zsh adds empty string to local parameter when += is used

zsh adds empty string to local parameter when += is used

[jamil bio]

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Hello good people,

It seems there is a bug which I will describe below.
I checked with usenet comp.unix.shell people and they said that is 
strange behavior.

I am using Z-shell 5.8. The problem is when I add elements to an array 
with += inside a function, an empty element is generated. The empty 
element is added only if I set array $files as local.

The following is an example code.


#!/bin/zsh
#!/bin/bash

func()
{
     local REPLY  files

     #unset files
     for REPLY in file1 file2 file3
     do
         files+=( "$REPLY" )
         echo "${#files[@]} -- ${files[@]}"
     done
}

func


If I set $files to local but unset it before the loop, the empty element 
is not added. Running the script with zsh returns:


2 --  file1
3 --  file1 file2
4 --  file1 file2 file3


While if we run the code with bash:


1 -- file1
2 -- file1 file2
3 -- file1 file2 file3


I was actually using a while read loop to read filenames, but the 
problem also occurs when I use the for loop.

So the workaround suggested is to set $files as local and then later 
define files=( ) which will prevent empty element being added somehow.

Thanks for all your efforts in making zsh so great!

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Re: zsh adds empty string to local parameter when += is used

[Roman Perepelitsa]

On Wed, Jan 20, 2021 at 5:17 PM jamil bio <jamilbio20@gmail.com> wrote:
>
> Hello good people,
>
> It seems there is a bug which I will describe below.
> I checked with usenet comp.unix.shell people and they said that is strange behavior.

I agree that it's strange. It's surprising to me (although I'm far
from being an expert in zsh).

Simpler test case:

    % zsh -fc 'typeset x; x+=(); typeset -p x'
    typeset -a x=( '' )


Roman.

Re: zsh adds empty string to local parameter when += is used

[Peter Stephenson]

> On 20 January 2021 at 16:16 jamil bio <jamilbio20@gmail.com> wrote:
> func()
> {
>      local REPLY  files
> 
>      #unset files
>      for REPLY in file1 file2 file3
>      do
>          files+=( "$REPLY" )
>          echo "${#files[@]} -- ${files[@]}"
>      done
> }
> 
> func
> 
> 2 --  file1
> 3 --  file1 file2
> 4 --  file1 file2 file3

Yes, this is a bit weird, but it's explicable.

"local files" makes the variable "files" into a scalar, so if it's ever
referred to it will return an empty string.

The first time the function executes

  files+-( "$REPLY" )

it retrieves the existing $files.  At this point that's just an empty
string.  Then it adds the new element and assigns back to files.  It's
only at this point that files becomes an array.

You can fix the problem by declaring files as an array,

local -a files

pws

Re: zsh adds empty string to local parameter when += is used

[Roman Perepelitsa]

On Wed, Jan 20, 2021 at 5:26 PM Peter Stephenson
<p.w.stephenson@ntlworld.com> wrote:
>
> Yes, this is a bit weird, but it's explicable.

Does the following look inconsistent though?

    % zsh -fc 'typeset x; x+=(); typeset -p x'
    typeset -a x=( '' )
    % zsh -fc 'typeset x; typeset -a x; typeset -p x'
    typeset -a x=(  )

Roman.

Re: zsh adds empty string to local parameter when += is used

[Daniel Shahaf]

jamil bio wrote on Wed, Jan 20, 2021 at 13:16:54 -0300:
>         echo "${#files[@]} -- ${files[@]}"
> 
> 2 --  file1
> 3 --  file1 file2
> 4 --  file1 file2 file3

For future reference, there's «typeset -p files» and «print -r -- "${(q)files}"».

Re: zsh adds empty string to local parameter when += is used

[jamil bio]

Thanks Peter S for your amazing explanation. That is enough for me.

I had given up on getting an explanation but that is excellent news.

I will be using `local -a` and your other suggestions in the future.

Cheers,

J

On 20/01/2021 13:31, Daniel Shahaf wrote:
> jamil bio wrote on Wed, Jan 20, 2021 at 13:16:54 -0300:
>>          echo "${#files[@]} -- ${files[@]}"
>>
>> 2 --  file1
>> 3 --  file1 file2
>> 4 --  file1 file2 file3
> For future reference, there's «typeset -p files» and «print -r -- "${(q)files}"».