A little puzzle

[Regan Heath]

On Mon, 19 Sep 2011 23:09:45 +0100, Simen Kjaeraas  
<simen.kjaras at gmail.com> wrote:

> On Mon, 19 Sep 2011 23:20:47 +0200, bearophile  
> <bearophileHUGS at lycos.com> wrote:
>
>> A tiny puzzle I've shown on IRC. This is supposed to create an inverted  
>> array of cards, but what does it print instead?
>>
>> import std.stdio, std.algorithm, std.range;
>> void main() {
>>     int[52] cards;
>>     copy(iota(cards.length - 1, -1, -1), cards[]);
>>     writeln(cards);
>> }
>
> Gawds, that's an ugly bug. For those who can't spot it,
> typeof(cards.length) == uint, hence -1 is converted to a uint
> (4_294_967_295, to be exact). iota then says 'that's fine, I'll just
> return an empty range for you.'
> Solution: knock some sense into integral promotion rules.
> Workaround: cast(int)cards.length.
>
> What was the rationale for having unsigned array lengths, again?

Well.. logically it makes sense as arrays cannot have negative lengths,  
but practically, this bug is what happens as a result.  This is the reason  
I typically use signed int for lengths, unless I expect the length to  
exceed max signed int.  It's also a good idea if you ever do any  
subtraction/calculation with them, as you don't want to underflow to max  
signed int ever.  It means that in my daily work I have to cast the return  
value of strlen() everywhere (cos my compiler complains, but only in 64  
bit mode..).. I mean, really, when are you going to have a string which is  
longer than max signed int?!  It's just nonsensical.

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