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PROLOG - How to round decimals of floating point numbers?

I have this line in the knowledge base:

height(Hipot,Y) :- Y is sin(pi/6)*Hipot.

which calculates one of the cathetus of a right triangle.

When asking Prolog for the value of Y, that is the cathetus, I get an inaccurate number:

?- height(1,Y).
Y = 0.49999999999999994.

But the real value is 1/2, so it should output 0.5. I guess that the inaccuracy is because of the use of pi, but I want to keep using it so how can I round Y to 0.5?

Solution

  • I am trying to learn prolog as well. The built-in round function only goes to the nearest integer, so I defined a rule that extends it to round to a certain number of digits:

    round(X,Y,D) :- Z is X * 10^D, round(Z, ZA), Y is ZA / 10^D

    I'm not sure if it's idiomatic, but it seems to work:

    ?- round(5.5555, Y, 2).
Y = 5.56.