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prologfibonaccitail-recursion

Calculate Fibonacci Serie in Prolog, Tail Recursive

I want to calculate Fibonacci Series in Prolog,in recursive tail mode.

fibonacci(0,0).
fibonacci(1,1).
fibonacci(N,Result) :-
   fibonacci(N,1,0).

fibonacci(N,Result,Count) :-
   Count < N,
   !,
   Count1 is Count + 1,
   Result1 is  Result + Count,
   fibonacci(N,Result1,Count1).  
fibonacci(N,Count,Count).

but the result is incorrect,what is the problem?

Solution

  • In the line Result1 is Result + Count you only add the Count the variable Result and add 0,1,2,... but in fibonacci you need to add two previous e.g 0,1,(1+0=1),(1+1=2),.... I suggest this implementation:

    fib(0, 0).
fib(1, 1).
fib(N,Result):-fibonacci(N,0,1,Result).

fibonacci(0,N,_,N).
fibonacci(N, Prev1,Prev2,Result):-
   N>0,
   New_Prev2 is Prev1+Prev2,
   N1 is N-1,
   fibonacci(N1,Prev2,New_Prev2,Result).