[closed] map sentence as array slice indexes

rsFalse has asked for the wisdom of the Perl Monks concerning the following question:

Hello,

I tried to understand why the following (with map sentence) gives an error. Can't understand.

use warnings;
use strict;

@_ = 'a' .. 'c';

print @_[ 
map $_ -1, 
grep { $_ > 0 and $_ <= 1 } 
map { $_ + 1 } -1 .. 1
];
[download]

Missing comma after first argument to map function at map_inside_splic
+e.pl line 12, near "]"
[download]

And the following (with map block) don't give an error:

use warnings;
use strict;

@_ = 'a' .. 'c';

print @_[ 
map {$_ -1} 
grep { $_ > 0 and $_ <= 1 } 
map { $_ + 1 } -1 .. 1
];
[download]

OUTPUT:

a
[download]

upd: thanks for answers below.

Comment on [closed] map sentence as array slice indexes Select or Download Code

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Re: map sentence as array slice indexes by BrowserUk (Patriarch) on Nov 25, 2015 at 12:08 UTC
I tried to understand why the following (with map sentence) gives an error. The not-very-helpful answer is: because you've created an ambiguous state in the parser. It can be avoided by: Omitting the space: `print @a[ map $_-1, grep { $_ > 0 and $_ <= 1 } map { $_ + 1 } -1 .. 1 ];` Or using a unary plus prefix: `print @a[ map +$_ -1, grep { $_ > 0 and $_ <= 1 } map { $_ + 1 } -1 .. 1 ];` Or (as you've identified) using the block form: `print @a[ map{ $_ -1 } grep { $_ > 0 and $_ <= 1 } map { $_ + 1 } -1 .. 1 ];` My guess is that because the parser fails to find a comma following the first argument ($_) to map, it attempts to treat that first argument as a block, and gets confused. Under 5.10 I get: `panic: ck_grep at ...`; but that panic seems to have been replaced with the error message you get in later versions. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :) In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]
Re: [closed] map sentence as array slice indexes by Discipulus (Canon) on Nov 25, 2015 at 12:09 UTC
Hello, in the docs for map i read: { starts both hash references and blocks, so map { ... could be either the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look ahead for the closing } it has to take a guess at which it's dealing with based on what it finds just after the {. Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the } and encounters the missing (or unexpected) comma. The syntax error will be reported close to the }, but you'll need to change something near the { such as using a unary + or semicolon to give Perl some help: So you need to help Perl doing the right guess, using a plus sign (and parentheses!): `use warnings; use strict; @_ = 'a' .. 'c'; print @_[ map +($_ -1), grep { $_ > 0 and $_ <= 1 } map { $_ + 1 } -1..1 ];` [download] HtH L* There are no rules, there are no thumbs.. Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.	[reply] [d/l]