integer - Divison and remainder in Prolog

Question

Trying to figure out how to write a recursive predicate divide_by(X, D, I, R) that takes as input a positive integer X and a divisor D, and returns the answer as the whole number part I and the remainder part R, however, I can't seem to get my head around Prolog. How would I go about doing this?

Answer 1

There are predefined evaluable functors for this.

(div)/2 and (mod)/2 always rounding down. Recommended by LIA-1, Knuth etc.

(//)/2 and (rem)/2 rounding toward zero (actually, it's implementation defined, but all current implementations do it like that). You can ask this via current_prolog_flag(integer_rounding_function, F) which gives in current implementations toward_zero.

The difference between those pairs shows only when negative numbers are involved. It is kind of a religious war which one to prefer. ISO/IEC 10967:2012 Language independent arithmetic (vl. LIA-1) only provides rounding down "due to proneness for erroneous use" (C.5.1.2.2) of toward_zero, whereas Fortran and followers like C go for toward_zero calling it "algebraic" (6.5.5). See also: Advantages of using truncation towards minus infinity vs towards zero