Re^3: Accidentally creating hash elements with grep { defined } on a hash slice

I was just asking myself exactly that question. Turns out grep requires lvalues, but sort doesn't.

>perl -MO=Concise -e"@b = grep f, @h{@a}"
...
a                 <@> hslice lKM ->b
...

>perl -MO=Concise -e"@b = sort @h{@a}"
...
a              <@> hslice lK ->b
...
[download]

Notice the "M" for "Modifiable" in the former.

So that begs the question: Why does grep require lvalues? The difference is:

>perl -e"@a = grep { $_='!'; 0 } @h{qw(i j)};"

>perl -e"@a = sort { $a='!'; 0 } @h{qw(i j)};"
Modification of a read-only value attempted at -e line 1.
[download]

But as far as I can tell, there is no good reason.

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Re^4: Accidentally creating hash elements with grep { defined } on a hash slice by wol (Hermit) on Nov 04, 2008 at 13:51 UTC
Why does grep require lvalues? So you can suprise people with what this does: `perl -e "@in = (1..5); @out = grep { $_++ } @in; print qq{@in}"` [download] But as far as I can tell, there is no good reason. Oh - a good reason? Sorry, haven't got one of them. Move along please, nothing to see here. -- .sig : File not found.	[reply] [d/l]
Re^5: Accidentally creating hash elements with grep { defined } on a hash slice by ikegami (Patriarch) on Nov 04, 2008 at 15:21 UTC
True, but it's a bad example since `@in` naturally returns lvalues. `>perl -le" @in = (1..5); @out = sort { $a=$b='!'; 0 } @in; print qq{@i +n}" ! ! ! ! !` [download] Note the absence of the "M" flag. `>perl -MO=Concise -le" @in = (1..5); @out = sort { $a=$b='!'; 0 } @in; + print qq{@in}" ... e <1> rv2av[t11] lK/1 ->f ...` [download] It will affect `pp_pos - Adds lvalue magic pp_substr - Adds lvalue magic pp_vec - Adds lvalue magic pp_aelem - Extends array pp_aelemfast - Extends array pp_aslice - Extends array pp_helem - Extends hash pp_hslice - Extends hash pp_padsv - No effect as far as I can tell pp_rv2sv - ??? pp_rv2av - ??? pp_rv2gv - ???` [download]	[reply] [d/l] [select]
Re^4: Accidentally creating hash elements with grep { defined } on a hash slice by ferment (Novice) on Nov 04, 2008 at 14:28 UTC
Thanks both for your replies. I understand the problem now :)	[reply]