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Re: Error on arithmetic evaluation of `~0`.


From: Chet Ramey
Subject: Re: Error on arithmetic evaluation of `~0`.
Date: Wed, 26 Dec 2018 12:10:24 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 12/23/18 12:01 PM, Bize Ma wrote:
> Chet Ramey (<address@hidden <mailto:address@hidden>>) wrote:
> 
>     >
>     > While this works:
>     >
>     > var=(hello); echo "${var[ ~0]}"
>     > hello
> 
>     Because negative array subscripts count backwards from the end of the
>     array.
> 
> 
> Doh!, yes. And, because of that: "${var[-1]}"
> should give the *last* element of array "var", shouldn't it?

Yes, if it's an array variable.

> Consequently,  this happens:
> 
>     $ unset var; var[0]=77; echo "${var[0]}"; echo "${var[-1]}"
>     77
>     77
>  
> The only value in var is at index 0, which means it is also the *last* value.

Correct.

> 
> The point being that a variable which has an scalar value "var=hello"
> should act (for most practical cases) as an array for which only the
> value at address 0 has been defined.
> 
> Both command line above should have printed "hello".

No. 0 is the only valid subscript for a non-array variable. The difference
between bash and other shells that implement this feature is that bash
warns about negative subscripts.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://tiswww.cwru.edu/~chet/



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