0, V1),
   call(G, X_12, ..., X_m2, V1, V2),
   ...
   call(G, X_1n, ..., X_mn, V<n-1>, Vn).

scanl behaves like a foldl that collects the sequence of values taken on by the Vx accumulator into a list.

Examples

Examples for scanl/N

?- scanl(plus, [1,2,3,4,5], 0, Sums).
Sums = [0, 1, 3, 6, 10, 15].

When considering the lists columns of a table, scanl/5 combines the accumulator with each row to produce the next value in the result list. We illustrate this using plus/4 which adds the accumulator with the values from the row to produce the next accumulator. We use trace/2 to illustrate how the values are combined.

plus(A,B,C,Result) :-
    Result is A+B+C.

?- trace(plus/4, exit).
%         plus/4: [exit]
?- scanl(plus, [1,2,3,4,5], [10,20,30,40,50], 0, Sums).
 T Exit: plus(1, 10, 0, 11)
 T Exit: plus(2, 20, 11, 33)
 T Exit: plus(3, 30, 33, 66)
 T Exit: plus(4, 40, 66, 110)
 T Exit: plus(5, 50, 110, 165)
Sums = [0, 11, 33, 66, 110, 165].