Memory tip

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

Guys I don't know if this is like a new discovery in here or what, using the join function does actually reduce the size of a particular array, i used Devel::Size to verify this and the results I got were 160 bytes each for @list, @result when

CASE1:
my @result = map $_*2, @list;
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and

@result = join (',',@result);
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a meagre increase in "@result" size was reported-168 bytes, i.e. only 8 bytes when

CASE2:
my @result = map $_*55, @list;
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and @result = join (',',@result); but when I removed the line

@result = join (',',@result);
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I got "@result" size increasing to 290 in the first case and 296 in the second case. Here is my Code, you can change the line 6 to map $_x-> (x=any number) to appreciate this

#!/usr/local/bin/perl
use strict;
use warnings;
use Devel::Size qw(size total_size);

my @list = qw(1 2 3 4);
my @result = map $_*55, @list;

@list = join(',',@list);
#@result = join (',',@result);

print qq(the size of "@list" is: ).total_size(\@list). " Bytes\n";
print qq(the size of "@result" is: ).total_size(\@result)." Bytes\n";
[download]

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Re: Memory tip by jrsimmon (Hermit) on Jun 30, 2009 at 16:14 UTC
I don't think you understand what you've observed. In your example, you are using join to turn the array (1,2,3,4) into the scalar "1,2,3,4".	[reply]
Re^2: Memory tip by biohisham (Priest) on Jun 30, 2009 at 16:38 UTC
which means the scalar has lesser space than the array, I know join gets into a string different elements but I did not know that there is a difference in the size when we are talking contexts of scalar and lists...	[reply]
Re^3: Memory tip by jrsimmon (Hermit) on Jun 30, 2009 at 17:09 UTC
Yes, they are different data structures and thus will have a different memory footprint. In your example, you are comparing the memory usage of a scalar to the memory usage of a list which contains 4 scalars ($list[0], $list1, $list2, $list3). The point I'm making is that you're comparing apples to oranages... I don't mean to be harsh, but perhaps you should peruse Which bad behaviour most deserves an electric shock? while paying specific attention to Re^2: Which bad behaviour most deserves an electric shock?.	[reply]
Re^3: Memory tip by SuicideJunkie (Vicar) on Jun 30, 2009 at 18:08 UTC
The scalar also contains less information, and the process is NOT generally reversible. It is quite easy to reduce the memory use when your algorithm is allowed to be lossy :) For example, consider `my @array = (1, 5, "hello,world!", 3.1415);` There is no way to restore the original array after doing a `join ',',@array` on it. `"1,5,hello,world!,3.1415"` could have been `(1, 5, "hello,world!", 3.1415)` or it could have been `(1, 5, "hello", "world!", 3.1415)`.	[reply] [d/l] [select]
Re: Memory tip by ikegami (Patriarch) on Jun 30, 2009 at 19:08 UTC
You might be interested in: `$ perl -MDevel::Size=total_size -wle' my @a; print total_size(\@a); @a=(1,2,3,4); print total_size(\@a); @a=(); print total_size(\@a); ' 56 <- Empty 136 <- With 4 numbers 72 <- Empty` [download] The internal buffer of arrays and strings isn't shrunk when the array or string is shrunk in anticipation that it will grow again. Also interesting: `$ perl -MDevel::Size=total_size -wle' sub foo { my @a; print total_size(\@a); @a=(1,2,3,4); } foo(); foo(); ' 56 <- Fresh variable 72 <- Fresh variable` [download] Variable aren't even deallocated when they go out of scope!	[reply] [d/l] [select]
Re: Memory tip by cdarke (Prior) on Jun 30, 2009 at 18:19 UTC
Demonstrates that Perl arrays are not a bit like C arrays. Try it with a hash.	[reply]