Why do I get an incorrect output from a modulus operation with negative number

Question

I tried this code on Dart: I get 28.5

void main() {
  double modulo = -1.5 % 30.0;
  print(modulo);
}

The same code in Javascript returns -1.5

let modulo =  -1.5 % 30;
console.log(modulo);

What is the equivalent of the javascript code above in Dart ?

Answer 1

The documentation for num.operator % states (emphasis mine):

Returns the remainder of the Euclidean division. The Euclidean division of two integers a and b yields two integers q and r such that a == b * q + r and 0 <= r < b.abs().

...

The sign of the returned value r is always positive.

See remainder for the remainder of the truncating division.

Meanwhile, num.remainder says (again, emphasis mine):

The result r of this operation satisfies: this == (this ~/ other) * other + r. As a consequence the remainder r has the same sign as the divider this.

So if you use:

void main() {
  double modulo = (-1.5).remainder(30.0);
  print(modulo);
}

you'll get -1.5.

Note that both values are mathematically correct; there are two different answers that correspond to the two different ways that you can compute a negative quotient when performing integer division. You have a choice between rounding a negative quotient toward zero (also known as truncation) or toward negative infinity (flooring). remainder corresponds to a truncating division, and % corresponds to a flooring division.

Answer 2

An issue was raised about this in the dart-lang repo a while ago. Apparently the % symbol in dart is an "Euclidean modulo operator rather than remainder."

An equivalent of what you are trying to do can be accomplished with

(-1.5).remainder(30.0)