How do you do modulo or remainder in Erlang?

Question

I\'m brand new to Erlang. How do you do modulo (get the remainder of a division)? It\'s % in most C-like languages, but that designates a comment in Erlang.

Several

Answer 1

According to this blog post, it's rem.

Answer 2

In Erlang, 5 rem 3. gives 2, and -5 rem 3. gives -2. If I understand your question, you would want -5 rem 3. to give 1 instead, since -5 = -2 * 3 + 1.

Does this do what you want?

mod(X,Y) when X > 0 -> X rem Y;
mod(X,Y) when X < 0 -> Y + X rem Y;
mod(0,Y) -> 0.

Answer 3

mod(A, B) when A > 0 -> A rem B;
mod(A, B) when A < 0 -> mod(A+B, B); 
mod(0, _) -> 0.

% console:
3> my:mod(-13, 5).
2

Answer 4

The erlang modulo operator is rem

Eshell V5.6.4  (abort with ^G)
1> 97 rem 10.
7

Answer 5

I used the following in elixir:

defp mod(x,y) when x > 0, do: rem(x, y);
defp mod(x,y) when x < 0, do: rem(x, y) + y;
defp mod(0,_y), do: 0

Please don't downvote this because it's another language than the question. We all live the dream, because we all have the beam.

Answer 6

The above Y + X rem Y seems to be wrong: either (Y + X) rem Y or Y + (X rem Y) yield incorrect results. Ex: let Y=3. If X=-4, the first form returns -1, if X=-3 the second form returns 3, none of which is in [0;3[.

I use this instead:

% Returns the positive remainder of the division of X by Y, in [0;Y[. 
% In Erlang, -5 rem 3 is -2, whereas this function will return 1,  
% since -5 =-2 * 3 + 1.

modulo(X,Y) when X > 0 ->   
   X rem Y;

modulo(X,Y) when X < 0 ->   
    K = (-X div Y)+1,
    PositiveX = X + K*Y,
    PositiveX rem Y;

modulo(0,_Y) -> 
    0.