On Tue, 16 Nov 2010 10:55:24 -0500, David Osborne <[email protected]>
wrote:
Hi,
I've noticed that the modulus operator acts differently when the divisor
is
the length of an array and the dividend is negative.
For instance, this code:
import std.stdio;
const int x = 4;
void main(){
int[x] arr = [0, 1, 2, 3];
writeln("Using arr.length");
for(int i = -3; i <= 0; i++){
writefln("%d mod %d = %d", i, arr.length, i % arr.length);
}
writeln("Using x");
for(int i = -3; i <= 0; i++){
writefln("%d mod %d = %d", i, x, i % x);
}
}
Produces this output:
Using arr.length
-3 mod 4 = 1
-2 mod 4 = 2
-1 mod 4 = 3
0 mod 4 = 0
Using x
-3 mod 4 = -3
-2 mod 4 = -2
-1 mod 4 = -1
0 mod 4 = 0
Which is a useful (but undocumented?) feature that lets you loop through
arrays backwards.
However, when the length of the array is odd...
const int x = 3;
...
int[x] arr = [0, 1, 2];
It looks like this:
Using arr.length
-3 mod 3 = 1 <-- this should be 0
-2 mod 3 = 2 <-- this should be 1
-1 mod 3 = 0 <-- this should be 2
0 mod 3 = 0
Using x
-3 mod 3 = 0
-2 mod 3 = -2
-1 mod 3 = -1
0 mod 3 = 0
Does anyone know of a reason for this? It doesn't seem like a bug, but I
don't know why it would do something like that.
Upon further investigation, I found that when arr.length is even, i acts
like (x*(abs(i)/x + 1) + i) , and when arr.length is odd, i acts like
(x*(abs(i)/x - 1) + i)
(I'm using dmd 2.049 on linux)
Simple, array.length is an unsigned value, so the signed operands are
promoted to unsigned through normal integer promotion:
cast(uint)-3 -> 4_294_967_293u
4_294_967_293u % 3u -> 1
I don't think this "undocumented" feature is valid. In fact, it may not
be valid for evens as well that aren't powers of 2.
-Steve
