why $ is need to access array [negative index]?

bachmeier no at spam.net
Fri Sep 18 20:57:04 UTC 2020

On Friday, 18 September 2020 at 20:48:47 UTC, bachmeier wrote:
> On Friday, 18 September 2020 at 19:53:41 UTC, mw wrote:
>> In Python it's such a convenience to be able to access array 
>> element from the end:
>> arr[-1]
>> in D, we can do that too, but need an extra $: arr[$-1]
>> I'm porting some code from Python to D:
>>   int[3] signs;          // sign: -1, 0, 1
>>   int sign = -1;         // for example
>>   writeln(signs[sign]);  // Range violation
>> // Error: array index 18446744073709551615 is out of bounds 
>> signs[0 .. 3]
>> (yes, I know I can use AA, int[int], but it just make things 
>> complicated)
>> Can we have a DIP remove / make optional `$` in this usage?
>> Thoughts?
> I'm inclined to say typing a single character is not a hardship 
> in exchange for extreme clarity of the code. Keep in mind that 
> Python is not the only language with a negative array index.
> For instance, C supports it:
> https://stackoverflow.com/questions/3473675/are-negative-array-indexes-allowed-in-c
> That's probably the place to look if you want to start, given 
> the relationship of C and D.
> Then there's R, for which x[-1] means to drop the first 
> element. That makes a lot of sense if x is treated as a vector 
> of data. Python's usage, on the other hand, is not at all 
> intuitive. Why -4 would mean the fourth to last element is 
> unclear. I believe it was copied from Perl. Ruby does the same 
> thing.
> Then there's PHP, which allows you to use a negative array 
> index as an arbitrary reference to an element: x[-2] could be 
> any of the elements.
> The strange one is Javascript, which has negative indexes that 
> are actually properties or something like that.
> Bottom line is that the Python approach is one of many, it only 
> makes sense if someone tells you what it means, and it saves 
> you a single character in return for less clear code.
> As noted, it's really easy to create a struct that operates 
> like Python if you want. That's the beauty of D.

I forgot to add when discussing C, that D already has negative 
index values, which mean something completely different than 
Python's usage:

double[] x = [1, 2, 3, 4];
double * y = &(x.ptr)[2];
writeln(y[-2]); // 1

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