Confusing grep behavior

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

I am trying to understand two snippets of code, that have different behaviors that I do not immediately grasp. Consider:

use strict;
use Data::Dumper;

my @res = grep $_ split(",", "a,b,c,d,,f,,h");
print Dumper(\@res);

my @res2 = grep { $_ } split(",", "a,b,c,d,,f,,h");
print Dumper(\@res2);
[download]

When ran, it returns

$VAR1 = [
          'a',
          'c'
        ];
$VAR1 = [
          'a',
          'b',
          'c',
          'd',
          'f',
          'h'
        ];
[download]

I understand the second example, as I am evaluating the "truth" of the elements I am iterating over. I do not understand the output of the first dump, when given just "$_". I did look at grep's perldoc, hoping for an ounce of clairvoyance, but to no avail.

Comment on Confusing grep behavior Select or Download Code

Replies are listed 'Best First'.
Re: Confusing grep behavior by FunkyMonk (Chancellor) on Dec 04, 2007 at 21:16 UTC
The first is a syntax error which caused the perl on my machine (Activeperl 5.8.8 build 822) to spit out: `panic: ck_grep at C:/Documents and Settings/bri/workspace/PerlMonks/te +mp.pl line 10 (#1) (P) Failed an internal consistency check trying to compile a grep. Uncaught exception from user code: panic: ck_grep at C:/Documents and Settings/bri/workspace/PerlMonk +s/temp.pl lin e 10. at C:/Documents and Settings/bri/workspace/PerlMonks/temp.pl line 10` [download] What did you really mean it to say?	[reply] [d/l] [select]
Re^2: Confusing grep behavior by dextius (Monk) on Dec 04, 2007 at 23:32 UTC
This code does not have any issues in v5.8.6 on Mac OS X.	[reply]
Re: Confusing grep behavior by grep (Monsignor) on Dec 04, 2007 at 21:19 UTC
You need to reread grep `my @res = grep $_ split(",", "a,b,c,d,,f,,h"); #is a syntax error.` [download] According to the POD when grep is not called with a block it requires a comma before the list you are working on. `grep EXPR,LIST` So this will work as you expect. `my @res = grep $_, split(",", "a,b,c,d,,f,,h");` [download] grep `One dead unjugged rabbit fish later...`	[reply] [d/l] [select]
Re^2: Confusing grep behavior by dextius (Monk) on Dec 04, 2007 at 23:33 UTC
You are quite right.. I should rephrase my question... On Mac OS X, the comma isn't required, and has far different behavior. Anyone happen to know what that is?	[reply]
Re^3: Confusing grep behavior by perrin (Chancellor) on Dec 04, 2007 at 23:52 UTC
It's not a Mac thing. 5.8.5 on Linux allows it as well. I'm guessing it's a bug that was fixed in 5.8.7 or 5.8.8.	[reply]
Re^4: Confusing grep behavior by dextius (Monk) on Dec 05, 2007 at 00:44 UTC
Re^3: Confusing grep behavior by grep (Monsignor) on Dec 05, 2007 at 00:44 UTC
No matter what OS you run it on the comma is still required, as noted by the POD and (quite noticeably) by the incorrect performance. The only difference(bug) is that it didn't throw an error. A crime is still a crime, whether you get caught or not :). grep `One dead unjugged rabbit fish later...`	[reply]
Re: Confusing grep behavior by lodin (Hermit) on Dec 04, 2007 at 21:17 UTC
You're missing a comma after `$_`. It should be `grep EXPR, LIST` [download] lodin	[reply] [d/l] [select]
Re: Confusing grep behavior by cdarke (Prior) on Dec 05, 2007 at 10:25 UTC
http://aspn.activestate.com/ASPN/Mail/Message/perl5-porters/2838617	[reply]

Back to Seekers of Perl Wisdom