A simple backticks question

return0 has asked for the wisdom of the Perl Monks concerning the following question:

Hello Monks! I was wondering if I can seek your wisdom as to how exactly the backticks work when in a foreach() iterator. is an anonymous array created? for instance, besides making a throw away array @files that I never modify, like:

my @files = `ls -l`
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Does Perl make something similar when I just do:

foreach(`ls -l`){ ... }
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and make an array that's simply not visible? Or is that too abstract? I was just wondering about it all as I usually do. I hope all is well and thank you in advance! :) ~return0

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Re: A simple backticks question by BrowserUk (Patriarch) on Mar 17, 2014 at 16:27 UTC
The for loop iterates a (potentially huge) list returned by the backticks. If you want to avoid creating that list, you can use open thusly: `open CMD, '-\|', 'ls -l' or die $!; while( my $fname = <CMD> ) { chomp $fname; #process the file names one at a time } close CMD;` [download] 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". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l]
Re: A simple backticks question by kennethk (Abbot) on Mar 17, 2014 at 16:33 UTC
The high order concept here is Context. The two major contexts are list and scalar. For example, calling an array in list context returns a list of the values in the array; calling an array in scalar context returns the length of the array. The parentheses following `foreach` are an indicator that you are calling your arguments in list context. From `STRING`: In list context, returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.	[reply] [d/l]
Re: A simple backticks question by no_slogan (Deacon) on Mar 17, 2014 at 16:18 UTC
Technically, the backtick operator returns a list, which is subtly different from an array. (You can't use it as the first argument to `push`, for example.) Arrays are automatically converted to lists when evaluated in list context.	[reply]
Re^2: A simple backticks question by LanX (Saint) on Mar 17, 2014 at 16:32 UTC
Not fully correct, the backticks are context sensitive, in scalar context `ls -l` will return a concat of all lines. (a ~~list~~ arrayą in scalar context returns the number of lines) backtick/qx in perlop In scalar context, it comes back as a single (potentially multi- line) string, or undef if the command failed. In list context, returns a list of lines (however you’ve defined lines with $/ or $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. Cheers Rolf ( addicted to the Perl Programming Language) updates ą) corrected, thanks to no_slogan. It very much depends on what the command within backticks returns.	[reply]
Re^3: A simple backticks question by no_slogan (Deacon) on Mar 17, 2014 at 16:36 UTC
Thanks for the correction, but again, not exactly correct. An array in scalar context evaluates to its number of elements. There is no such thing as a list in scalar context.	[reply]
Re^4: A simple backticks question by LanX (Saint) on Mar 17, 2014 at 16:49 UTC
Re: A simple backticks question by return0 (Acolyte) on Mar 17, 2014 at 19:17 UTC
Thank you all :) You guys are great!	[reply]

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