I'm trying to code this formula in Prolog :
"str" is input number as a string "base" is the base of the input number.
Result is,
(base)^0 * str[len-1] + (base)^1 * str[len-2] + (base)^2 * str[len-3] + ...
I'm new to Prolog and I have this right now:
calc([],_,0):- !. calc([H|T],Base,Res):- length([H|T],Long), Long >= 0, Size is Long - 1, power(Base , Size, Res), Res1 is Res * H, calc(T,Base,Res1). but it doesn't work properly.
You can do something like this:
value(String, Base, Value) :- string_chars(String, Digits), value(Digits, Base, 0, Value). value([], _, Value, Value). value([Digit|Digits], Base, Accumulator, Value) :- atoi(Digit, Number), NewAccumulator is Base*Accumulator + Number, value(Digits, Base, NewAccumulator, Value). atoi(Char, Int) :- % convert ASCII code to integer char_code(Char, Code) , Int is Code - 48. The predefined predicate string_chars converts a string into a list of chars:
?- string_chars("1101", Chars). Chars = ['1', '1', '0', '1']. The predicate atoi converts a character representing a digit into a corresponding integer:
?- atoi('3', Integer). Integer = 3. Supposing that [1,1,0,1] is a list of integers (representing a number in base 2), its corresponding value in base 10 can be computed as following:
Digit Accumulator - 0 1 2 x 0 + 1 = 1 1 2 x 1 + 1 = 3 0 2 x 3 + 0 = 6 1 2 x 6 + 1 = 13 Here are some examples:
?- value("1101", 2, V). V = 13. ?- value("1201", 3, V). V = 46. Alternative solution Suppossing that you already have a list of integers representing the digits of a number, the solution is even simpler:
value_integers(Digits, Base, Value) :- value_integers(Digits, Base, 0, Value). value_integers([], _, Value, Value). value_integers([Digit|Digits], Base, Accumulator, Value) :- NewAccumulator is Base*Accumulator + Digit, value_integers(Digits, Base, NewAccumulator, Value). Here are some examples:
?- value_integers([1,1,0,1], 2, Value). Value = 13. ?- value_integers([1,2,0,1], 3, Value). Value = 46. ?- value_integers([1,2,0,1], 10, Value). Value = 1201. 
