What does _ mean?

bichonfrise74 has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: What does _ mean? by ikegami (Patriarch) on Feb 02, 2009 at 18:50 UTC
Read the docs for `-d` If any of the file tests (or either the `stat` or `lstat` operators) are given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or `stat` operator) is used, saving a system call. A few notes and examples follow.	[reply] [d/l] [select]
Re^2: What does _ mean? by bichonfrise74 (Vicar) on Feb 02, 2009 at 20:17 UTC
Thanks!	[reply]
Re: What does _ mean? by merlyn (Sage) on Feb 04, 2009 at 15:18 UTC
It means I've confused yet another generation of budding Perl programmers with a microoptimization that I called for back in the Perl 3 alpha days, and probably can be safely ignored now. There are very few pieces of Perl that are uniquely mine. This is one of them. Sorry. :) -- Randal L. Schwartz, Perl hacker	[reply]
Re^2: What does _ mean? by holli (Abbot) on Feb 04, 2009 at 16:24 UTC
So how much more expensive is `-e($file) && -f(_)` than `-e($file) && -f($file)`? holli When you're up to your ass in alligators, it's difficult to remember that your original purpose was to drain the swamp.	[reply] [d/l] [select]
Re^3: What does _ mean? by tirwhan (Abbot) on Feb 04, 2009 at 17:05 UTC
`strace -tt -f -e '$file="filename";-e($file) && -f($file)'` (as well as the alternative) says it's exactly one stat system call more expensive, unsurprisingly. Which on the system I tested it on was something like 0.0001 seconds but that will obviously vary. All dogma is stupid.	[reply] [d/l]
Re^3: What does _ mean? by massa (Hermit) on Feb 04, 2009 at 22:59 UTC
Four characters. Six, if you also rip out the parentheses. :-) Seriously, it's not just about saving one system call or half a dozen source code chars (altough I am all in favour of both) -- `-d $filename && -x _ && -k _ && -u _` is simpler, cleaner, and more expressive than the alternative IMHO. BTW, I happen to like even more the perl6ish `$filename ~~ :d & :x & :k & :u`. []s, HTH, Massa (κς,πμ,πλ)	[reply] [d/l] [select]
Re^4: What does _ mean? by ikegami (Patriarch) on Feb 04, 2009 at 23:21 UTC
Re: What does _ mean? by pileofrogs (Priest) on Feb 02, 2009 at 18:58 UTC
I think it means you might want to smack the writer of said code up the head and give them a copy of the PBP book. Update: Heh... As I read it, the whole book was about writing clear code. "Don't use $_" etc... (Yes, my statement in support of clarity was totally unclear... sigh...)	[reply]
Re^2: What does _ mean? by runrig (Abbot) on Feb 02, 2009 at 20:26 UTC
And what does PBP say? To make unnecessary calls to stat (I hope not)?	[reply]
Re^3: What does _ mean? by jvector (Friar) on Feb 03, 2009 at 14:27 UTC
I looked up '_' in PBP's index and it referred me to (underscore). Looked up 'underscore', subtopic 'best practice' : p.473: Use underscores to improve the readability of long numbers. No reference to the file operations in any of the other subtopics in the index either. This signature will be ready by Easter	[reply]
Re^4: What does _ mean? by ikegami (Patriarch) on Feb 03, 2009 at 19:50 UTC
Re^3: What does _ mean? by BenHopkins (Sexton) on Jun 21, 2010 at 21:31 UTC
It doesn't specifically address this, but I'd apply the general principle of commenting any obscure usage. Of course, one man's obscure is another man's clear. I ran into this exact thing in an old program I had to modify and added this comment after figuring out what it meant: `######### FINALLY FIGURED OUT WHAT _ IS!!! # QUOTE: # # If any of the file tests (or either the stat() or lstat() operators +) are given # the special filehandle consisting of a solitary underline, then the +stat structure # of the previous file test (or stat operator) is used, saving a syste +m call. # # Isn't that something? All I gotta do now figure out what or where i +s the previous # test??` [download]	[reply] [d/l]
Re^2: What does _ mean? by ikegami (Patriarch) on Feb 03, 2009 at 19:51 UTC
the whole book was about writing clear code. How is "`_`" unclear? It has one very specific use.	[reply] [d/l]
Re^3: What does _ mean? by pileofrogs (Priest) on Feb 03, 2009 at 23:30 UTC
How about obscure?	[reply]
Re^4: What does _ mean? by ikegami (Patriarch) on Feb 03, 2009 at 23:38 UTC
Re^5: What does _ mean? by pileofrogs (Priest) on Feb 04, 2009 at 00:52 UTC
Some notes below your chosen depth have not been shown here
Re^2: What does _ mean? by bichonfrise74 (Vicar) on Feb 03, 2009 at 01:57 UTC
What would be the "best practice" in this case? How would this be re-written?	[reply]
Re^3: What does _ mean? by runrig (Abbot) on Feb 03, 2009 at 17:13 UTC
pileofrogs is probably confusing the "_" file test operation argument with implicit usage of the `"$_"` variable, which is discouraged by some. "_" used appropriately as a file test argument is fine.	[reply] [d/l]