Re: Strange behaviour using negative index in arrays

[Koichi Murase]

Happy new year!

2021年1月2日(土) 20:44 felix <felix@f-hauri.ch>:
> ( Under 4.2.37(1)-release, I get: ``-bash: a[$i]: bad array subscript'' )
>
> But with ``my'' 5.0.3(1)-release, as with new 5.1.4(1)-release, no
> error...

Negative array subscripts are indeed a new feature of Bash 4.3.  From
the changelog (http://tiswww.case.edu/php/chet/bash/CHANGES):

> This document details the changes between this version,
> bash-4.3-alpha, and the previous version, bash-4.2-release.
>
>
> [...]
>
> 3.  New Features in Bash
>
> [...]
>
> x.  The shell now allows assigning, referencing, and unsetting
>     elements of indexed arrays using negative subscripts (a[-1]=2,
>     echo ${a[-1]}) which count back from the last element of the
>     array.

----

> Where 5th value is 45 instead of -12 and 6th is 36 instead of -21
>  45 + 12 = 57,  36 + 21 = 57 ... (highest submited value was 56!)??

The index -1 corresponds to the highest index, so a negative index
"-O" corresponds to the index "H-O+1" where H is the highest index, so
the behavior that -12 and -21 correspond to 56-12+1=45 and 56-21+1=36,
respectively, is the correct one. Or in another way of thinking, when
the highest index is 56, the number of elements in a hypothetical
dense array a[0]..a[56] is N=57, so -12 and -21 correspond to N-12=45
and N-21=36.

--
Koichi