Pascal's triangle in Prolog
I have written a function for returning the next row in Pascal's triangle given the current row:
pascal_next_row([X],[X]).
pascal_next_row([H,H2|T],[A|B]):-
pascal_next_row([H2|T],B),
A is H + H2.
I want to be able to find the nth row in the triangle, e.g. pascal(5,Row)
, Row=[1,5,1,0,1,0,5,1]
. I ha开发者_Go百科ve this:
pascal(N,Row):-
pascalA(N,[1,0],Row).
pascalA(N,R,_Row):-
N > 0,
M is N-1,
next_row([0|R],NR),
pascalA(M,NR,NR).
Obviously Row
should be the last one found before n==0
. How can I return it? I tried using the is
keyword, i.e. Row is NR
but that isn't allowed, apparantly. Any help?
Trying to use is
on lists gets me:
! Domain error in argument 2 of is/2
! expected expression, but found [1,4,6,4,1,0]
! goal: _23592586 is[1,4,6,4,1,0]
Do the base case, N > 0
cancels your calculation...
pascalA(N,R,_Row):-
N > 0, %% this evaluates to false so the calculation gets canceled
M is N-1,
next_row([0|R],NR),
pascalA(M,NR,NR).
pascalA(0,R,R). %% this should be the base case... hope I got it correct...
pascalA(N,R,_Row):-
M is N-1,
next_row([0|R],NR),
pascalA(M,NR,_Row).
You need a base case for pascalA where N = 0.
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