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Re: simply appending to a scalar...
by japhy (Canon) on Jun 25, 2006 at 13:27 UTC
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$x = $x . $y can be written as $x .= $y, and yes, that's the "best" way to append to a scalar.
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Re: simply appending to a scalar...
by davido (Cardinal) on Jun 25, 2006 at 16:36 UTC
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There are seven ways I can immediately think of, and you've picked one of the clearest and easiest. Here is the list I can come up with:
- The . (dot) operator: concatenation.
- The .= operator: append.
- substr: Substring manipulation.
- join: Joining two or more strings.
- s/(...)/$1$string/: Substitution.
- The qq/...../ or "...." operator: interpolation.
- open: open my $fh, '>+', \$variable or die $!;: Print to an in-memory filehandle.
I'm sure I've missed a few, but these ways jump to mind immediately. The last method I listed; open, is pretty obfuscatory in nature. There aren't many situations I can think of where it would be a favorable approach, especially if simple concatenation is your goal. But it's there, so I mentioned it.
dot (.) and dot-equals (.=) are definitely the simplest and clearest approaches.
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Examples of mentioned solutions:
- $data = $data . $more;
- $data .= $more;
- substr($data, length($data), 0, $more);
- substr($data, length($data)) = $more;
- $data = join('', $data, $more);
- $data =~ s/\z/$more/;
- $data =~ "$data$more";
- { open(my $fh, '>+', \$data); print $fh $more; } (Requires 5.8)
Other useful solutions:
- $data = sprintf('%s%s', $data, $more);
- $data = pack('a*a*', $data, $more);
- $data = pack('(a*)*', $data, $more); (Requires 5.8)
- $data = do { local $"; my @array = ($data, $more); "@array" };
Other text formatting functions:
- format formats text sent to a file handle.
- formline is the guts of format.
And that's only using tools meant for text manipulation and/or formatting. You could do weird stuff like:
- $data = reverse scalar reverse $data, $more;
- $data = do { local $;; my %hash; $hash{$data, $more}++; (keys(%hash))[0] };
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The example of the open function you've mentioned has become of some interest also.
I am able to open and print to the in-memory filehandle, though unable to syswrite to it and i don't know why :(
thank you for your patience,
Isaac.
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Because syswrite operates at the system level, bypassing Perl's higher-level file IO.
Let me just say this however: Of the seven ways I listed for appending information to a string, in-memory filehandles are the most difficult, least Perlish one; the one that I strongly urge you to consider not using. There are three ways down from the top floor of the Sears Tower building: You can take the elevator, you can take the staircase, or you can jump off the roof. I would advise against jumping off the roof. But doing so will get you to the bottom, pronto.
There is a very limited set of problems for which using in-memory filehandles will be the optimal choice. ...very limited, and you rarely see them in run-of-the-mill everyday code. It's a little like symbolic references (though even less useful); they exist, they have uses, but you will rarely see them in code, and will probably never actually need to implement them yourself.
You may have one of those fairly uncommon situations where the in-memory filehandle leads to better code. If you do, I'd love to see what that application is. Then the next time someone asks where it's a good idea to use them, I'll have at least one good example. lol
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Re: simply appending to a scalar...
by Hue-Bond (Priest) on Jun 25, 2006 at 13:40 UTC
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use Benchmark qw/cmpthese/;
my $append = 'B' x 50;
cmpthese (100, {
dot => sub { my $c = 'A' x 50; $c = $c . $append f
+or 1..1e5; },
dot_eq => sub { my $c = 'A' x 50; $c .= $append f
+or 1..1e5; },
substr => sub { my $c = 'A' x 50; substr $c, length $c, 0, $append f
+or 1..1e5; },
});
__END__
Rate substr dot dot_eq
substr 16.5/s -- -35% -37%
dot 25.5/s 55% -- -2%
dot_eq 26.0/s 58% 2% --
Update: Erm, had to put the 1e5 inside each sub to reduce the overhead of the my declaration.
Update2: After playing with it for a while, I find interesting what happens when $append has a size that is a power of 2. Size of $c doesn't matter:
use Benchmark qw/cmpthese/;
my $append = 'B' x 256;
cmpthese (100, {
dot => sub { my $c = 'A' x 50; $c = $c . $append f
+or 1..1e5; },
dot_eq => sub { my $c = 'A' x 50; $c .= $append f
+or 1..1e5; },
substr => sub { my $c = 'A' x 50; substr $c, length $c, 0, $append f
+or 1..1e5; },
});
__END__
Rate substr dot_eq dot
substr 6.99/s -- -35% -49%
dot_eq 10.7/s 53% -- -21%
dot 13.6/s 95% 27% --
Size of $c doesn't matter. So if you are working with 8 Kb chunks, you better use $data = $data . $more.
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So if you are working with 8 Kb chunks...
... you're doing IO and microbenchmarks of Perl operators really don't matter. If you really need all the speed you can get, and if you've already optimized the rest of the slow parts in your program, tune your buffer size.
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substr: >6.80
dot_eq: 10.2 .. 10.7
dot: 13.3 .. 13.6
This is perl, v5.8.8 built for i486-linux-gnu-thread-multi on Debian/Linux.
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Re: simply appending to a scalar...
by bart (Canon) on Jun 25, 2006 at 15:34 UTC
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$_ = "Old buffer ";
sysread DATA, $_, 4, length;
print;
__DATA__
datadatadata
Result:
Old buffer data
So this effectively can be used to append. | [reply]
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bingo, i'm very gratefull for the answers i've received,
indeed sysread() can do all that i need and i overlooked that.
thank you :)
Isaac.
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