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digitalmars.D.learn - Prefer Signed or Unsigned in D?

reply "John Carter" <john.carter taitradio.com> writes:
C/C++ discussion here....

    http://blog.robertelder.org/signed-or-unsigned-part-2/

D rules here...

    http://dlang.org/type.html#integer-promotions
Sep 01 2015
next sibling parent reply "BBasile" <bb.temp gmx.com> writes:
On Tuesday, 1 September 2015 at 23:06:50 UTC, John Carter wrote:

 C/C++ discussion here....

    http://blog.robertelder.org/signed-or-unsigned-part-2/

 D rules here...

    http://dlang.org/type.html#integer-promotions


It depends on the context.

You should take care of blending signed and unsigned:
comparison error, a is > b but...
---
uint a = 1;
int b = -1;
assert(a < b); // does not throw
---

You should take care to the index type in a loop:
loop that doesn't run at all because of an infered unsigned 
index...
---
auto array = new int[](8);
for (auto i = array.length - 1; i > -1; i--)
     array[i] = 8;
assert(array[0] == 0); // does not throw
---
Sep 02 2015
parent "Dominikus Dittes Scherkl" writes:
On Wednesday, 2 September 2015 at 09:47:16 UTC, BBasile wrote:

 On Tuesday, 1 September 2015 at 23:06:50 UTC, John Carter wrote:

 C/C++ discussion here....

    http://blog.robertelder.org/signed-or-unsigned-part-2/

 D rules here...

    http://dlang.org/type.html#integer-promotions


 It depends on the context.

 You should take care of blending signed and unsigned:
 comparison error, a is > b but...
 ---
 uint a = 1;
 int b = -1;
 assert(a < b); // does not throw
 ---

 You should take care to the index type in a loop:
 loop that doesn't run at all because of an infered unsigned 
 index...
 ---
 auto array = new int[](8);
 for (auto i = array.length - 1; i > -1; i--)
     array[i] = 8;
 assert(array[0] == 0); // does not throw
 ---


I wish the following would be the standard D behaviour (for T == 
U or if floats are involved neither C nor D have a problem)):

int opCmp(T, U)(const(T) a, const(U) b) pure  safe  nogc nothrow
    if(isIntegral!T && isIntegral!U && !is(Unqual!T == Unqual!U))
{
    alias C = CommonType!(T, U);
    static if(isSigned!T && isUnsigned!U && T.sizeof <= U.sizeof)
       return (a < 0) ? -1 : opCmp(cast(U)a, b);
    else static if(isUnsigned!T && isSigned!U && T.sizeof >= 
U.sizeof)
       return (b < 0) ? 1 : opCmp(a, cast(T)b);
    else return opCmp(cast(C)a, cast(C)b);
}

this is really almost equally fast, but always correct.
Sep 02 2015
prev sibling parent reply "ponce" <contact gam3sfrommars.fr> writes:
On Tuesday, 1 September 2015 at 23:06:50 UTC, John Carter wrote:

 C/C++ discussion here....

    http://blog.robertelder.org/signed-or-unsigned-part-2/

 D rules here...

    http://dlang.org/type.html#integer-promotions


Everything Bjarne said still applies equally to D code, since 
integer promotion is identical with C from what I understand.
Sep 02 2015
parent reply "John Carter" <john.carter taitradio.com> writes:
On Wednesday, 2 September 2015 at 11:03:00 UTC, ponce wrote:

 On Tuesday, 1 September 2015 at 23:06:50 UTC, John Carter wrote:

 C/C++ discussion here....

    http://blog.robertelder.org/signed-or-unsigned-part-2/

 D rules here...

    http://dlang.org/type.html#integer-promotions


 Everything Bjarne said still applies equally to D code, since 
 integer promotion is identical with C from what I understand.


Hmm. What Robert Elder says also applies still. And in my world 
his argument about undefined behavior carries weight.

Bugs that emerge from different optimizations / different 
versions of the compiler / on different CPU's are nightmares in 
my domain.

Here is a bug that existed in the JDK for 9 years (and probably 
in many other places....)
http://googleresearch.blogspot.co.nz/2006/06/extra-extra-read-all-about-it-nearly.html

So given the advice on mixing unsigned and signed, and the 
complexity of Dominikus's code to get signed and unsigned code to 
compare sanely....

Maybe if his code becomes standard in D...

Yup, mixing signed and unsigned is A Bad Thing, and you best go 
with whatever your base libraries give you.

The whole problem....
    http://www.di.unipi.it/~ruggieri/Papers/semisum.pdf
...makes me feel vaguely ill and long for a Numerical Tower.

I really must get around to benchmarking BigInt....
Sep 02 2015
parent reply "ponce" <contact gam3sfrommars.fr> writes:
On Wednesday, 2 September 2015 at 21:22:59 UTC, John Carter wrote:

 On Wednesday, 2 September 2015 at 11:03:00 UTC, ponce wrote:

 Everything Bjarne said still applies equally to D code, since 
 integer promotion is identical with C from what I understand.


 Hmm. What Robert Elder says also applies still. And in my world 
 his argument about undefined behavior carries weight.


I'd like to point out that while Robert Elder has remarkably 
documented articles, his conclusions are still contrary to most 
C++ experts (Bjarne, Meyers, Andrei...).

Additionally, I was said weeks ago on this NG that and signed 
overflow in D is not actually Undefined Behaviour.
Sep 02 2015
parent "John Carter" <john.carter taitradio.com> writes:
On Wednesday, 2 September 2015 at 21:43:17 UTC, ponce wrote:


 Additionally, I was said weeks ago on this NG that and signed 
 overflow in D is not actually Undefined Behaviour.


Interesting.. The reference is fairly terse on exactly what 
happens, is it more formally specified anywhere? In which case 
Elder's objection melts away for D and becomes explicitly "prefer 
signed for D, but don't mix".


 For integral operands, the *, /, and % correspond to multiply, 
 divide, and modulus operations. For multiply, overflows are 
 ignored and simply chopped to fit into the integral type.
Sep 02 2015