in reply to How Perl Optimize your code & some code TIPS ;-P

You give 3 examples of concatenating a string: 
    $var = "foo" ;
    $var .= "bar" ;             # 1

    $var = "foo$var" ;          # 2

    substr($var,0,0) = "foo" ;  # 3

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Of these ways to do it, I find number 2 the most readable. you can see immediately what is going on. But you claim that method 3 is the fastest, because you don't have to "rewrite" $var.

For fun, I did a little benchmark: 

#!/usr/bin/perl -w

use Benchmark;

timethese(1_500_000, {
    Interpolate => q/$var = "String"; $var = "foo$var";/,
    Concatenate => q/$var = "String"; $var = "foo" . $var;/,
    Substr      => q/$var = "String"; substr($var, 0, 0) = 'foo'/
});

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On a 1.6GHz P4 machine, I got the following results (Perl 5.8.0): 
Benchmark: timing 1500000 iterations of Concatenate, Interpolate, Subs
+tr...
Concatenate:  1 wallclock secs ( 1.56 usr +  0.00 sys =  1.56 CPU) @ 9
+60000.00/s (n=1500000)
Interpolate:  1 wallclock secs ( 1.59 usr +  0.00 sys =  1.59 CPU) @ 9
+45812.81/s (n=1500000)
    Substr:  2 wallclock secs ( 2.07 usr +  0.01 sys =  2.08 CPU) @ 72
+1804.51/s (n=1500000)

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Perl 5.6.1 and 5.005 give similar results; the substr trick is consistently the slowest.

So, optimizations can be very useful, but I doubt that it is in this case. I value readability over performance, within reasonable limits. Most of the time, use of another algorithm can yield far better results than micro-optimizations.

Arjen

Update: I made a little mistake in showing the examples. Now, #1 and #2 are IMO equally readable, depending on what you're doing.

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Re: Re: How Perl Optimize your code & some code TIPS ;-P
by autarch (Hermit) on Jan 24, 2003 at 20:34 UTC

    In fact, they're identical. Perl rewrites this: 

    my $bar = "hello, $foo";

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    into this: 
    my $bar = "hello, " . $foo;

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      In fact, they're identical. Perl rewrites this:

      my $bar = "hello, $foo";

      into this:

      my $bar = "hello, " . $foo;

      $ perl -MO=Terse -e '$var = "String";  $var = "foo$var"'

      $ perl -MO=Terse -e '$var = "String";  $var = "foo" . $var'

      Strip out the addresses (0x*) and do a diff. Almost the same :-)

      Arjen

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        No really, it's the same. The null operations are never seen by runtime perl and only exist in the optree as artifacts. I took the output from both of your lines, stripped out the null operations and they are 100% identical. Incidentally B::Concise provides better output than B::Terse.

        $ perl -MO=Terse -e '$var = "String"; $var = "foo$var"'
LISTOP leave [1]
    OP enter
    COP nextstate
    BINOP sassign
        SVOP const PV "String"
        SVOP gvsv  GV *var
    COP nextstate
    BINOP sassign
        BINOP concat [1]
            SVOP const PV "foo"
            SVOP gvsv  GV *var
        SVOP gvsv  GV *var

$ perl -MO=Terse -e '$var = "String"; $var = "foo" . $var' 
LISTOP leave [1]
    OP enter
    COP nextstate
    BINOP sassign
        SVOP const PV "String"
        SVOP gvsv  GV *var
    COP nextstate
    BINOP sassign
        BINOP concat [1]
            SVOP const PV "foo"
            SVOP gvsv  GV *var
        SVOP gvsv  GV *var

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        Seeking Green geeks in Minnesota

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