Integer modulo operation is not specified

[DartBot]

This issue was originally filed by [email protected]

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Behavior of int.operator% is not specified.
Apparently it is not the same as ECMA-262 modulo

[floitschG]

Yes. the documentation has been lost over time...
Dart's modulo should follow the Euclidean definition (see http://en.wikipedia.org/wiki/Modulo_operation)

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Added Area-Library, Triaged labels.

[DartBot]

This comment was originally written by [email protected]

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There appears to be a bug with large numbers in the Dart standalone VM when the first argument is negative:

void main() {
  print((-922337203685477580 % 284112972158697727).toString());
  print((-9223372036854775808 % 2841129721586977273).toString());
}

prints:

214114684949313328
-699982872093843989

The first result is expected since it satisfies 0 <= (a % b) <= abs(b). The second is a bug.
 

[DartBot]

This comment was originally written by [email protected]

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The Euclidean definition of mod is somewhat less useful without a corresponding Euclidean division operator. Together, the operators '/' and '%' should maintain the defining Euclidean properties:

q = a / b
r = a % b
q * b + r == a
0 <= r < |b|

To take a concrete example, let a = -8 and b = 3. Then the Euclidean modulus must be 1, since

q * b + r == a
q * 3 + r == -8
q * 3 == -8 - r

and 1 is the only value for r in the range [0, 2] that makes (-8 - r) a multiple of 3.

But then:

q * 3 + 1 == -8
q * 3 == -9
q == -3

In other words, a Euclidean division operator must yield -8 / 3 == -3. However, Dart's integer division yields -8 ~/ 3 == -2 since it truncates (rounds to 0).

So, while in some ways I appreciate the convenience of a '%' operator that always returns a non-negative result (e.g., due to its utility for accessing table entries using an expression like 'index % length'), it violates the natural expectation that (a / b) * b + (a % b) ==a.

[floitschG]

Updated the documentation with r20942.

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Added Fixed label.