Anonymous user
Pascal's triangle: Difference between revisions
→{{header|Prolog}}
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true.
</lang>
===An alternative===
The above use of difference lists is a really innovative example of late binding. Here's an alternative source which, while possibly not as efficient (or as short) as the previous example, may be a little easier to read and understand.
<lang prolog>%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
% Produce a pascal's triangle of depth N
%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
% Prolog is declarative. The predicate pascal/3 below says that to produce
% a row of depth N, we can do so by first producing the row at depth(N-1),
% and then adding the paired values in that row. The triangle is produced
% by prepending the row at N-1 to the preceding rows as recursion unwinds.
% The triangle produced by pascal/3 is backwards and lacks the last row,
% so pascal/2 prepends the last row to the triangle and reverses it.
% Finally, pascal/1 produces the triangle, iterates each row and prints it.
%~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pascal_row([V], [V]). % No more value pairs to add
pascal_row([V0, V1|T], [V|Rest]) :- % Add values from preceding row
V is V0 + V1, !, pascal_row([V1|T], Rest). % Drops initial value (1).
pascal(1, [1], []). % at depth 1, this row is [1] and no preceding rows.
pascal(N, [1|ThisRow], [Last|Preceding]) :- % Produce a row of depth N
succ(N0, N), % N is the successor to N0
pascal(N0, Last, Preceding), % Get the previous row
!, pascal_row(Last, ThisRow). % Calculate this row from the previous
pascal(N, Triangle) :-
pascal(N, Last, Rows), % Retrieve row at depth N and preceding rows
!, reverse([Last|Rows], Triangle). % Add last row to triangle and reverse order
pascal(N) :-
pascal(N, Triangle), member(Row, Triangle), % Iterate and write each row
write(Row), nl, fail.
pascal(_).
</lang>
=={{header|PureBasic}}==
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