Anonymous array indexing

Anonymous array indexing

[Steve Talley]

Hello,

Is there a compact version of the following:

    fred=(one two three)
    export FRED=$fred[2]
    unset fred

I am looking for something like

    export FRED=(one two three)[2]

so that I can avoid having to use the temporary variable fred.
The above doesn't work.  Is there something equivalently short I
can use?  I'm running zsh 3.1.9.

Thanks,

Steve

Re: Anonymous array indexing

[Dan Nelson]

In the last episode (Dec 12), Steve Talley said:
> Is there a compact version of the following:
> 
>     fred=(one two three)
>     export FRED=$fred[2]
>     unset fred
> 
> I am looking for something like
> 
>     export FRED=(one two three)[2]

Assuming "one two three" is actually stored in a shell variable (we'll
call it a):

export FRED=${${=a}[2]}

will work. = turns on SH_WORD_SPLIT, essentially turning the variable
into an array temporarily.


-- 
	Dan Nelson
	dnelson@emsphone.com

Re: Anonymous array indexing

[Bart Schaefer]

On Dec 12,  6:41pm, Steve Talley wrote:
} 
} Is there a compact version of the following:
} 
}     fred=(one two three)
}     export FRED=$fred[2]
}     unset fred
} 
} so that I can avoid having to use the temporary variable fred.

The general trick is that the nameless parameter ${} always expands to the
empty string, so ${:-value} always expands to "value".  Then build up from
there.

} I am looking for something like
} 
}     export FRED=(one two three)[2]

The value in ${:-value} always starts out as a scalar (a string), so you
have to explicitly convert to an array before you can index it.

If the individual words of the array do not contain spaces, you can combine
word splitting ${=scalar} with ${:-value} to get:

	export FRED=${${=:-one two three}[2]}

If the words will have embedded spaces, you'll have to resort to some other
kind of splitting.  E.g., ${(f)...} splits at newlines, so you can do:

	export FRED=${${(f):-$'one\ntwo has spaces\nthree'}[2]}

This uses $'...' to turn \n into newlines, then splits on them.  However,
the $'...' syntax is not available in 3.0.x.  In all versions you can
instead split on some other character, e.g. on period:

	export FRED=${${(s(.)):-one.two has spaces.three}[2]}

I don't think there is a fully general solution that works when you can't
choose in advance a character on which to split.  However, I'm guessing
that the strings in the array are known in advance, and that it is the
subscript that might vary, so you should be able to work something out.

-- 
Bart Schaefer                                 Brass Lantern Enterprises
http://www.well.com/user/barts              http://www.brasslantern.com

Zsh: http://www.zsh.org | PHPerl Project: http://phperl.sourceforge.net   

Re: Anonymous array indexing

[Steve Talley]

Thanks Bart/Dan for the replies!

This _almost_ solves my problem, although I think I
oversimplified.  The actual usage of this is in:

    # Find a value for TERMINFO by looking for xterm package
    delegates=(5.9 5.8.1 5.8 5.7 5.6)
    terminfo=( $HOME/$^delegates/pkgs/xterm/lib/terminfo(/) )
    export TERMINFO=$terminfo[1]
    unset terminfo

Unfortunately this doesn't work well with your solution, ie.

    export TERMINFO=${${=:-$HOME/$^delegates/pkgs/xterm/lib/terminfo(/)}[2]}

doesn't even come close to working.  Any ideas for this slightly
more complicated example?

Thanks,

Steve


Bart Schaefer wrote:

> On Dec 12,  6:41pm, Steve Talley wrote:
> } 
> } Is there a compact version of the following:
> } 
> }     fred=(one two three)
> }     export FRED=$fred[2]
> }     unset fred
> } 
> } so that I can avoid having to use the temporary variable fred.
> 
> The general trick is that the nameless parameter ${} always expands to the
> empty string, so ${:-value} always expands to "value".  Then build up from
> there.
> 
> } I am looking for something like
> } 
> }     export FRED=(one two three)[2]
> 
> The value in ${:-value} always starts out as a scalar (a string), so you
> have to explicitly convert to an array before you can index it.
> 
> If the individual words of the array do not contain spaces, you can combine
> word splitting ${=scalar} with ${:-value} to get:
> 
> 	export FRED=${${=:-one two three}[2]}
> 
> If the words will have embedded spaces, you'll have to resort to some other
> kind of splitting.  E.g., ${(f)...} splits at newlines, so you can do:
> 
> 	export FRED=${${(f):-$'one\ntwo has spaces\nthree'}[2]}
> 
> This uses $'...' to turn \n into newlines, then splits on them.  However,
> the $'...' syntax is not available in 3.0.x.  In all versions you can
> instead split on some other character, e.g. on period:
> 
> 	export FRED=${${(s(.)):-one.two has spaces.three}[2]}
> 
> I don't think there is a fully general solution that works when you can't
> choose in advance a character on which to split.  However, I'm guessing
> that the strings in the array are known in advance, and that it is the
> subscript that might vary, so you should be able to work something out.
> 
> -- 
> Bart Schaefer                                 Brass Lantern Enterprises
> http://www.well.com/user/barts              http://www.brasslantern.com
> 
> Zsh: http://www.zsh.org | PHPerl Project: http://phperl.sourceforge.net   

Re: Anonymous array indexing

[Bart Schaefer]

On Dec 13, 10:05am, Steve Talley wrote:
} Subject: Re: Anonymous array indexing
}
}     # Find a value for TERMINFO by looking for xterm package
}     delegates=(5.9 5.8.1 5.8 5.7 5.6)
}     terminfo=( $HOME/$^delegates/pkgs/xterm/lib/terminfo(/) )
}     export TERMINFO=$terminfo[1]
}     unset terminfo
} 
} Unfortunately this doesn't work well with your solution, ie.
} 
}     export TERMINFO=${${=:-$HOME/$^delegates/pkgs/xterm/lib/terminfo(/)}[2]}

You've got two problems.  One is that you're trying a file glob, which
isn't going to work in the same shell-word with TERMINFO= (unless you
resort to using `setopt globassign').  Two is that =:- is in the wrong
place.

For the second problem, you'd want

    $HOME/${^${=:-5.9 5.8.1 5.8 5.7 5.6}}/pkgs/xterm/lib/terminfo(/)

You can get around the glob problem by using $(print ...), but I don't
know why avoiding a temp parameter would be important enough to fork a
new process.

    export TERMINFO=${$(print \
    $HOME/${^${=:-5.9 5.8.1 5.8 5.7 5.6}}/pkgs/xterm/lib/terminfo(/))[1]}

-- 
Bart Schaefer                                 Brass Lantern Enterprises
http://www.well.com/user/barts              http://www.brasslantern.com

Zsh: http://www.zsh.org | PHPerl Project: http://phperl.sourceforge.net