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Re: using function output ananymous array vs. named array in foreach

by BrowserUk (Pope)
on Dec 26, 2012 at 11:48 UTC ( #1010365=note: print w/ replies, xml ) Need Help??


in reply to using function output ananymous array vs. named array in foreach

Neither snippet will do what you think it will do. chomp returns whatever is chomped off, not the line, so $a, will end up being a bunch of delimiters.

In terms of performance, there will be little to choose between them; they'll both be quite slow.

In terms of memory, the first will hang on to the memory used by @lines until you explicitly clean it.

You might find this does what you are after more efficiently:

my @lines = foo( 'bar' ); chomp @lines; my $a = join '', @lines; my $b; $b *= getVa( $_ ) for @lines; undef @lines;

As for scaling: if fun() can returns very large numbers of lines, building an array inside the function and then returning that array as a list will scale badly.

You could either pass a reference to the array into foo() and having it populate it; or return a reference to the array constructed internally; this avoiding the construction of the list and the copying of the array.

But better yet would be to move the chomping, concatenation and parsing inside the loop where the lines are read or generated, and return the concatenated string and the accumulated total; thus avoiding the construction of the array completely:

sub foo{ my $bar = shift; my( $a, $b ); ... while( my$line = getLine() ) { chomp $line; $a .= line; $b *= getVa( $line ) } return $a, $b; } my( $a, $b ) = foo( 'bar' );

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.


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Re^2: using function output ananymous array vs. named array in foreach
by Marshall (Prior) on Dec 26, 2012 at 15:48 UTC
    chomp returns whatever is chomped off, not the line, so $a, will end up being a bunch of delimiters.

    Well, actually chomp() returns the number of "characters" that were chomped - not the delimiters. So the OP's code is not going to work or I will admit that I am having a hard time understanding what this thing it supposed to do.

    A short demo of chomp() on my Windows machine:

    #!usr/bin/perl -w use strict; #chomp demo #running on Windows XP 32 bit... # chomp... "It removes any line ending that corresponds # to the current value of $/ (also known as # $INPUT_RECORD_SEPARATOR in the English module)." # # chomp() returns the total number of "characters" removed. # The "number of characters" depends.... my $count; print "Count\t Text\n"; my $x ="blah...with CRLF (Windows)\n"; $count = chomp ($x); print "$count \t\t $x\n"; print "setting \$/ to \\r\n"; $/ = "\r"; my $y ="blah...with CR (like Old Mac)\r"; $count = chomp ($y); print "$count \t $y\n"; print "This won't work...An \"extra new line\" remains...\n"; my $z ="blah again with CRLF (Windows)\n"; $count = chomp ($z); print "$count \t\t $z\n"; print "set \$/ back to \\n \n"; $/ = "\n"; $z ="blah again with CRLF (Windows) but 2 CRLF's\n\n"; $count = chomp ($z); print "only one of the \\n's is chomped\n"; print "$count \t\t $z\n"; print "end of demo\n"; __END__ Count Text 1 blah...with CRLF (Windows) setting $/ to \r 1 blah...with CR (like Old Mac) This won't work...An "extra new line" remains... 0 blah again with CRLF (Windows) set $/ back to \n only one of the \n's is chomped 1 blah again with CRLF (Windows) but 2 CRLF's end of demo
      actually chomp() returns the number of "characters" that were chomped - not the delimiters.

      You're right of course. (But your over elaborate demonstration added nothing to the simple statement of fact.)

      I remember discovering early on that the return from chomp wasn't useful and I've never concerned myself with what non-useful thing it actually returned ever since.

      So the OP's code is not going to work...

      That was the point I was making when I said: "Neither snippet will do what you think it will do.".


      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.
        Yes, I was overly elaborate.
        Yes, I have never used the return value from chomp().
        That's it and I think we agree about that.
Re^2: using function output ananymous array vs. named array in foreach
by ISAI student (Scribe) on Dec 27, 2012 at 09:03 UTC
    You are probably right, but I can't touch foo at the moment... It stands for a number of objects' methods that I cannot change. I am given this deck of cards, and deal with it... I guess that for the sake of simplicity, brevity (and less duration of memory footprint), I'll use the anonymous array method.
      for the sake of simplicity, brevity (and less duration of memory footprint), I'll use the anonymous array method.

      Neither of your OP snippets uses an anonymous array.

      Your function returns a list, which you either: assign to an array which you then process using for; or you process the list directly with for.

      But yes, there is no purpose to assigning to the array, unless you needed to retain the returned values beyond the loop.


      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.

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