Re: [eigen] Indexes: why signed instead of unsigned?

[Mark Borgerding <mark@xxxxxxxxxxxxxx>]

Using unsigned types for indexing is much less forgiving of different 
styles of for loops ( as your example shows ). 
I've been programming since I was twelve (nearly 30 years now) and I 
still often write "for" loops that don't work well with unsigned types. 
The loops look correct and in most cases behave correctly.

Here are a few examples that fail with unsigned N:

   for (k=0;k<N-3;k+=4)
      // ... deal with 4 elements at a time. 
      // This fails spectacularly with unsigned N<3.  It works with 
signed N

   while (--N > 0) {
      // fails with unsigned N=0. It works with signed N
   }

Even trickier, there can be latent bugs that only appear when 
sizeof(unsigned int) != sizeof(void*) 
(this one really happened to me)
   unsigned int  smallOffset = N-k
   buf[ largeOffset + smallOffset ]  = 42;
If unsigned N<k then smallOffset wraps around to around 4e9 (with a 32 
bit int).  
On a 32 bit OS, the index/dereference of buf wraps back around, doing 
what the programmer intended (latent bug).
On a 64 bit OS, this actually has the address space to offset buf by 4e9 
elements and fails spectacularly.
If smallOffset and N were signed, then there would be no problem.


-- Mark Borgerding


Benoit Jacob wrote:
> There's a new thread on the forum about that (sigh).
> http://forum.kde.org/viewtopic.php?f=74&t=87883
>
> Let's settle this once and for all. There are 2 debates:
> 1) signed or unsigned?
> 2) int (that is 32 bits) or same-sizeof-as-pointers (e.g. 64bit on
> 64bit platforms)?
>
> For 2) signed, we could do ptrdiff_t, i guess. Unless you're sure that
> 'long' is actually on all platforms the size of a pointer. I still
> dont know whether 2) matters. Certainly not for cubic-complexity
> algorithms (would take forever). But for plain "level 1" operations,
> perhaps it's plausible. Feels like we shouldn't arbitrarily restrict
> sizes to 32 bits. Opinions?
>
> For 1), I don't know. the forum poster mentions a reasonable way to
> write decreasing for loops. It's true that such loops are not too
> frequent anyway. I dont know. Asking for opinions. Dont want to impose
> upon you a decision made 3 years ago when I was a "noob".
>
> Gael? Hauke? Jitse? Thomas? everybody?
>
> Benoit
>
>