Elixir has `rem(dividend, divisor)` to calculate the remainder of an
integer division. However, as you probably know, there are different ways
to handle remainders with negative numbers:
`rem` will take the remainder, but use the sign of the dividend, which
means that the following function definition:
def is_odd(n)
rem(n, 2) == 1
end
will fail for -1, -3, etc, as `rem(-3, 2)`, for instance, has as answer *-1*
.
It is also possible to make a *modulo* function, which uses the sign of the
divisor. This has the advantage over the remainder function that it maps
the whole domain of integers unto a smaller, cyclic, range of numbers. This
is highly useful in many algorithms dealing with numbers or indices.
Many programming languages both have a remainder and a modulo function. I
have seen multiple people roll their own definitions of `mod` in their
Elixir libraries, because they needed it.
I propose the addition of the modulo function, `mod(a, n)` to Elixir.
A possible implementation is as follows:
defmacro mod(a, n) do
quote do
unquote(a) - (unquote(n) * div(unquote(a), unquote(n))
end
end
This implementation is guard-safe, so it can be used anywhere where `div`
and `rem` are also allowed.
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